Author Topic: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown  (Read 36559 times)

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Online EEVblog

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EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« on: November 06, 2013, 07:57:25 am »
What's inside a vintage Fluke 5450A Resistance Calibrator used in cal labs to calibrate multimeters?
Coto reed relay: http://www.farnell.com/datasheets/1486738.pdf
High Resistance measurement with a relay matrix:



 

Offline daddario

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #1 on: November 06, 2013, 09:00:56 am »
I've always found precision resistors absolutely fascinating.
When I first opened up my Cambridge Instrument co. 5 'digit' decade box for cleaning, I've must have spent at least half an hour just examining all the hand-wound resistors and the stupendous switches in it. No painting or piece of architecture can match up the beauty and and the art of these things. Amazing.
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Online sipo75

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #2 on: November 06, 2013, 09:51:36 am »
I stumbled upon the Fluke 5450A only last  week on eBay and was tempted to buy one. But they are still on the high side for transatlantic shipping.

As usual a great teardown.

 

Online Vgkid

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #3 on: November 06, 2013, 09:59:01 am »
What a great teardown. I love seeing precision resistors in test gear. It will be interesting if this video drives up prices. If I was rich I could have bought one over the summer, but I bought a broken multimeter instead.
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Offline Dave

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #4 on: November 06, 2013, 11:10:40 am »
Yep, Sprague are definitely still around. Does Vishay Sprague ring any bells?

Great video, mate. I really love precision test gear, be it old or young. :D
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Online EEVblog

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #5 on: November 06, 2013, 11:19:32 am »
Does anyone else a see an ad below the embedded youtube video above?
I've never seen that before. I don't know if that's youtube or the forum plugin?
 

Offline jebcom

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #6 on: November 06, 2013, 11:35:06 am »
Does anyone else a see an ad below the embedded youtube video above?
I've never seen that before. I don't know if that's youtube or the forum plugin?

I don't see an ad below it. A small Fluke ad showed up at the bottom of the video itself briefly, but that's common.
 

Offline CesarEscudero

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #7 on: November 06, 2013, 11:37:15 am »
No ads, with and without adblock.
 

Offline V42bis

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #8 on: November 06, 2013, 12:54:37 pm »
I wonder if they just get as close as they can first time on the low value wire wound 1 ohm or if there is a tweek while building them. It would be impossible to get the right wire length first time.

Great work Dave
 

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #9 on: November 06, 2013, 01:08:42 pm »
Here is what I mean
 

Offline Orpheus

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #10 on: November 06, 2013, 01:14:09 pm »
It occurs to me that by feeding the wire through a conductive guide or nozzle, they could measure its resistance AS it was wound. No guesswork needed.
 

Online BravoV

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #11 on: November 06, 2013, 01:25:34 pm »
No embedded ads here at my Chrome at my PC, and also Safari and Chrome at ipad.

Example from my Chrome at PC :

 

Online Vgkid

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #12 on: November 06, 2013, 01:28:47 pm »
No embedded ads on IE either.
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Offline Treehouseman

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #13 on: November 06, 2013, 01:40:18 pm »
Here is what I mean
Dave, I'm afraid you need to check your PC for an infection. I noticed some text overlaid on the bottom of the player and it said "Deal Finder". After looking it up, came across this (http://www.bleepingcomputer.com/forums/t/497925/removal-of-deal-finder-malware/) where someone posted an image of it that the ad looks very similar to yours. There should be info on how to remove it there, if there isn't sufficient information or the method listed doesn't work I'll see if I can dig something else up.
 

Offline NiHaoMike

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #14 on: November 06, 2013, 01:40:31 pm »
Why aren't the resistors mounted inside a thermal oven to reduce the effect of ambient temperature and sealed in a hermetic chamber with desiccant to prevent humidity from changing the values?
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Offline Kryoclasm

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #15 on: November 06, 2013, 01:41:58 pm »
Those 1 ohm and 1.9 ohm resistors look like they are wrapped on mica sheets

I'm betting those cost a very pretty penny each.




Wiki snippet about Mica Sheets...
These sheets are chemically inert, dielectric, elastic, flexible, hydrophilic, insulating, lightweight, platy, reflective, refractive, resilient, and range in opacity from transparent to opaque. Mica is stable when exposed to electricity, light, moisture, and extreme temperatures. It has superior electrical properties as an insulator and as a dielectric, and can support an electrostatic field while dissipating minimal energy in the form of heat; it can be split very thin (0.025 to 0.125 millimeters or thinner) while maintaining its electrical properties, has a high dielectric breakdown, is thermally stable to 500 °C, and is resistant to corona discharge.

Sheet mica is used in electrical components, electronics, isinglass, and atomic force microscopy.
« Last Edit: November 06, 2013, 01:51:45 pm by Kryoclasm »
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Offline acstd90

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #16 on: November 06, 2013, 02:57:20 pm »
The 1 and 1.9 ohm resistors are mounted on Mica card
The % for the resistors and the nominal value is a specification for the actual resistance value around the nominal cardinal value. The actual specification of the resitance used for calibration purposes is the absolute specification and the TC specification. The ppm / % is used in the entry mode. The entry mode is to enter the value seen on the meter under test and the 5450 will reply with the difference of the device under test with respect to the standard in ppm or %. Relays on the sockets allow for ease of replacement but they also keep heat from the coil farther away from the PCB that has the resistors mounted on it. Same with the regulator mounted on the flex strip.
 

Offline kaushleshchandel

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #17 on: November 06, 2013, 03:42:22 pm »
Does anyone else a see an ad below the embedded youtube video above?
I've never seen that before. I don't know if that's youtube or the forum plugin?

I don't see any ads either.. Only links to your own videos...
 

alm

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #18 on: November 06, 2013, 03:45:14 pm »
I agree that this looks like Spyware/Adware on Dave's computer.
 

Offline Lightages

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #19 on: November 06, 2013, 04:04:47 pm »
It looks to me that the regulator that is connected to the board with that flat flex cable was to thermally isolate it from the rest of the circuit. I think it is obvious why one would do that.
 

Online BravoV

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #20 on: November 06, 2013, 04:25:57 pm »
Really surprising to see that new "branded"  ::) benchtop DMM just can't do auto ranging decently, isn't that strange ?  :-DD
 

Offline calexanian

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #21 on: November 06, 2013, 05:27:49 pm »
Potter and Brumfield relays have been around since Moby Dick was a guppy, Guardian relays are just about as old. Also that does appear to be mica in those low value resistors, You never forget that broken edge appearance. The reason for the sockets on those relays was multi fold. First it was unheard of and frowned upon to directly solder those relays as many of them used polystyrene in the casing for low leakage and it would melt when soldered quite easily. Also I recall people would be nervous about the limited switching life numbers for those relays. Some of the various platings had ridiculously  low cycle counts before replacement like 10,000 times, particularly for gold plating which these most likely are. Somewhere I have a bucket of those things from our OLD products and I used to play with them when I was a kid. Then one day we started buying PC mount relays from Hasco (And still do 30 years later) Those appear to be Micalex or Micanol plastic in those sockets, fancy fancy expensive mil spec. Its typically that mint green color.
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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #22 on: November 06, 2013, 05:40:34 pm »
Really surprising to see that new "branded"  ::) benchtop DMM just can't do auto ranging decently, isn't that strange ?  :-DD
No. What's the problem? Any decent autoranging system will be designed with some hysteresis to prevent the meter from nervously switching between two ranges. In general a stable reading with one digit less is preferable to the meter switching range every few seconds due to noise. If you want to force the meter to choose one particular range, use manual ranging. Calibration procedures will always call for manual ranging.
 

Offline ddrl46

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #23 on: November 06, 2013, 07:02:39 pm »
Here is what I mean

I've had that happen before, some shitty extensions in Google Chrome (I assume you are using Google Chrome as well) add that under images / videos. Try disabling all your extensions and enabling them one by one to figure out which one is causing it. A virus scan probably isn't a bad idea either.

If I remember correctly that deal finder thing is called Superfish.

« Last Edit: November 06, 2013, 07:08:42 pm by ddrl46 »
 

Offline Stonent

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #24 on: November 06, 2013, 08:17:08 pm »
Those 1 ohm and 1.9 ohm resistors look like they are wrapped on mica sheets

I'm betting those cost a very pretty penny each.




Wiki snippet about Mica Sheets...
These sheets are chemically inert, dielectric, elastic, flexible, hydrophilic, insulating, lightweight, platy, reflective, refractive, resilient, and range in opacity from transparent to opaque. Mica is stable when exposed to electricity, light, moisture, and extreme temperatures. It has superior electrical properties as an insulator and as a dielectric, and can support an electrostatic field while dissipating minimal energy in the form of heat; it can be split very thin (0.025 to 0.125 millimeters or thinner) while maintaining its electrical properties, has a high dielectric breakdown, is thermally stable to 500 °C, and is resistant to corona discharge.

Sheet mica is used in electrical components, electronics, isinglass, and atomic force microscopy.

Mica is one of the few materials that can provide a physical barrier, yet allow alpha radiation through. It's pretty much the exclusive material used in high sensitivity geiger probes.
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Online PA0PBZ

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #25 on: November 06, 2013, 08:53:10 pm »
Mica is one of the few materials that can provide a physical barrier, yet allow alpha radiation through. It's pretty much the exclusive material used in high sensitivity geiger probes.

Also used in microwave ovens to prevent the food from entering the wave guide, and supporting the heating wire in toasters (at least the older ones).
Keyboard error: Press F1 to continue.
 

