Electronics > Projects, Designs, and Technical Stuff
I built a bridge
(1/3) > >>
BradC:
So I've been playing with working towards building Conrads Mini-Metrology Lab, and I got sidetracked comparing resistors.
I work in a relatively noisy environment, and I was getting a lot of noise in the meter when getting close to a balance, so I built a dedicated bridge in an aluminium box.

My construction needs some work. I had it all laid out and prepared for the arrival of the resistors, and I then went and soldered half of them in, bent them up and realised I was trying to connect them incorrectly resulting in some resistor-lead-yoga trying to get them to fit without putting any stress on the resistor bodies.

I used a 10k/10k Rc/Rd & open Ra/Rb.
Rd is a 10k/50k in parallel (8.333k), in series with a 2k in parallel with a 1k pot & 9.53k in series (1.653k-1.680k). So Rd is variable from about 9.986k to 10.014k.

Both Rc & Rd are TE UPW50 series wirewounds (3ppm/C). The 50K & 2K are TE UPF50 metal film (5ppm/C) and the 10 turn pot is a Vishay 20ppm wirewound.
The capacitors on the input and output are 0.001uF MKT 100V and the 220k resistor is just a 600mW 1% Metal film. The output clipping diodes are 1n4148.
The toggle switch is a Apem unit with gold plated silver contacts rated to a .01 ohm maximum resistance.

The reason for this post is the toggle switch. I wired it such that it changes the input polarity to Rc/Rd, effectively swapping the resistors. The theory was once settled I could flick the switch and if I balanced the bridge such that the output didn't change when I flicked the switch, then in theory the Rc/Rd side of the bridge should be relatively equally balanced, meaning the differential is due to the ratio difference between Ra/Rb.

The idea was being able to relatively accurately center Rc/Rd without needing anything more accurate than the meter being used as the differential detector.

I tried it using a spare 0.1% 10K UPF50 resistor as Rb, and a Genrad 1434 Decade as Ra. I deliberately set the decade off to unbalance the bridge and then alternated the switch polarity while adjusting the pot to get equal magnitudes. Once that was stable, I adjusted the decade to get as close to a null as I could, then separately measured the resistor and the decade on a hp3457a. They agreed +/- a couple of counts, so I figured the concept was sound, and thought I'd ask about it here as I'd likely find people with experience using bridges to measure/compare resistors in an absolute sense.

Is this a sane way of balancing one side of the bridge? Aside from the ability to move the bridge about significantly for resistor matching, I wanted the ability to be able to know relatively accurately that the a/b side of the bridge was as equal as I could possibly make it without resorting to exotic resistors or witchcraft.

guenthert:
Nice work there.

--- Quote from: BradC on August 29, 2018, 01:52:11 pm ---[..]
The toggle switch is a Apem unit with gold plated silver contacts rated to a .01 ohm maximum resistance.

The reason for this post is the toggle switch. I wired it such that it changes the input polarity to Rc/Rd, effectively swapping the resistors. The theory was once settled I could flick the switch and if I balanced the bridge such that the output didn't change when I flicked the switch, then in theory the Rc/Rd side of the bridge should be relatively equally balanced, meaning the differential is due to the ratio difference between Ra/Rb.

The idea was being able to relatively accurately center Rc/Rd without needing anything more accurate than the meter being used as the differential detector.
[..]

--- End quote ---

What are you using as null detector?  Isn't the circuit a bit ill-defined during the switching?  I wonder whether there's a time when only one of the resistors is connected.  A DVM might not care much, but a more traditional null meter might not take it lightly, if instead of the expected few mV or uV, it sees a couple of Volt, even for a brief moment.  Oh, I guess, that's what the diodes are there for.

Isn't it more common to toggle the polarity of the supply voltage?  It won't assure that the balance is achieved, but at least show you which influence lingering thermal EMF has.
Conrad Hoffman:
Nice! A matching bridge is a handy thing to have, at least for a voltnutter. I've never built a dedicated one, but should. My preference would be to switch the DUTs, but it's probably OK to switch the reference side. Just switching voltage doesn't help- you have to reverse the relationship between the DUT side and the reference side, as I think you've done. People don't even recognize them today, but these are what used to be used for bridge work- https://www.worthpoint.com/worthopedia/vintage-leeds-northrup-electric-knife-535680819

The old engineering and metrology books show some commercial matching bridges, but I've never seen one in the wild.
BradC:

--- Quote from: guenthert on August 29, 2018, 04:07:10 pm ---What are you using as null detector?  Isn't the circuit a bit ill-defined during the switching?  I wonder whether there's a time when only one of the resistors is connected.  A DVM might not care much, but a more traditional null meter might not take it lightly, if instead of the expected few mV or uV, it sees a couple of Volt, even for a brief moment.  Oh, I guess, that's what the diodes are there for.
--- End quote ---

I'm using either a HP3478a or a HP3457a, so yeah the point of the diodes was to clamp the output at ~600mV. The meters don't mind that so much, but they do tend to report overrange on the odd switch. I don't think I'd do that with a proper null meter.


