Author Topic: DIY Precision AC-RMS to DC Transfer Standard  (Read 25671 times)

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Offline Kleinstein

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #25 on: April 17, 2016, 08:33:44 pm »
You can consider the wires are low frequency wave guides - so in this way AC current is low frequency radiation, just not free space but near field and bound to the cables as waveguides.
 

Offline branadic

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #26 on: February 27, 2018, 08:12:29 pm »
Hi,

I was linked to multijunction thermal converter (MJTC by NIST) by a member. When I searched for it, I found:

New AC Voltage Devices: Factor of Three Improvement in Uncertainty

AC-DC Difference

This looks pretty much like what is known to be a thermal mass flow sensor:



So I wonder, has anyone experience with thermal mass flow sensor elements such as FS2 (IST AG)



or MFS02 (IST AG)



or a discrete assembly with a hf usable resistor and two NTCs on a polyimide foil as a ac transfer standard? I do have realized such a thermal mass flow sensor only a few days ago, but had no idea up to now, that similar assemblies are used to measure ac rms voltage.

-branadic-
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Offline Vtile

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #27 on: February 27, 2018, 08:47:30 pm »
Have anyone tried to use something like LTZ1000A or LM399 as the OPs diode & resistor pair. They both have the resistor? (or is it something else) and diode (which is not even thermally stable).  >:D
 

Offline Vgkid

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #28 on: February 27, 2018, 08:58:41 pm »
Have anyone tried to use something like LTZ1000A or LM399 as the OPs diode & resistor pair. They both have the resistor? (or is it something else) and diode (which is not even thermally stable).  >:D
Here is a lm399 being used as a anemometer.
http://www.gellerlabs.com/MGTA%20kits.htm
If you own any North Hills Electronics gear, message me. L&N Fan
 
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Offline branadic

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #29 on: February 27, 2018, 08:59:57 pm »
Have anyone tried to use something like LTZ1000A or LM399 as the OPs diode & resistor pair. They both have the resistor? (or is it something else) and diode (which is not even thermally stable).  >:D

Seems like you haven't read the linked publications or no idea of how a thermal mass flow sensor works, otherwise you would have understood that this is nothing different. The only question is, to which degree this works, as a thermal mass flow sensor element is not optimized for hf, but the other parameters like good thermal coupling between heater and temperature sensor as well as poor thermal conductivity are similar.
You compare apples with oranges.

-branadic-
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Offline Vtile

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #30 on: February 27, 2018, 09:32:16 pm »
Yes I'm not too familiar with the thermal mass converter, resistive element thermally connected to temperature sensor. Preferably weightless, purely linearly resistive, without thermal lag, fully isolated from sensor etc. Used for obtaining real RMS values. ( ..But like said no I'm not expert of the area and didn't read this time the white papers. Plus reading from phone at the time I should already considering to get some rest for next day.)

 Isn't those integrated oven references at some level just that especially if modified (case removed) atleast they have better thermal coupling and less mass (bare chip) than OPs neat resistor and smd BOC device.. Just asking around since interested without equipment.
« Last Edit: February 27, 2018, 09:35:29 pm by Vtile »
 

Offline ap

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #31 on: February 28, 2018, 10:00:44 am »
A thermal mass flow sensor (TMFS) and a TVC work on different principles. Moreover, a TMFS is not specified for AC behavior and also not very accurate.
If you need a DIY solution for a precision AC transfer standard, I would recommend Nikkohm devices.
Metrology and test gear and other stuff: www.ab-precision.com
 
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Offline branadic

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #32 on: February 28, 2018, 06:46:32 pm »
A thermal mass flow sensor (TMFS) and a TVC work on different principles. Moreover, a TMFS is not specified for AC behavior and also not very accurate.
If you need a DIY solution for a precision AC transfer standard, I would recommend Nikkohm devices.

Well, no! The principle is not that different, but I agree that a TMFS element is not optimized for the use as MJTC application and already stated that especially the heater is not designed for hf. However, I don't agree that they are not accurate, as the TMFS like a MJTC are only sensor elements, thus both need to be calibrated within the complete system ;)

-branadic-
« Last Edit: February 28, 2018, 08:44:25 pm by branadic »
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Offline 2N3055

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #33 on: February 28, 2018, 08:33:31 pm »
A thermal mass flow sensor (TMFS) and a TVC work on different principles. Moreover, a TMFS is not specified for AC behavior and also not very accurate.
If you need a DIY solution for a precision AC transfer standard, I would recommend Nikkohm devices.

Hello!
Would you by any chance know where Nikkohm LP34TW could be ordered...?

Thanks!!

Regards,
Sinisa
 

Offline ap

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #34 on: March 01, 2018, 01:19:46 pm »
Under 'contacts' the distributers are listed on their website. Shoudl you get a quote, would you publish this here please. I did request a quote for the TVC some time ago, it was arround 400 IIRC. The LP34 should be less.
Metrology and test gear and other stuff: www.ab-precision.com
 

Offline 2N3055

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #35 on: March 07, 2018, 04:06:32 pm »
Under 'contacts' the distributers are listed on their website. Shoudl you get a quote, would you publish this here please. I did request a quote for the TVC some time ago, it was arround 400 IIRC. The LP34 should be less.

I did send a request to few distributors in EU.. I got quotes ranging from 310 USD to 605 € for one piece....
 

Offline Moon Winx

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #36 on: March 07, 2018, 08:07:09 pm »
By the way, you can buy one of those NIST thermal converters from directly from NIST for a relatively dirt-cheap price (~$2k). Those guys don't get out of bed for less than $2k so actually getting a NIST-cal'd MJTC for that price isn't a bad deal.

https://www.nist.gov/sri/standard-reference-instruments/sri-6002-multi-junction-thermal-converter

 

Offline ap

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #37 on: March 07, 2018, 08:53:02 pm »
I did send a request to few distributors in EU.. I got quotes ranging from 310 USD to 605 € for one piece....

That price is for the LP34?
Metrology and test gear and other stuff: www.ab-precision.com
 

Offline 2N3055

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #38 on: March 07, 2018, 09:12:13 pm »
I did send a request to few distributors in EU.. I got quotes ranging from 310 USD to 605 € for one piece....

That price is for the LP34?

Yep, LP34TW 50 Ohm .. That's dual, bridge mode converter..

Regards,
Sinisa
 

Offline Conrad Hoffman

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #39 on: May 07, 2018, 05:11:16 pm »
I've got a 540B and it seems to work fine. My understanding was that each individual unit was supplied with a correction factor and they aren't inherently as perfect as you'd think. I don't have that number for mine. Does anybody know more about this?

When making sensors you'd be surprised at how much heat will conduct down even quite fine copper wire. Use constantan or manganin instead. The slight resistance is probably less of an issue than the conductivity of copper.

I always wanted to play with the LT part, but cost prevented it then and availability prevents it now. If there had been more demand it would have survived, so we have to assume the market is small.
 

Offline branadic

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #40 on: May 07, 2018, 05:39:13 pm »
I have sent some assemblies to Frank for testing.
One setup is build of NTCs (0402) directly mounted on heaters/resistors (0603) of different values, but also NTCs and resistors assembled on a foil substrate very close to each other. Looking for the first results that Frank provides.

-branadic-
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Offline Qmavam

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #41 on: August 17, 2019, 06:13:15 pm »
Has this topic died or has anyone built a circuit we can copy.
 I recently purchased two HP400E AC voltmeters, I was happy when I got the
first one, then when I got the second one the readings don't agree, by 15%.
 I would like a cheap way to calibrate it up to 2Mhz, but 10 would be great.
 I'd be happy with an accurate AC to DC converter, type circuit, rather than an AC reference.

                                  Thanks, Mikek
 

Offline Qmavam

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #42 on: August 17, 2019, 06:56:40 pm »
OK, sense I revived this thread, let me see if I have any understanding.
In the very first post the OP thermally connected a 100 ohm resistor and a
a C-B junction of a SMD NPN transistor.
 Then he says, "forward biased where you get about -2mV/C coefficient"
I would ask, How much current needs to flow in the C-B junction to measure the -2mV/C?
And, it would seem it needs to be very low to prevent internal heating.
 So how is that -2mV/C being measured?
I'm thinking I can't just set my DVM to diode test, apply 1Vdc, take a reading,
and the then switch to 1Vac and they would read the same.
 I think they should, except for the DVM supplied current.
   Help me out guys.
                                    Mikek
 btw, might it be better to use a 50 ohm resistor to properly load a sig-gen.
 Maybe sandwich the C-B junction between two 100ohms resistors in parallel.
 
 

Offline 2N3055

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #43 on: August 17, 2019, 07:19:25 pm »
Read the post 5 and 6, there is an explanation..
 

Offline Qmavam

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #44 on: August 17, 2019, 08:06:57 pm »
Ok, I read 5 and 6. He used a LM358 as an amplifier.
I see no mention of how much current he used as bias in the diode.
 Anyone have an idea, my concern is self heating of the diode,
 Am I just being over critical or is that a concern?

 Do I have the theory correct,  in that I can measure a dc voltage with good accuracy, I apply 1Vdc across the
100 ohm resistor and read the diode drop voltage.  Then I put AC across the 100 ohm resistor,
and adjust the amplitude until it reads the same as the 1Vdc did. Now I have 1Vac.

I don't understand the 100*C target, is that a maximum temp?

Also this "we can easily get 100MHz+ opamps that should have a flat enough frequency response to get DC to 1kHz accuracies of 0.01% easily."
 Why 100MHz opamp, aren't we amplifying a DC voltage from the C-B junction?
                                    Thanks, Mikek
 PS. I want to calibrate an AC meter up to 10 MHz, or at least 2MHz. (HP400E) I bought two and they don't agree.
I was happy when I only had one! :-)
 

Offline Kleinstein

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #45 on: August 17, 2019, 08:43:53 pm »
For using a diode temperature sensor, a common current level is at 0.1 mA, for local use less current, like 10 µA is possible to.
For a diode the differential impedance is  at some 26 mV/ current. So 100 µA would result in 260 Ohms - so already a low impedance signal.

100 µA * 0.6 V gives a power level of 60 µW. This is not much compared to 1 V at 100 Ohms -> 10 mW.
 

Offline dietert1

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #46 on: August 17, 2019, 08:58:21 pm »
About 30 years ago i made a converter that still exists. It is based on a 10 or 20 KOhm NTC in a constant resistance regulator bridge (stability by self heating). If you can get that bridge to balance using an outer oven, then you have a very sensitive power detector that works by compensation. I mean the regulator will reduce self heating as much as you apply external heat. Mine has a conversion formula on its label: P = U² * 33 uW/V². So it works up to about 1 mW.
It may not be a precision device, though. It has a 50 Ohm resistor glued to the NTC for AC/DC conversion. Don't know where the schematic went. Nowadays one could make the outer oven with a digital PID regulator.

Regards, Dieter
 

Offline 2N3055

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #47 on: August 17, 2019, 09:24:39 pm »
Ok, I read 5 and 6. He used a LM358 as an amplifier.
I see no mention of how much current he used as bias in the diode.
 Anyone have an idea, my concern is self heating of the diode,
 Am I just being over critical or is that a concern?

 Do I have the theory correct,  in that I can measure a dc voltage with good accuracy, I apply 1Vdc across the
100 ohm resistor and read the diode drop voltage.  Then I put AC across the 100 ohm resistor,
and adjust the amplitude until it reads the same as the 1Vdc did. Now I have 1Vac.

I don't understand the 100*C target, is that a maximum temp?

Also this "we can easily get 100MHz+ opamps that should have a flat enough frequency response to get DC to 1kHz accuracies of 0.01% easily."
 Why 100MHz opamp, aren't we amplifying a DC voltage from the C-B junction?
                                    Thanks, Mikek
 PS. I want to calibrate an AC meter up to 10 MHz, or at least 2MHz. (HP400E) I bought two and they don't agree.
I was happy when I only had one! :-)

You're supposed to read both application notes by Jim Williams. All theory and schematics for LT1088 are directly applicable...
 
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Offline Qmavam

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #48 on: August 17, 2019, 10:28:38 pm »
Ok, I read 5 and 6. He used a LM358 as an amplifier.
I see no mention of how much current he used as bias in the diode.
 Anyone have an idea, my concern is self heating of the diode,
 Am I just being over critical or is that a concern?

 Do I have the theory correct,  in that I can measure a dc voltage with good accuracy, I apply 1Vdc across the
100 ohm resistor and read the diode drop voltage.  Then I put AC across the 100 ohm resistor,
and adjust the amplitude until it reads the same as the 1Vdc did. Now I have 1Vac.

I don't understand the 100*C target, is that a maximum temp?

Also this "we can easily get 100MHz+ opamps that should have a flat enough frequency response to get DC to 1kHz accuracies of 0.01% easily."
 Why 100MHz opamp, aren't we amplifying a DC voltage from the C-B junction?
                                    Thanks, Mikek
 PS. I want to calibrate an AC meter up to 10 MHz, or at least 2MHz. (HP400E) I bought two and they don't agree.
I was happy when I only had one! :-)

You're supposed to read both application notes by Jim Williams. All theory and schematics for LT1088 are directly applicable...
Off to do that, warning, that will probably generate more questions. :-)
                             Thanks, Mikek
 

Offline Qmavam

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Re: DIY Precision AC-RMS to DC Transfer Standard
« Reply #49 on: August 17, 2019, 11:13:20 pm »
For using a diode temperature sensor, a common current level is at 0.1 mA, for local use less current, like 10 µA is possible to.
For a diode the differential impedance is  at some 26 mV/ current. So 100 µA would result in 260 Ohms - so already a low impedance signal.

100 µA * 0.6 V gives a power level of 60 µW. This is not much compared to 1 V at 100 Ohms -> 10 mW.

 After reading part of AN22, I see Jim Williams circuit uses about 5 milliamps of bias on the LT1088.
Now we are up to a number (3mW)  of power compared to 10mW of RF power.

Any thoughts about that?

 But, what if I try to measure a 4mv ac signal, (is that possible) now my power across a 50 ohm resistor
would be 0.32 µW .

 I'm beginning to think this might not work well to check the calibration on a 10mV range of a AC voltmeter.

Any thoughts.

Can any on calculate heat rise of 4mv across a 50 ohm SMD with styrfoam as the insulator.
I'm going down fast :-(
 


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