Online EEVblog

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #26 on: November 06, 2013, 10:27:24 pm »
Why aren't the resistors mounted inside a thermal oven to reduce the effect of ambient temperature and sealed in a hermetic chamber with desiccant to prevent humidity from changing the values?

Because they are already low enough tempco to meet the desired spec.
 

Offline rs20

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #27 on: November 06, 2013, 10:45:05 pm »
Bit of constructive criticism -- I think an opportunity was missed in the video to point out that, when the 4-wire method is used, low contact resistance relays are pointless. Doesn't matter on the current supply side, and there's practically zero current on the sense side, so all around it doesn't matter. And if you're using this bit of kit without using four wires*, you lose out on performance in a big way: referring to the manual, using the 5450A in 2-wire mode at 1 ohm introduces a 25 milliohm error (90 days, after the 2 wire compensation, using test leads that were included in the calibration procedure, etc, etc), which works out to 25000 ppm, a thousand times more than the 4-wire mode specs you pointed out.

Throughout the video, whenever you were talking about contact resistances, or surprise and the non-soldered relay sockets, or the mention of amazing gold plated relay sockets, you should have heard me shouting "only in 2 wire mode!?!?". Those are not 25ppm in 2 wire mode relays or sockets. And 25ppm in 4 wire mode is achievable with a rusty nail.

Along the same lines, the relays on the top and bottom of that schematic aren't the rows and columns of a matrix (without a link to that earlier video you mentioned, I can't verify what you meant by "matrix", so I may be putting words in your mouth here, sorry), they are the separate output and sense lines, specifically separately relayed to make relay contact resistance completely, completely irrelevant**. You could connect a 1 ohm resistor using 5 ohm contact resistance relays all around, and it'd work perfectly fine**.

That minor nitpick aside, outstanding video as always; amazing components on show in there.

* Sure, you can calibrate a multimeter that only supports 2 wires, but in that case, all the plug contact resistances and lead resistances are probably going to dwarf even a half-decent relays' contacts? So again, what's the point in using million dollar relays?
** In 4 wire mode.
 

Online BravoV

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #28 on: November 06, 2013, 11:49:44 pm »
Really surprising to see that new "branded"  ::) benchtop DMM just can't do auto ranging decently, isn't that strange ?  :-DD
No. What's the problem? Any decent autoranging system will be designed with some hysteresis to prevent the meter from nervously switching between two ranges. In general a stable reading with one digit less is preferable to the meter switching range every few seconds due to noise. If you want to force the meter to choose one particular range, use manual ranging. Calibration procedures will always call for manual ranging.

So we just can not use this expensive benchtop DMM say like in logging mode at a DUT that it's resistance may wildly jumps across ranges ?  :o
« Last Edit: November 06, 2013, 11:52:47 pm by BravoV »
 

Offline rs20

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #29 on: November 07, 2013, 12:05:57 am »
So we just can not use this expensive benchtop DMM say like in logging mode at a DUT that it's resistance may wildly jumps across ranges ?  :o

He said hysteresis, not a complete refusal to ever change.

What that means is that is that the measurement ranges have slight "overlaps"; if the measured value is within that overlap region, it won't change range and just stay put. On the other hand, if the measured value changes 10x, 100x, or probably even 2x, it will most certainly change and follow.
 

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #30 on: November 07, 2013, 12:08:13 am »
Along the same lines, the relays on the top and bottom of that schematic aren't the rows and columns of a matrix (without a link to that earlier video you mentioned, I can't verify what you meant by "matrix"

I'm used to using the word "matrix" because in a former day job I spent years designing countless "crosspoint" type relay matrices to do all sorts of stuff for production test systems, it's just a term that generically pops out of my mouth whenever I see any form of relay switching system with a lot of relays.
 

Offline cengland0

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #31 on: November 07, 2013, 03:37:47 am »
Dave said something that interested me.  There were four 40 ohm resistors in parallel to make 10 ohms.  Each resistor was at 0.05% tolerance and he said having four of them makes that better than 0.05%.  Why would that be?  Obviously later in the video he discovers they are all specially matched which could make this true but he said this before knowing that detail.

So each 40 ohm resistor could have a resistance between 39.98 and 40.02 ohms and still be within tolerance.  If each resistor just happened to be 39.98, you would have a total resistance of 9.995.  That is still 10 ohms at 0.05%.
 

Online Vgkid

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #32 on: November 07, 2013, 04:48:27 am »
^^^ I believe that these would be binned to reduce the errors.
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Offline free_electron

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #33 on: November 07, 2013, 05:36:32 am »
the electrols are used in the theithley electrometers as well.
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Offline SeanB

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #34 on: November 07, 2013, 05:58:15 am »
I got a Google itself ad. and I am running this site as the only one not adblocked ( gives the clicks to Dave). Looks like they are monetising almost every video these days.
 

Offline SeanB

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #35 on: November 07, 2013, 06:17:04 am »
The boards most likely were made, tested and then stored in a CA store until needed, and then they were pulled out of storage, cleaned and assembled.

The Mica used to make those low value resistors was likely mined in South Africa, funnily enough in a mine situated in the Lowveldt right next to a small village called appropriately enough Mica. Nice town, with some beautiful scenery and some lovely road cuttings through the actual mica bands.
 

Offline cengland0

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #36 on: November 07, 2013, 06:30:28 am »
The boards most likely were made, tested and then stored in a CA store until needed, and then they were pulled out of storage, cleaned and assembled.

The Mica used to make those low value resistors was likely mined in South Africa, funnily enough in a mine situated in the Lowveldt right next to a small village called appropriately enough Mica. Nice town, with some beautiful scenery and some lovely road cuttings through the actual mica bands.

Doing some cave exploring in Italy, I found some naturally occurring mica.  It was one of my best geological finds and it remains in my rock collection today.  It's only about a square inch, oddly shaped, about 2mm thick, and very fragile.
 

Offline wavebits

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #37 on: November 07, 2013, 06:58:59 am »
I'm thinking the shielded box around the low ohm wirewound resistors is not so much to prevent electrical noise but to decrease thermal differentials caused by drafts within the enclosure. The noise suseptability of this low impedance part of the circuit would be minimal. Temperature gradients however, would be an important consideration.
 

Offline Stonent

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #38 on: November 07, 2013, 08:00:02 am »
I'm thinking the shielded box around the low ohm wirewound resistors is not so much to prevent electrical noise but to decrease thermal differentials caused by drafts within the enclosure. The noise suseptability of this low impedance part of the circuit would be minimal. Temperature gradients however, would be an important consideration.

I figured it was to just keep something from falling on them and shorting or affecting the measurement.
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Offline RadoK

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #39 on: November 07, 2013, 08:27:55 am »
Here is what I mean

Hi Dave,
I had the same Ad problem in Firefox a week ago. If your problem is firefox related then check firefox Add-ons menu. In my case there was one add-on that was showing ads below videos. After removal everything was ok.

Rado
 

Offline nivek

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #40 on: November 07, 2013, 09:58:02 am »
Here is what I mean

Are you per-chance using the smooth gestures plugin in Chrome?  They recently updated it to include ad's that look identical to that... you can disable them in the settings for the extension (Hidden at the bottom of course).
 

Offline Razor512

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #41 on: November 07, 2013, 11:46:04 am »
How do companies come up with a reference to begin with, for example how did they come up with the first resistance reference and what makes it so special and accurate?

For a bunch of calibration equipment, they seem to be calibrated off of a bunch of other calibration equipment is a weird cycle, but there is never ant decent info on how the reference came to be in the first place and how was that first one made to be accurate when there is nothing else to compare it to?
« Last Edit: November 07, 2013, 12:32:18 pm by Razor512 »
 

alm

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #42 on: November 07, 2013, 12:10:10 pm »
This gets into experimental physics and metrology. You could define 1 Ohm as the resistance of a certain length and volume of mercury (artifact calibration), which is what they did in the past. You can also define the Ohm in terms of current and voltage, assuming you have standards for 1 A and 1 V. The current definition of resistance is based on quantum physics (search for integer quantum Hall effect). Since this is not a very practical definition, they will make a (supposedly) very stable standard resistor and characterize its value and stability using the quantum Hall effect. They will then use this resistor to calibrate other resistance standards.
 

Offline John Coloccia

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #43 on: November 07, 2013, 01:47:35 pm »
Hey, did anyone notice what looks like silver Mica caps around the Z80?  I wonder what function they serve that they would need to use mica caps?
 

Offline codeboy2k

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #44 on: November 07, 2013, 03:16:51 pm »
This gets into experimental physics and metrology. ...they will make a (supposedly) very stable standard resistor and characterize its value and stability using the quantum Hall effect. They will then use this resistor to calibrate other resistance standards.

This stuff is pretty cool, actually.  I just read up what I could on it.  You start with a MOSFET with high electron mobility (HEMT), sometimes also called an HFET. For making a quantum hall resistor (QHR), they will make a device similar to a typical MOSFET, using GaAs and AlGaAs, but with specially constructed terminals for making a 4-wire resistance measurement.  At ~1K temperature and in the presence of a magnetic flux density of 7-9 Teslas (the actual value depends on the silicon structure), then the 2D electron gas (2DEG) layer (which is tightly confined beneath the gate) will completely quantize, and a resistance between the source and drain is formed that is exactly 25,812.807 ohms !!

In other words, applying a fixed, known accurate constant current source from drain to source, it will develop fixed voltage plateaus (called the Hall Voltage VH) across the drain to source at each quantization level as the magnetic flux density is increased from 0T up to 8 or 9T and higher.  Each voltage plateau VH(i) actually represents a different resistance called the Hall resistance RH(i) (because the current is constant). Later experiments related this to the universal constant h, Planks constant and e, the elementary charge on an electron, as  RH(i) = h/ie2. The physicist who did this research and related this quantum hall effect to the universal constants was Klaus von Klitzing. He received a Nobel prize for this discovery in 1985.  In 1990 they (SI standards body) chose i = 1 and thus 25,812.807 ohms as the resistor standard and called it RK-90 (K in honor of von Klitzing).

And then, as alm said, this resistance is not practical to use, so they transfer it to a multitude of standard resistors like 1 ohm, 100 ohm, 1000 ohm and upwards using various methods, usually involving a very accurate null bridge and cryogenics.
 

alm

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #45 on: November 07, 2013, 04:16:51 pm »
And the fun thing is that current is currently defined in terms of Ohm's law and the definition of resistance (in terms of the QHE) and voltage (defined based on the Josephson effect). As far as I know they haven't come up with a QM definition of current, yet. So just put a known voltage from a Josephson junction array across a QHR (via some transfer standards) and you get your current standard ;).
 

Offline AG6QR

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #46 on: November 07, 2013, 04:20:57 pm »
Dave said something that interested me.  There were four 40 ohm resistors in parallel to make 10 ohms.  Each resistor was at 0.05% tolerance and he said having four of them makes that better than 0.05%.  Why would that be?  Obviously later in the video he discovers they are all specially matched which could make this true but he said this before knowing that detail.

So each 40 ohm resistor could have a resistance between 39.98 and 40.02 ohms and still be within tolerance.  If each resistor just happened to be 39.98, you would have a total resistance of 9.995.  That is still 10 ohms at 0.05%.

The resistors will have something like a "bell curve" normal distribution.  If they've got a standard normal distribution, the chance of getting all four of them at the very bottom or very top of the range is very slim.  The very worst case would be that the percentage error in the combined resistance would be as bad as the percentage error on one of them.  But the average case is much better than that, even if you're not specially selecting the resistors in matched sets.

If you're randomly selecting the resistors, you can only put a statistical confidence level on the likelihood of the worst case not happening.  But if you individually select the resistors, combining high ones with low ones and medium ones with each other, you can guarantee that the worst possible case doesn't happen.
 

Offline rarthur22

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #47 on: November 07, 2013, 04:28:48 pm »
I noticed that you didn't mention that the relay contacts were bifuricated.  We use those when running very low current inputs on control systems.  Standard relays have too high a minimum load to keep the contacts clean.  When switching low mA loads these type are necessary.   The minimum current for single contact relays would be too high for this type of application to get reliable operation.
 

Offline Bored@Work

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #48 on: November 07, 2013, 04:40:21 pm »
And the fun thing is that current is currently defined in terms of Ohm's law and the definition of resistance (in terms of the QHE) and voltage (defined based on the Josephson effect).

The SI ampere is still defined via the force between two conductors. But that is impractical to realize, so standard labs derive it via Ohm's law. There is some work going on to redefine the SI ampere based on the charge of a proton. But the BIPM members are discussing that for years now, and they don't seem to be in a hurry to do the change.
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alm

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #49 on: November 07, 2013, 04:58:04 pm »
The resistors will have something like a "bell curve" normal distribution.  If they've got a standard normal distribution, the chance of getting all four of them at the very bottom or very top of the range is very slim.  The very worst case would be that the percentage error in the combined resistance would be as bad as the percentage error on one of them.  But the average case is much better than that, even if you're not specially selecting the resistors in matched sets.
You can't assume a normal distribution (although the Fluke engineers might have had additional information), especially not one centered around the nominal value, so worst case is that you should assume a uniform distribution within the stated tolerance. Still, the standard deviation (and any confidence interval you care to calculate) of the parallel (or series) combination will be smaller than the single resistor. I believe the freely available book 'Analog SEEKrets' by Leslie Green contains a decent discussion of this topic.
 

Offline ddavidebor

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EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #50 on: November 07, 2013, 05:15:47 pm »
Why not? The grey beard woukd try to make it's best to hit the desired value!

This would end in a bell type distribution. (gauss curve)
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Online Dr. Frank

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #51 on: November 07, 2013, 07:39:58 pm »
The SI ampere is still defined via the force between two conductors. But that is impractical to realize, so standard labs derive it via Ohm's law. There is some work going on to redefine the SI ampere based on the charge of a proton. But the BIPM members are discussing that for years now, and they don't seem to be in a hurry to do the change.


Naa, that's not correct. The Proton experiment was too unprecise so it's not followed up.

The proposed and really planned change of the SI system is described here:
http://www.bipm.org/en/si/new_si/why.html  (Go through the details on the left side)

It begins with the redefinition of the kg by a (1) Si sphere and (2) the Watt balance, then defining fixed (exact) values of e, h and NA.

As soon as the new kg definition can be realized to a better level than the kg prototype in Sèvres (currently looses weight compared to copies), i.e. as soon as those two experiments agree better than 2x 10^-8, this new SI definition will be set active by BIPM. The status of the experiments  is checked every two years.

The Ampere would be then automatically defined by counting electrons (e.g. charge pump), and at a first step realized = mise en pratique by the - only then - exact definitions of Volt and Ohm by Josephson and von-Klitzing quantum standards.

Currently, Volt and Ohm are precise within SI to a level of about 3x10^-7 only (!), but can be practically realized to levels of about 10^-19 to 10^-9, depending on whether you compare e.g. different Volt standards on cryogenic level (SQUID) or at room temperature (problem of thermo voltages).

In a second, later step, if the charge pump experiments (counting electrons, SET) some day delivers currents big enough (nA) and without any missing electron counts, it would be possible to close the Metrological Triangle, i.e the values for e and h would be over determined by the measurements of U ~ f x h/e, R ~ h/e^2, I ~ e x f, and their numerical values could be measured exactly depending on the definition of the Second, instead of initially fixing their values arbitrarily.

As the current SI defintion of the Ohm is precise to 10^-7 Until only, such Ohm standards as the venerable Fluke 5450A and others, are fully sufficient.

Frank
« Last Edit: November 07, 2013, 08:45:41 pm by Dr. Frank »
 

Offline rs20

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #52 on: November 07, 2013, 08:55:18 pm »
The resistors will have something like a "bell curve" normal distribution.  If they've got a standard normal distribution, the chance of getting all four of them at the very bottom or very top of the range is very slim.  The very worst case would be that the percentage error in the combined resistance would be as bad as the percentage error on one of them.  But the average case is much better than that, even if you're not specially selecting the resistors in matched sets.
You can't assume a normal distribution (although the Fluke engineers might have had additional information), especially not one centered around the nominal value, so worst case is that you should assume a uniform distribution within the stated tolerance. Still, the standard deviation (and any confidence interval you care to calculate) of the parallel (or series) combination will be smaller than the single resistor. I believe the freely available book 'Analog SEEKrets' by Leslie Green contains a decent discussion of this topic.

When fluke describes the resistors as matched sets, they're doing something far better than relying on reducing standard deviations. As mentioned in the video, they'll be binned, that is, very accurately measured and grouped ("binned") according to measured value. Take a resistor from the 50.004 ohm bin and pair it with a resistor from the 49.996 bin, and hey presto, you're doing much, much better than the root-4 improvement you can expect to get from random chance.
 

alm

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #53 on: November 07, 2013, 09:04:38 pm »
If you read the question AG6QR was responding to, you'll note it was about Dave mentioning the improvement in tolerance before he discovered they were selected, i.e. picking your resistors at random. Obviously you can get the error arbitrary small if you spend enough time selecting them.
 

Offline cengland0

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #54 on: November 07, 2013, 09:05:46 pm »
Dave said something that interested me.  There were four 40 ohm resistors in parallel to make 10 ohms.  Each resistor was at 0.05% tolerance and he said having four of them makes that better than 0.05%.  Why would that be?  Obviously later in the video he discovers they are all specially matched which could make this true but he said this before knowing that detail.

So each 40 ohm resistor could have a resistance between 39.98 and 40.02 ohms and still be within tolerance.  If each resistor just happened to be 39.98, you would have a total resistance of 9.995.  That is still 10 ohms at 0.05%.

The resistors will have something like a "bell curve" normal distribution.  If they've got a standard normal distribution, the chance of getting all four of them at the very bottom or very top of the range is very slim.  The very worst case would be that the percentage error in the combined resistance would be as bad as the percentage error on one of them.  But the average case is much better than that, even if you're not specially selecting the resistors in matched sets.

If you're randomly selecting the resistors, you can only put a statistical confidence level on the likelihood of the worst case not happening.  But if you individually select the resistors, combining high ones with low ones and medium ones with each other, you can guarantee that the worst possible case doesn't happen.
Not sure I agree with this because the 0.05% is already a worse case scenario on a single resistor and I can see the manufacturing process having an entire batch being either high or low.

I don't even know how to do the math on this one.  So assuming you are correct, if I connect four 0.05% resistors in parallel, what is the formula to determine the final tolerance of all combined?
 

Offline cengland0

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #55 on: November 07, 2013, 09:12:37 pm »
When fluke describes the resistors as matched sets, they're doing something far better than relying on reducing standard deviations. As mentioned in the video, they'll be binned, that is, very accurately measured and grouped ("binned") according to measured value. Take a resistor from the 50.004 ohm bin and pair it with a resistor from the 49.996 bin, and hey presto, you're doing much, much better than the root-4 improvement you can expect to get from random chance.
I understand that but my question was about Dave's comment before he noticed the schematic called for specially matched sets.  I would then have thought the resistors were randomly selected and would still have a better tolerance than a single resistor alone.  I'm asking for help understanding the math on that aspect because I can see how if you select a random set of four, on average the resistance should be on a normal distribution but each resistor is probably on a normal distribution too.  Only a few would be on the extreme value but that's why there's a normal curve instead of them all being an exact value.  However, can it be stated mathematically that if you combine four items on a normal curve, the final product will have a non-normal curve but will be more narrow?  How does that math work?
 

alm

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #56 on: November 07, 2013, 09:42:58 pm »
I'm asking for help understanding the math on that aspect because I can see how if you select a random set of four, on average the resistance should be on a normal distribution but each resistor is probably on a normal distribution too.  Only a few would be on the extreme value but that's why there's a normal curve instead of them all being an exact value.  However, can it be stated mathematically that if you combine four items on a normal curve, the final product will have a non-normal curve but will be more narrow?  How does that math work?

I believe the freely available book 'Analog SEEKrets' by Leslie Green contains a decent discussion of this topic.
See chapter 3. Google should give you a link to the (legal) PDF version within a few seconds, hosted on this very site. It explains when and when not to use statistical tolerances, and gives some rules of thumb.

But I'll explain the basic case with the assumption that the resistors follow a normal distribution, which is not necessarily a fair assumption (see the book for reasons why and how to handle that). To keep it simple I'll first explain the series case. Assume the resistance of the 40 Ohm resistors is normally distributed with a mean of 40 Ohm and a standard deviation (or any multiple of the standard deviation, like a 95% confidence interval) of 0.05% * 40 Ohm, or 0.02 Ohm. The probability distribution of the total resistance of the four resistors in series is the sum of four normal distribution, which is another normal distribution with mean = sum of the means and variance (which is the standard deviation squared) = sum of the variances. If the total variance is the sum of the variances, then the total standard deviation is the root of the sum of squared standard deviations. So in this case the mean total resistance would be 160 Ohm with a standard deviation of sqrt(4*0.02^2) = 0.04 Ohm. This is 0.025% of the mean, i.e. half that the tolerance of the original resistors. A quicker way to calculate this for identical standard deviations is sd / sqrt(n), where sd represents the standard deviation of the individual resistor and n the number of resistors.

For the parallel case you can show that for normal distributions which are well above zero, the distribution of the reciprocal value is another normal distribution with mean 1/40 Ohm and standard deviation 0.05% * 1/40 Ohm. You can then add conductances to get the conductance of the four resistors in parallel, and again get a factor sqrt(4) improvement in standard deviation.

Note that this is about what the majority (eg. 99%) of the circuits is doing, as you note the worst case does not improve.
 

Offline cengland0

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #57 on: November 07, 2013, 09:54:30 pm »
If you were to design a resistance reference box like this today, wouldn't it be better to use solid state relays instead?  You wouldn't have to worry about contact resistance issues and the leakage is very low like 0.006 pA at 80 degrees C.  And only 0.003 pA at 40 degrees C.  And if you're concerned about that small leakage, putting a couple in series couldn't hurt because they are only around $1 each.

Without doing much research about the best ones to use, I picked a random one and looked at the specs and they are pretty good compared to a conventional relay.  http://www.clare.com/home/pdfs.nsf/www/CPC1016N.pdf

With conventional relays, the contact resistance can be low in the beginning but increase in time as the contacts become worn and seems that can mess with the calibration as you use it.  With the solid state relays, I would think the resistance would be consistent throughout the life of the product.  The internal resistance of the IC could be compensated with different value resistors if that is important.
 

Offline cengland0

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #58 on: November 07, 2013, 10:09:03 pm »
See chapter 3. Google should give you a link to the (legal) PDF version within a few seconds, hosted on this very site. It explains when and when not to use statistical tolerances, and gives some rules of thumb.

But I'll explain the basic case with the assumption that the resistors follow a normal distribution, which is not necessarily a fair assumption (see the book for reasons why and how to handle that). To keep it simple I'll first explain the series case. Assume the resistance of the 40 Ohm resistors is normally distributed with a mean of 40 Ohm and a standard deviation (or any multiple of the standard deviation, like a 95% confidence interval) of 0.05% * 40 Ohm, or 0.02 Ohm. The probability distribution of the total resistance of the four resistors in series is the sum of four normal distribution, which is another normal distribution with mean = sum of the means and variance (which is the standard deviation squared) = sum of the variances. If the total variance is the sum of the variances, then the total standard deviation is the root of the sum of squared standard deviations. So in this case the mean total resistance would be 160 Ohm with a standard deviation of sqrt(4*0.02^2) = 0.04 Ohm. This is 0.025% of the mean, i.e. half that the tolerance of the original resistors. A quicker way to calculate this for identical standard deviations is sd / sqrt(n), where sd represents the standard deviation of the individual resistor and n the number of resistors.

For the parallel case you can show that for normal distributions which are well above zero, the distribution of the reciprocal value is another normal distribution with mean 1/40 Ohm and standard deviation 0.05% * 1/40 Ohm. You can then add conductances to get the conductance of the four resistors in parallel, and again get a factor sqrt(4) improvement in standard deviation.

Note that this is about what the majority (eg. 99%) of the circuits is doing, as you note the worst case does not improve.
Hey, thanks.  I actually understand that explanation and it makes sense. 
 

Offline rs20

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #59 on: November 07, 2013, 10:48:24 pm »
If you were to design a resistance reference box like this today, wouldn't it be better to use solid state relays instead?  You wouldn't have to worry about contact resistance issues and the leakage is very low like 0.006 pA at 80 degrees C.  And only 0.003 pA at 40 degrees C.  And if you're concerned about that small leakage, putting a couple in series couldn't hurt because they are only around $1 each.

Without doing much research about the best ones to use, I picked a random one and looked at the specs and they are pretty good compared to a conventional relay.  http://www.clare.com/home/pdfs.nsf/www/CPC1016N.pdf

With conventional relays, the contact resistance can be low in the beginning but increase in time as the contacts become worn and seems that can mess with the calibration as you use it.  With the solid state relays, I would think the resistance would be consistent throughout the life of the product.  The internal resistance of the IC could be compensated with different value resistors if that is important.

Sorry about misinterpreting your earlier question about resistor combinations.

Where are these 0.006pA leakage current solid state relays? The one you linked has 1uA leakage, which is certainly unacceptable. I'd be pleasantly surprised if you could find a reasonably priced solid state relay that had a reasonably low on-state resistance (all the ones on digikey are basically immediately too high for 2-wire measurement; as I mentioned in an earlier post, though, on-state resistance is irrelevant in four-wire mode unless the relays are burning out or loading down the current source) and low leakage current (beware the distinction between control-to-relay leakage, and off-state leakage).

One point that comes to mind, at least for two-wire measurement, is that the on-state resistance would be a function of relay drive voltage. If you want the on-state resistance to be very repeatable (a far greater concern than absolute value), this is a very bad trait. At least a relay is very much either on or off.
 

Offline sync

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #60 on: November 08, 2013, 12:49:19 am »
Without doing much research about the best ones to use, I picked a random one and looked at the specs and they are pretty good compared to a conventional relay.  http://www.clare.com/home/pdfs.nsf/www/CPC1016N.pdf
It has an on-resistance of max. 16 ohm. A mechanical replay can get easily 16 milliohm. That's 3 orders of magnitude better. And the resistance of the solid state relay changes with temperature. That's unacceptable for a calibrator.
 

Offline cengland0

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #61 on: November 08, 2013, 01:58:05 am »
Where are these 0.006pA leakage current solid state relays? The one you linked has 1uA leakage, which is certainly unacceptable. I'd be pleasantly surprised if you could find a reasonably priced solid state relay that had a reasonably low on-state resistance (all the ones on digikey are basically immediately too high for 2-wire measurement; as I mentioned in an earlier post, though, on-state resistance is irrelevant in four-wire mode unless the relays are burning out or loading down the current source) and low leakage current (beware the distinction between control-to-relay leakage, and off-state leakage).

One point that comes to mind, at least for two-wire measurement, is that the on-state resistance would be a function of relay drive voltage. If you want the on-state resistance to be very repeatable (a far greater concern than absolute value), this is a very bad trait. At least a relay is very much either on or off.
It says it in the graph titled "Typical Leakage vs. Temperature Measured Across Pins 3&4"  The pA was too small on the pdf so I had to copy and paste it into a text editor to confirm it was pA and not uA or mA.  Regarding repeatable on-state resistance, a relay doesn't seem repeatable because it degrades as the contacts become worn.

It has an on-resistance of max. 16 ohm. A mechanical replay can get easily 16 milliohm. That's 3 orders of magnitude better. And the resistance of the solid state relay changes with temperature. That's unacceptable for a calibrator.
I did see that the on-resistance was 16 ohms and I also stated you can compensate for that with different value resistors.  I would be more worried about conventional relays having an unpredictable resistance as time goes on as the device gets used.  At least this shouldn't change for a solid state device.  I did not initially see the graph that showed the solid state relay resistance changing with temperature and now that I see that, I can understand why they are not used.  Consider that a stupid question now.  Oops.
 

Offline ralph

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #62 on: November 08, 2013, 02:33:18 am »
It says it in the graph titled "Typical Leakage vs. Temperature Measured Across Pins 3&4"  The pA was too small on the pdf so I had to copy and paste it into a text editor to confirm it was pA and not uA or mA.

On that graph I read :
- 0.006µA @ 80°C
- 0.002µA @ 20°C

 

Offline cengland0

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #63 on: November 08, 2013, 02:57:48 am »
It says it in the graph titled "Typical Leakage vs. Temperature Measured Across Pins 3&4"  The pA was too small on the pdf so I had to copy and paste it into a text editor to confirm it was pA and not uA or mA.

On that graph I read :
- 0.006µA @ 80°C
- 0.002µA @ 20°C
Sorry to beat a dead horse on this but how do you know it's uA and not pA?  I couldn't see it clearly on the graph so I copied the text and pasted into a text editor and it says PA in all caps.  I'm curious if the greek letters are being pasted into my text editor incorrectly now.  Can you copy and paste and tell me what you get?
 

Offline stackoverflow

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #64 on: November 08, 2013, 03:24:11 am »
Great video!! @ 27min I'm guessing the low resistor are wound on mica card.
 

Offline codeboy2k

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #65 on: November 08, 2013, 06:14:12 am »
@cengland0:  I see uA too
graph attached.
 

Offline cengland0

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #66 on: November 08, 2013, 06:43:30 am »
@cengland0:  I see uA too
graph attached.
Thank you for posting the enlarged image.  It's clear that it is uA but could you do me a favor and copy that and paste it into notepad, wordpad, ultraedit or some other text editor?  All three that I tried on both Mac and PC paste it as PA.  The only thought that I have is it's an ASCII 80 but in a special font that doesn't correlate with the regular alphabet.  When pasting ASCII 80 into a regular text editor, it loses font detail.  If someone else confirms it happens to them, I might be able to sleep at night knowing I'm not going crazy.
 

Online Dr. Frank

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #67 on: November 08, 2013, 09:27:11 am »
Dave,
great video again, thanks for showing the entrails of that delicate box!

I just love those old designs/schematics and layouts, that's great engineering.
Today, that would cost too much, so those things are not done any more, I fear.

Anyhow, some comments on you video:

The 5450A is not a transfer standard, as you have stated.

It delivers 17 absolute Ohm values, but it is not capable of doing resistance transfers.

A transfer standard normally has less absolute uncertainty, but is capable of transferring one absolute value to another random value with very high uncertainty, perhaps one decade apart.

Examples of transfer standards:

ESI SR1010: transfers Ohm values per decade, uncertainty 1ppm.
Fluke 720A: transfers voltage to random values to 0.1ppm of input. Ex.: 10V to 1.018659V is transferred to 1ppm uncertainty of input (+/-10µV).
Fluke 752A transfers voltages to 10:1 and 100:1 levels. Example: 1000V can be compared against a 10V reference standard (i.e. 0.5ppm of input, much better than the 720A!)
HP3458A is able to transfer volt and ohm to an uncertainty of 0.05 ... 0.02 ppm of input, better than the 720A.
The Fluke 5440, 57xx an the Gertsch/Singer AC transformers/Bridges are further examples of transfer standards.

The 5450A could be redesigned much more compact and more stable today :
The processor board would be a one chip embedded solution, occupying <1/20 of the inital area.
The resistor standards board would also make use of relays and Teflon stand ups, but perhaps more compact bistable ones.

The FLUKE ww resistors would be replaced by more stable (vs. T and t) industrial metal foil resistors from Vishay.

Flukes was great on making selected sets of precision resistors.
They were selected for uncertainty, but mainly for identical temperature coefficients.
So, those 4 paralleled resistors could have a selected T.C. of < 1ppm/K
Today, a single Vishay metal foil types can achieve that at least without selection.
And Vishay oil filled type resistors timely drift would be < 2ppm/6yr, compared to the mediocre 12.5ppm/year of the old style Fluke technology.

Btw.: Thanks for the link to the FLUKE manual of the 5450A, it's not visible in EU, currently.

Frank
« Last Edit: November 08, 2013, 06:48:21 pm by Dr. Frank »
 

Offline Bored@Work

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #68 on: November 08, 2013, 05:02:48 pm »
The SI ampere is still defined via the force between two conductors. But that is impractical to realize, so standard labs derive it via Ohm's law. There is some work going on to redefine the SI ampere based on the charge of a proton. But the BIPM members are discussing that for years now, and they don't seem to be in a hurry to do the change.


Naa, that's not correct. The Proton experiment was too unprecise so it's not followed up.

... then defining fixed (exact) values of e, ...

But that is the charge of a proton, isn't it? I thought the idea was fixing e (one value in Coulomb = As), and then "simply" deriving the Ampere from the charge.
I delete PMs unread. If you have something to say, say it in public.
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Online Dr. Frank

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #69 on: November 08, 2013, 06:46:31 pm »
But that is the charge of a proton, isn't it? I thought the idea was fixing e (one value in Coulomb = As), and then "simply" deriving the Ampere from the charge.


Yes, you are right, in that could also define the charge of an electron, and therefore the Ampère, theoretically.

But it's not easy to make the experiment ( mise en pratique) with high enough precisision...

Therefore, BIPM and all metrology institutions decided on going the way described in the resolution I have linked.

Frank
 

Offline nitro2k01

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #70 on: November 09, 2013, 12:44:12 pm »
Great video!! @ 27min I'm guessing the low resistor are wound on mica card.
Is there any particular reason why it would be mica, other than just to use the absolute best material? I can't see any particular justification to use it for electrical resistivity, thermal conductivity or dielectric properties in this application.
Whoa! How the hell did Dave know that Bob is my uncle? Amazing!
 

Offline ddavidebor

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EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #71 on: November 09, 2013, 05:06:09 pm »
Because not only is the best but also is cheap and practical
Davide Bortolami,
Fermium LABS srl
 

Offline Ronald1962

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #72 on: November 10, 2013, 05:35:49 am »
Great tear down!!!

Dave talked about special connectors for input and sense.

Does anybody knows what type / brand it is?
 

Offline ddavidebor

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EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #73 on: November 10, 2013, 06:12:09 am »
It's a tellurium-copper low emf connector
Davide Bortolami,
Fermium LABS srl
 

Offline CHexclaim

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #74 on: November 11, 2013, 12:42:50 pm »
I have just finished watching the video but looking at the big relays, and taking into account that this is a precision gear, it is understandable to see the big Sprague filter capacitor there. Think when all relays are on plus the digital logic.

Great teardown.

CH!
 
 

Offline Chipguy

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That Fluke 5450A gave me an idea : Where to get custom value resistors
« Reply #75 on: November 18, 2013, 12:54:27 am »
Hey folks,

Dave's video gave me an idea to research something:
The question was : Can you order custom made resistors with affordable prices / small numbers.
The reason why I asked myself the question is that I have an upcoming project where I need resistor values like:
1 / 2 / 4 / 8 / 16 / 32 / 64 / 128 Ohm .... you can see, it's binary ;)

I found this:
http://www.vishaypg.com/foil-resistors/videos/?video=33

They make you 0805/1206/2010 and 2512 resistors with 0.01% tolerance and 0.2 ppm TC.
They can be ordered via digikey for example.

I have not found other distributors than Digikey that process the order for end customers.... yet.
Where is that smoke coming from?
 

Offline ddavidebor

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #76 on: November 18, 2013, 01:46:14 am »
how much this little jewels cost?
Davide Bortolami,
Fermium LABS srl
 

Offline Chipguy

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #77 on: November 18, 2013, 01:56:53 am »
how much this little jewels cost?

About 11 to 19 EUR each.
Where is that smoke coming from?
 

Offline ddavidebor

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EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #78 on: November 18, 2013, 06:36:26 am »
Well, that's pretty good
Davide Bortolami,
Fermium LABS srl
 

Offline codeboy2k

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #79 on: November 18, 2013, 06:36:39 am »
I'd like to have on hand some of those bulk, uncut resistors :)

I can measure myself and have my own microscope. 

 

Offline hpux735

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #80 on: December 03, 2013, 05:40:23 am »
Hi everyone! 

When I saw this video and Dave's call for information about the resistors, I immediately thought of my friend Tom's brother.  He has worked at Fluke for 25 years, and improved a process for building high-precision resistors for the standards industry.  He hasn't had a hand in this product, or it's resistors, but he personally knows the people that almost assuredly assembled them.  This thanksgiving, Tom got a raft of information about the process and the story of one of the nude virgins in utopia-land (which is Everett, Washington in case you were wondering.). 

As several of the responders have mentioned, the form used on the low value resistors is mica, not plastic.  Also, those resistors are almost certainly out of spec now that the case is open.  Resistances that low are very sensitive, and can be changed by dust on the surface.  Not that Dave probably cares that much, I assume he bought that unit for our viewing pleasure (thanks, Dave!).

The can resistors are wire-wound using enameled nichrome wire.  They are made individually, by hand, using lathes.  The resistance per foot of the wire is well known, and used as a first pass for the desired resistance.  The resistors are hermetically sealed in glass that has the same coefficient of thermal expansion as the nichrome wire.  I'm not clear on the details, but they are terminated using a technique called "water welding".  I can't really figure out what that is, but apparently it's '60s era tech.  The final resistance is fine-tuned by adjusting the length of the lead that is terminated.  Apparently, this is where the comment about the nude virgins in utopia-land was hilarious because, predominately, the women that make these resistors are about 60.  One woman in particular has been making resistors for Fluke since she was 18.  After about 40 years, she's getting close to retiring.  Also, the nichrome spool that they've been using to make the resistors is about exhausted, so they will become awfully hard to source in the near future. 

It's possibly that I'll get some of the raw materials in the mail soon.  If I do, I'll take high quality pictures and post them.

With regard to the pairing of the resistors:  They are indeed paired, but not for the reason you may think.  They select resistors based on the TCR (Temperature coefficient of resistance, I assume).  They determine that across a +/- 20 degree C range (let's assume) that 3 resistors have a +.3 ohm change and one has a -1 ohm change.  When you place these 4 resistors together, they will average near a null temp-co over the range.  The nichrome wire used in these resistors is terrible for temp-co.  If they are heated +20C, the wire expands and decreases the resistance.  It may remain this way for a week or two before it returns to spec.  This is why resistor pairing for temp-co is so important in this application.

In case you were wondering about the resistors used in late-model standards, they are still hermetically sealed in glass, but have a MUCH better temp-co.  The resistive material is sputtered (I believe) onto one side of the glass package.  On another sheet of glass, an inert coating is applied.  These pieces of glass are mounted to each other face-to-face (coatings on the inside).  The perimeter is also sealed with glass.  At this point, the resistance is roughly correct.  To get it perfect they laser ablate the resistive material to trim the resistor.  His contribution to the process is tuning the chemistry of the materials such that a minimum of glass is ablated with the resistive material, which increased yield from about 5 to 80 percent.
 

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #81 on: December 24, 2013, 09:08:21 am »
Awesome info, thanks!
I was coming to the conclusion that the Vishay precision resistor group did these resistors. Great to hear Fluke did them!, and that the group is still going?
 

Offline hpux735

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #82 on: December 24, 2013, 09:32:33 am »
Yes, the group is still going. It seems they're still considering whether they should buy another spool of the nichrome wire...  That's assuming they can get a new crop of nude virgins, that is.  The hermetically-sealed laser-trimmed resistors are still made in house at Fluke in Everett (and also Scotland, I think).
 

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #83 on: December 24, 2013, 03:02:54 pm »
@hpux735, thank you, what a great infos you have there, and please keep them coming !  :-+
 

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #84 on: December 24, 2013, 05:09:06 pm »
Yes, the group is still going. It seems they're still considering whether they should buy another spool of the nichrome wire...  That's assuming they can get a new crop of nude virgins, that is.  The hermetically-sealed laser-trimmed resistors are still made in house at Fluke in Everett (and also Scotland, I think).

I'm very surprised the group hasn't been given the arse by the "The Danaher System", as equivalent resistors could be outsourced to Vishay.
 

Offline SeanB

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #85 on: December 24, 2013, 05:36:05 pm »
Don't think the Vishay ones are accurate enough or stable enough to use as a primary standard. Might be some legislation calling for a certain thing, like with RAMBUS.
 

Offline hpux735

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #86 on: December 25, 2013, 06:57:48 am »
Yes, the group is still going. It seems they're still considering whether they should buy another spool of the nichrome wire...  That's assuming they can get a new crop of nude virgins, that is.  The hermetically-sealed laser-trimmed resistors are still made in house at Fluke in Everett (and also Scotland, I think).

I'm very surprised the group hasn't been given the arse by the "The Danaher System", as equivalent resistors could be outsourced to Vishay.

From what I understand, the Danaher purchase didn't change the group all that much.  I know that the employee perks (such as the employee store) got much worse.  It used to be that employees could buy equipment a vast discounts.  Now, it's more like a yearly auction format.
 

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EEVblog #544 - Fluke 5450A Resistance Calibrator - review part 1
« Reply #87 on: December 17, 2015, 11:00:21 pm »
As I recently acquired a Fluke 5450A, I'd like to revive this old thread, adding some interesting information.

My instrument came from the German Air Force, last calibrated in 2003 (coincidentally by Air Force cal lab, where myself worked 35 years ago).
It was obviously stored in a mining tunnel depot since then, and now sold as surplus.

The manufacturing date is about 1987/88, 2 years older than Daves instrument.

When I checked the resistors by a 34465A and a 3458A, I found the same drifts, as Dave did, mostly all lower resistance values drifted upwards, 1k and 1k9 more than the others, and the high ohmic ranges, 10M, 19M and 100M read extremely low, between -1000 and -5000 ppm.

So I quickly found the fault by 'nosing': The transport box and the instrument itself smelled like an old, musty cellar.
It was obviously stored under high humidity conditions for an extended period of time, which might well cause leakage paths.

When I opened  the instrument, see picture 1, it contained moulded ELECTROL reed relays for these high ranges, instead of the COTO types, like in Daves instrument.
Moulded components can very well suck water vapour inside, which would explain, where the leakage is located. 

So I heated the relays indirectly by an incandescent lamp to about 55°C, for about 5h.

These gross errors now have vanished, still reading a bit low, to the same level as shown in Daves video.
I will come back to that point later.


During the heating period, I monitored the 10k value, which was extremely stable to < 2ppm, meanwhile the temperature of these resistors also had increased by about 15°C.
That means, they really have a T.C. of around 0.15ppm/K.
Great performance, but how is this accomplished?

I now had a closer look & calculation on the T.C. matching, which is really very tricky.
You can see in the pictures, that the individual T.C.s are written on the resistors, with sign and in multiples of 0.25ppm/K.

So, the two 450k resistors are marked P0.75 and N0.50, which denotes +0.75ppm/K, and -0.50ppm/K.
For the calculation of the combined T.C. of the resulting 900k resistor, the difference of their T.C.s has to be averaged also,  so the resulting T.C. is 0.125ppm/K only, opposed to the expected 0.25ppm/K.

For the lowest resistor arrays of this chain, 10 Ohm, 9 Ohm, 81 Ohm, one can easily see the individual T.C. markings.
The paralleling of four resistors gives 4 fold improvement of the T.C., and also mitigates self heating effects when higher measurement currents are applied.

These big vias are used as the resistor reference points with Kelvin connection on the PCB bottom side.

This pairwise matching / averaging 'trick' can be found in all Fluke instruments, like the 332B/D, 752A, and the 742A, which explains their extremely low T.C.s, even after all these years.


I took the readings of all resistors from 10 Ohm to 9 MOhm and calculated the overall T.C.s.

So the 5450A resistors have T.C.s of -0.54ppm/K worst case, and 0.1ppm/K for the 10 kOhm resistor, which is in full agreement with my rough measurement.
« Last Edit: December 18, 2015, 12:20:26 am by Dr. Frank »
 
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Online Dr. Frank

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EEVblog #544 - Fluke 5450A Resistance Calibrator - part 2 - calibration
« Reply #88 on: December 17, 2015, 11:01:45 pm »
Part 2: Calibration

The 5450A consists of two nested, multifold  10:1 resistor dividers.
Therefore in CAL mode, it can very conveniently be calibrated, by using a stable DCV source and a precise 10:1 transfer instrument.

Instead of the bulky setup in the manual, I only used the 5442A calibrator and the 3458A, using its extremely linear A/D in DCV mode.

You first compare an external 10k reference resistor to the 5450A 10k and 19k resistors, and calculate their absolute values.

After that, the lower or upper decade values (i.e. 1k or 100k) are measured by making precise 10V:1V voltage comparisons.
The 3458A is capable of making 0.3ppm precise transfers.

Within 4 steps, the extreme values are calibrated, at about 1.2ppm accuracy, due to the cumulative ratio error.

The comparison against the 12 year old calibration revealed a typical drift of less than 1ppm/year.

On the highest resistors, 100 MOhm, leakage currents of the 3458A (up to 20pA) may deteriorate the accuracy, up to +/- 180ppm.

By using 50V for the excitation voltage (which can be measured precisely to < 1ppm), this error is reduced to about 36ppm.

Paralleling the 3458A alternatively to the upper or lower resistor at 50V and 5,55555V, giving the same ~ 5V reading (generating identical leakage current), allows to make precise corrections for the real resistor values, and also to solve for the leakage current.
Latter was estimated to be between 2 and 11 pA.
(See attached xls calculation.)

So even the 100M resistor could be determined to an uncertainty on the order of about 10ppm.. The fully corrected and the 50V excitation method differed by 5ppm only!

As a surprise, the 3 upper resistors were now in accordance to their nominal values, a few ten ppms off only.

Despite that result, the HP instruments both still measure very low values.
These are about within their specified accuracy of 50 and 500ppm

I assume a systematic problem with the HP resistance circuits, more probable than residual internal leakage currents in the 5450A.

I further assume, that the internal isolation of the HP instruments is too low, as the whole Ohm measurement circuitry involves the front / rear switch, the PCB, overload protection, and so forth.


As a conclusion this 5450A resistance calibrator performs much better than its specification:

The T.C. is about 5..10 times better.

The uncertainty specification may also be better, to the 1ppm level.

This is simply due to the nowadays easily available Ohm Offset Compensation method.
Impacts from thermal voltages were back then included into the spec, instead of removing them during qualification.
 
Frank   
   
« Last Edit: December 18, 2015, 01:38:32 am by Dr. Frank »
 

Offline 3roomlab

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #89 on: December 30, 2015, 02:26:19 pm »
very nice info dr Frank, did you need to physically apply IPA to clean anything inside?
Hayao Miyazaki - "there is no nuclear power here (at studio ghibli)"
Feynman - "What Do You Care What Other People Think?"
 

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #90 on: December 30, 2015, 08:24:13 pm »
very nice info dr Frank, did you need to physically apply IPA to clean anything inside?

Definitely not!
I did not touch anything inside.
The PCB was relatively clean, but something like dust or metallic particles were visible all over the top side and around the reed relays.
I removed that carefully with a fine, clean brush.

Frank
 

Offline zlymex

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #91 on: March 08, 2016, 12:36:41 am »
This is the first time I read this thread, a lot of useful info, thanks every body!
I have three of these 5450A, all in good condition. One of the amazing thing about it is the 81 meg WW, I cannot image how the hell they made it. Even the diameter is 0.00004", 5km of wires is needed!
For the past 10 years, I've been gathering information on these hermetically sealed WW resistors. I believe its still unrepeatable in many aspects. It's true(and pity) that fluke, the main company used so many of these, no longer use these resistors any more except in some hi-end products such as 732B(6.2k, 11.42k, 180, 1k, 8.823k), 742A(20.01 Ohm, 39.992k) and 752A(119.8k, 120k).

Modern calibrators such as Fluke 5720A(the grand son of 5450A, if I may say so) inherent many things from 5450A. Here is the photo of the Ohms board of 5720A and fragment schematics from it's serving manual


WW have been replaced with those film resistors, and big relay replaced by small latching relays, basic principle remains the same. As for the performance, some range better but others inferior. They still use WW resistors for 1 and 1.9 Ohm range.
 

Offline RobK_NL

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #92 on: March 09, 2016, 06:53:49 am »
Interesting!

That 90M part looks like it's a standard Caddock device; probably TF series.
Tell us what problem you want to solve, not what solution you're having problems with
 

Offline zlymex

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #93 on: March 10, 2016, 12:47:47 am »
Interesting!

That 90M part looks like it's a standard Caddock device; probably TF series.
I'm sure you are right. I don't have front photo of the 90M, but I have a back photo of Caddock TF656N.
 

Offline quarks

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #94 on: March 10, 2016, 02:44:53 am »
Does any know where to get the 4.5 MOhm resistors or a replacement?
 

Offline zlymex

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #95 on: March 10, 2016, 01:13:40 pm »
Does any know where to get the 4.5 MOhm resistors or a replacement?
That would be very difficult because hi-value WW resistors are difficult to make. Afaik, they only obtainable by taken out from other 5450A. Fluke replace those 4.5M with film type in their new design.
Here are some of my old stuff.
 

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #96 on: March 10, 2016, 08:06:30 pm »
Hello lymex,

it's very nice to read about your great experience here @ eevblog!


Did you measure a noteworthy change on these high ohm resistors from the Fluke 5450A, i.e. the 4.5MOhm and 81 MOhm resistors, towards smaller values, being about 200ppm or even 500ppm lower than specified?

And how did you measure these resistors, as for example all the HP DMMs are not capable of making precision measurements on high Ohm resistors?

As I wrote, I calibrated these resistors inside the 5450A by using 50V as a stimulus and measuring the 10:1 ratio .
This gave calibration values, which were in the correct ballpark, and the annual drift even on the 10M and 100M resistors was not so high.

When measured with my 3458A in 2W Ohm mode, I found the same big decrease in the 10M and 100M values, like quarks and also Dave did, which I still cannot explain, especially not the discrepancy between the 50V calibration method and the 5 V Ohm method inside the 3458A... Possible leakage currents inside the 5450A should be the very same in both configurations.


Has anybody of you, lymex and quarks, measured the 5450A high Ohm resistors with a Fluke 8508A or similar, using its high precision high Ohm mode, which sources up to 200V as compliance voltage?
This 8508A is really superior over the 3458A, concerning such resistor measurements...

Frank
« Last Edit: March 11, 2016, 08:50:29 pm by Dr. Frank »
 

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #97 on: March 11, 2016, 12:25:24 am »
Hi Frank,

That is a good question, and had been my headache for sometimes. You are right, the resistance of 3458A has been the weak point not only for high ohmetic but even 10k is inferior than other 8.5 digits DMM. Therefore, I use my 3458A mainly at its best - 10V range.

I had a plan several years ago to make a guarded high precision hi-resistance bridge, to compare resistor in 10M to 1T range(but only decades), testing voltage would be 100V, which is the standard test voltage in here for hi-resistance. The idea is based on the Guarded Active-Arm Bridge(see page 31 of NIST TN1458). I'm also material ready for build guarded hamon transfer standard of 10M, 1G and 100G, but I just bite off more than I can chew. I vanished from this forum for about a year and also very quite in that period. Since I come back, I'll finish what have been left over.

I build a very stable 100V standard before by cascading 10 LT1021B to facilitate the 100V required to measure mega ohms, but again, no serious measurements done on those 4.5M resistors.

I did measured those hermetic resistors in the box in lower voltage that most of them show a slight positive increase of about 50 to 360ppm. No negative change of more than 100ppm ring a bell. But I cannot remembered that I use 3458A to measure 5450A for 1M or above.
« Last Edit: March 11, 2016, 12:31:21 am by zlymex »
 

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #98 on: March 11, 2016, 05:44:42 am »
Has anybody of you, lymex and quarks, measured the 5450A high Ohm resistors with a Fluke 8508A or similar, using its high precision high Ohm mode, which sources up to 200V as compliance voltage?
This 8508A is really superior over the 3458A, concerning such resistor measurements...

Hello Frank,

no I did not use "HiVOhm" of the 8508A because max. allowed voltage is 50V (at least that is stated in the 5450 manual). 
But I am glad you asked, because of that I meassured the Fluke 5450A with my Fluke 8508A and the Fluke 5440A-7002 cables again and found all values to be in spec.

My guess is, although I used the very same cable, most likely my setup (1 or 2 years ago) was just not clean enough. Because today I cleaned my 5450 inside and also the cables. 

bye
quarks

 
« Last Edit: March 11, 2016, 05:48:50 am by quarks »
 

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #99 on: March 12, 2016, 03:45:55 am »
Hello Frank,

no I did not use "HiVOhm" of the 8508A because max. allowed voltage is 50V (at least that is stated in the 5450 manual). 
But I am glad you asked, because of that I meassured the Fluke 5450A with my Fluke 8508A and the Fluke 5440A-7002 cables again and found all values to be in spec.

My guess is, although I used the very same cable, most likely my setup (1 or 2 years ago) was just not clean enough. Because today I cleaned my 5450 inside and also the cables. 

bye
quarks

Hello quarks,

that's an interesting information, that your 5450A NOW seems to be in specification.

Would you mind to test, if there is a difference in reading, between the 8508A and the 3458A for 10M and 100M?

I still suspect the 3458A on these high ohm ranges, especially as I found this contradiction, i.e. the volt ratio method (10:1 divider at 50V) gave reasonable values for these resistors, instead.

HP had defined barn-door wide high Ohm specs, for some good reasons, I assume..

And a last comment: The 10k to 1M Ohm ranges in contrary are quite precise on the 3458A, and not as bad, as lymex stated.
You have to use the instrument properly, e.g. by using appropriate delays on OCOMP.

Cheers, Frank
 

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #100 on: March 12, 2016, 05:54:27 pm »
Hello Frank,

please give me the optimal settings for 3458A you want me to do this.

Bye
quarks
 

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #101 on: March 12, 2016, 06:37:52 pm »
Hello quarks,

that's easy, just use your Fluke super cable set, 2W Ohm, guard connected, open at the 3458A, NPLC 100, and just measure the 10M, 19M and 100M range, and compare these  to the values you measured with your 8508A.
It takes some time on the 3458a to settle to a stable value

Maybe you also have done a regular calibration already , using the ratio method?
Then these values were also interesting.

Thank you very much!

Frank
 

Offline quarks

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #102 on: March 12, 2016, 10:17:33 pm »
2W Ohm, guard connected, open at the 3458A, NPLC 100, and just measure the 10M, 19M and 100M range, and compare these  to the values you measured with your 8508A.
Hello Frank,

ok, I thought you wanted special settings for the 3458A (OHMF, OCOMP ON, delay, ...)

settings/invironment:
all warmed up >4h
20 °C 50% relh
8508A OHM Zero 2w? 20M? range (1µA <20V), 200M? range (100nA <20V)
3458A OHM NPLC 100, used ACAL before measuring
same cable used, guard connected to 5450A but not to DMM

            10 M       19 M       100 M
---------------------------------------------------------------------   
3458A   +78,3   +266,5      +373,0   ppm deviation to 8508A

I have not done a regular calibration on both DMMs.

bye
quarks

« Last Edit: March 12, 2016, 10:22:29 pm by quarks »
 

Online Dr. Frank

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #103 on: March 13, 2016, 12:39:39 am »
Hello quarks.
That's really interesting!
The 3458A values are off from the 8508a in the same ballpark, as the deviations Dave has seen on his 5450A, and me on mine, but obviously your readings were too high, whereas Daves and mine were too low.

I suppose, that the Ohm mode on the 3458A suffers from leakage currents in either direction, located maybe in the front / rear switch, or in the overload protection.

Could you please publish the individual values of the 5450A, the 8508A and the 3458A, so one can get an idea, how far the instruments deviate from the nominal values of the DMMs?

Thanks a lot!

Frank.
« Last Edit: March 13, 2016, 12:59:58 am by Dr. Frank »
 

Offline quarks

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #104 on: March 13, 2016, 03:45:19 am »
Hello Frank,

here you go

           10 M?           19 M?         100 M?
----------------------------------------------------------------------------------------   
5450A   10,00064      19,00115         99,999    stored values ( >15 year old and I do not have a cal document to proof if the values where true back then)
----------------------------------------------------------------------------------------
8508A   9,9985807     18,9921478     99,989104   
3458A   9,999364      18,99721         100,02641   
-----------------------------------------------------------------------------------------
             78,3             266,5              373,0   ppm deviation to 8508A
-----------------------------------------------------------------------------------------
8508A   -205,9          -473,8             -99,0   ppm deviation to 5450A values
3458A   -127,6          -207,4             274,1   ppm deviation to 5450A values

Please let me know how this will help, because we still do not know any real values.
We only see, that both DMMs disagree.
That is the bad thing when you have more then one "good" meter.
You never know which one is closer to the "truth".

Besides a real "PTB" calibration, maybe the best (cost effective) thing would probably be, if we find "travelling" Transfer Standards with good confidence in the actual values.

bye
quarks
 

Offline pelule

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #105 on: March 13, 2016, 05:11:01 am »
What confuses me is the deviation of measured values between 8508A and 3458A, at the 10MOhm ~79ppm.
This means both are at the worst case expected difference.
I would trust the Fluke more than the 3458A in case of resistance
Worst 1year spec
8508A = @ 365days (5°C, Normal Mode 20MOhm range) ~20ppm +5.
3458A = @ 365days (1°C) ~50ppm +10

PeLuLe
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Online Dr. Frank

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #106 on: March 13, 2016, 09:30:06 pm »
Hi quarks,
Both measurements  really have a big variance, i have to confirm, what PeLuLe states.
On the other hand, the 5450A does not need to be calibrated by an external calibration lab.

You already have everything to calibrate it on your own, i.e. an ESI SR 104, plus your 8508A for 10k reference
Then you just follow the calibration description in the 5450A manual, and you can transfer the uncertainty of the 10k standard to 10k => 100k => 1M=> 10M => 100M.
If you use your 4808 calibrator at 50V, and your 3458A, maybe also your 845A, you can simplify the procedure and increase the transfer uncertainty, and you will get a 10M/100M uncertainty much lower than the DMMs ones.

Then you can decide on your own, which DMM measures correctly; or if the deviation is still too high, you mayidentify another error source, instead.

Frank
« Last Edit: March 14, 2016, 05:04:36 am by Dr. Frank »
 

Offline quarks

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #107 on: March 15, 2016, 01:04:30 am »
Hello Frank,

as soon as I have enough time I would like to try to calibrate my 5450A.
To be prepared I made a calibration worksheet (see att. pic. like table 4-3 in the manual)
 
Also I have read
http://www.eevblog.com/forum/blog/eevblog-544-fluke-5450a-resistance-calibrator-teardown/75/
but so far I did not really understand how you did your calibration with only one source and the 3458A.
It would be great if you could share a kind of a "dummies guide".

thanks
quarks
 

Online Dr. Frank

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Fluke 5450A - Mysterious problem identified, not yet solved
« Reply #108 on: January 15, 2017, 03:29:52 am »
I calibrated my 5450A again, after one year, see column G
And again, the verification of 10M, 19M and 100M were way off, compared to the verification by 3458A, column I.



I opened the instrument, and measured these high Ohm values directly inside the unit, at the resistors, as indicated in the picture. The 5450A is set to OPEN, which de-energizes all relays.



From the measured values, "direct", in column K and M, you can see, that now all the high-Ohm readings were very well matching the before calibrated values.


As soon as I engaged the appropriate range, the readings dropped to the faulty values, between 50 to 300pm low. That's displayed in the picture above.

Therefore, there has to exist a leakage path inside the 5450A.

I de-soldered the high output/sense PTFE cables, as indicated here:



The measurement of the 100M stabilized much faster, but was faulty as before.
When unsoldering the cable to relay K30, the correct reading applied. K30 is open for the 100M Ohm range.

So I disassembled the K30 relay socket, and measured its leakage by applying 50V between two open contacts and monitoring the current, being <1pA. Therefore, the socket should have > 5 * 10^13 Ohm. In parallel to 100MOhm, that will create an error of a few ppm at most.

Anyhow, I cleaned the socket and its top and bottom area of the PCB with methyl alcohol. This did not yet cure the error.

So I investigated on relay K30.
Between isolated pins, @ 50V, I measured leakage currents on the order of 20..40pA, i.e. between two open relay contacts, between switch contact and guard (relay metal body), and between switch contact and coil contacts.
That gives leak resistors of about 10^12 Ohm each, giving an error for the 100MOhm resistor of several 100pm, as these paths add up.
It can also be observed, that the 5V digital supply may even source leakage currents into the network.



At a closer look on relay K30, you may see the base plate and the isolation washers, which are made from some plastic, which I assume not to be PTFE.
This plastic will give rise to the leakage currents across all the contacts, as indicated by the green arrows. I assume, that this plastic material contains filler / softening agent, which has outgassed or diffused after 30 years, and creates this leakage. Maybe the plastic material itself deteriorates, and the cracked products are conducting.

I disassembled the relay , removed the coil, and washed the contacts and base plate using benzine and methyl alcohol. This gave no improvement, the leakage currents remained the same.


The leakage of all mechanical relay K15..K20, K30 and K31 may affect the high Ohm readings.
If I pull these out, and engage the high Ohm ranges, the reading now will not drop any more.



Everybody can measure this summed leakage by applying 50V between case ground and guard. I observed about 450pA leakage current, so about 10^11 Ohm isolation only between analog and digital, and the resistor chain.. that's an order of 1000ppm error related to the 100M resistor.

The explanation, why it's possible to calibrate correctly, is indicated in the schematics. You see, that the 50V test voltage across 100M is divided to about 5V. The ratio between both voltage measurements gives the calibration ratio from 10M to 100M. The crucial leakage current does not affect the 100M resistor in this volt - measurement mode, but will definitely decrease its value during any normal resistance measurement.
There's only a leakage in parallel to the 10M resistor, which affects this ratio measurement, but an order of magnitude smaller, i.e. on the order of about 10ppm.



This error description is independent from brand of DMM and from measurement voltage, 10V or 100V, or else. So, even the highly prestigious 8508A will show the exact same error, in normal or in high voltage resistance mode.


So I conclude, that the HP instruments measure high Ohm very well, inside specification, or better.

The 5450A itself causes this fault in the high Ohm ranges. The same observations have been made for the instruments of eevblog (#544, 33:52 .. 34:10 min), PeLuLe, quarks, and mine, and maybe also by zlymex.

The Fluke PWW resistors are really great, but these mechanical relays suck.

Therefore, this is a systematic error after many years.

These relays may be un-obtainium as new.
Old stock is very expensive, I've seen about 80$ for 1 EA, but they will have definitely the same problem.
TYCO still offers these standard R10 relays, but isolation is specified the same, 10^11 Ohm, which is 3 orders of magnitude short.


In retrospect, the Fluke engineers faced the problem that these mechanical relays should isolate to 10^14 Ohm, but also would be able to carry up to 500mA of current. In this design, it was not possible to solve this contradiction. Maybe a look into the similar 5720A design might reveal, how it can be done better.
Or these newer instruments will face the same problem, after more than 10 years time.


To solve this problem in the 5450A, I think over separating higher and lower ranges by interrupting the path between the high ohm reed relays and the lower ohm mechanical relays, and connecting the high ohm ranges (1M ..100M) to the front jacks, and the lower Ohm ranges (1 Ohm .. 190 kOhm) to the rear.

Maybe later I'll find some really highly isolated reed relays, 5V coil and 100mA carriage current, to replace the failing relays.

Some final remarks about the  uncertainty of the calibration process.

I used the 5442A as a DCV source and the 3458A for the ratio measurements.
In the standard procedure, which follows the manual, the measured voltages and offsets were recorded in the calculation Excel-sheet, starting from my external 10k standard. The fields marked in green then display the calibrated value, which is consecutively used for the next upper/lower range.



As the 3458A may create additional leakage currents for the high Ohm ranges, I used an additional procedure, with two measurements for each resistance value, to cancel these DVM currents.
That gives very similar readings as the the standard process.



I also processed this latter method directly inside the open instrument, which also gave consistent results to the calibration from outside.
So I conclude, that the normal calibration process is really not much effected by the relays leakage.


Frank
« Last Edit: January 15, 2017, 08:14:20 pm by Dr. Frank »
 
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Offline pelule

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #109 on: January 15, 2017, 04:45:47 am »
Great investigation  :-+ :-+
I did the last verification of my 5450A in Dec 2015 against 1281 & 3458A. Both DMMs showed similar readings and  also against in in Oct 2014 calibrated values.
My annual verification is due, thus I will check and report my findings.
Hope I don't have that issue (yet).
I guess your front/rear solution my suitable, but has disadvanmtages (I personally prever to stay as close as possible to original function to prevent mishandling).
How is the idea of following solution/fixing:
- use low leakage COTO relays mounted in a fly over the board solution
There should be enough room/volume in the 5450A
BR
PeLuLe
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Online Dr. Frank

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #110 on: January 15, 2017, 05:41:06 am »
I guess your front/rear solution my suitable, but has disadvanmtages (I personally prever to stay as close as possible to original function to prevent mishandling).
How is the idea of following solution/fixing:
- use low leakage COTO relays mounted in a fly over the board solution
There should be enough room/volume in the 5450A
BR
PeLuLe
Yes, this splitting isn't nice, but it will be a reversible change.

COTO does not produce 10^14 Ohm insulating relays anymore, obviously.
Old stock costs 40$/EA.
But I also thought to do it like you propose, i.e. disassembling all affected mechanical relays and sockets, 8EA, I think, and assembling the reed relays hanging in the breeze.

Any other good ideas are welcome!
 

Offline pelule

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Re: EEVblog #544 - Fluke 5450A Resistance Calibrator Teardown
« Reply #111 on: January 15, 2017, 09:01:17 am »
I just wouldn't disassembe the relay sockets, if they are not affected (at least I understood your description of problem that way).
I would use the old relay contacts (only that), solder the connectiong wires to, and use it as plug-in to the sockets for the connection of the add-on card. So, in case you find by luck replacement parts, you could easy remove your changes.
An out of my view suitable alternative is DigiKey 374-1389-ND / EUR 13,74 + VAT
Hermetic sealed reed releay with high insulation relay coil, contact 100 TOhm (1e14) High Leakage Distance

I use that for my precision analog switching also.
BR
PeLuLe

PS: HP 3458A and Datron 1281 are warmin up to check the 5450A..
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