--- Quote from: guenthert on August 29, 2018, 04:07:10 pm ---Isn't it more common to toggle the polarity of the supply voltage?  It won't assure that the balance is achieved, but at least show you which influence lingering thermal EMF has.

--- End quote ---

Yes, but I'm not really looking for the lingering EMF here, I'm more interested in balancing the bridge as best as possible.


--- Quote from: Conrad Hoffman on August 29, 2018, 05:03:44 pm ---Nice! A matching bridge is a handy thing to have, at least for a voltnutter. I've never built a dedicated one, but should. My preference would be to switch the DUTs, but it's probably OK to switch the reference side. Just switching voltage doesn't help- you have to reverse the relationship between the DUT side and the reference side, as I think you've done. People don't even recognize them today, but these are what used to be used for bridge work- https://www.worthpoint.com/worthopedia/vintage-leeds-northrup-electric-knife-535680819

The old engineering and metrology books show some commercial matching bridges, but I've never seen one in the wild.
--- End quote ---

Yes, I was aiming to only switch one side of the bridge to use that to set the reference side as closely as possible to dead center. I thought about switching the DUT side but figured at some point I might want to do something out of the ordinary with the 4 binding posts connected to some other piece of kit and figured it'd be more predictable to keep the polarity outside the box constant.

Now I've had some time to use it, I realise I've made a bit of a blunder. I set the pot up with the intention of roughly +/- 1000ppm for resistor matching. The reality is that isn't enough to get a balance on a set of 1% metal films, and its so wide that I'm trying to balance the pot between windings when centering the reference side for a comparison measurement. So it's too narrow to be useful for resistor matching and too wide to be accurate in centering the reference. I think I'll re-jig it to narrow it right down to allow accurate reference centering and use my decade box as the reference resistor for comparison matching. That'll get me +/- 10ppm on 10K metal films, which would mean I can narrow the adjustment down far enough to be useful for comparison measurements.

Still, that's why we do these things.

<edit>
So I re-worked the pot from near enough +/-1500ppm to +/- 118ppm. Much more usable for getting a good zero. I can probably wind it even lower, but I'll play with this for a while and get a feel for it.

In all reality with a 27V input I now can easily get centered within 1ppm on a 10K pair (so ~7uV). I only have to move around in my chair and I get more noise than that. I might have to try more of a low pass filter on the output.

This is actually pretty cool as it becomes frightfully easy to see (and to some degree measure) all the other factors that inject noise, instability and variability into the system.
BradC:
Nothing I'm going to put here will come as any surprise to anyone versed in the art of sub-ppm measurements, and I've known about all of this, but knowing about it in theory and seeing it happen on the bench is a whole other world of "wow".

I've been using my HP3457a as a null detector on the 30mV range. While I was on a long flight recently I put a bit of a spreadsheet together to calculate the relative accuracy of each range, and noticed the 3V range is by far the most accurate and quiet. I already knew that, but it never really twigged (the other thing it demonstrated is the hp3457a just less than twice as accurate as the hp3478a, which was a bit of an eye opener when looking at the error budgets. Those 3478a are a really good buy for the accuracy!). I digress.

Looking at the specs, on the 3V range I get a 1uV resolution, which is all I can reliably deal with anyway given the noise in my environment and the stability of the components.
So I started measuring on the 3V range. A couple of advantages :
- I no longer get intermittent overrange when flipping the reference side polarity as the 600mV diode clip is well within the range.
- I get a much more stable reading without the distraction of a whole heap of fluctuating digits I don't need to pay attention to.

Winner!

I also re-worked the balance control again to further reduce its range, and put more of the resistance in the 10K wirewound which has helped the temperature stability and made it much easier to balance (I've now got something like 5ppm per turn of the pot).

I can now compare resistors to within about 1/4ppm, which is another issue itself as each time I breathe the resistors change value (or I see thermoelectric offsets in the cheap binding posts I used). So high resolution, but not necessarily accurate.

The other thing I've noticed is the triboelectric noise in the coax I'm using to connect the bridge to the meter is HUGE! Bump the cable and there is a spike of between 10's to 100's of uV before it settles back down.

This has started me on a good learning exercise.
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod