Hamon Project - next step is calibration - Page 1
Forum section: electronics / general

author: enut11
author_rank: Super Contributor
timestamp: November 03, 2022, 10:10:41 pm
content: I have completed my latest project, a Hamon Divider, inspired by Conrad Hoffman's Mini Metrology Lab articles.Thanks Conrad, you have inspired many a would-be metrologistThis instrument has both 10:1 and 100:1 divisions.The plywood fascia adds a hint of nostalgia to the styling and the box is from a dead Tek probe.The resistors are a mixture of Fluke and Russian items that I had on-hand. The galvanometer was added for a bit of fun. It works but is not sensitive enough to be of real use.Now it is time to get serious.A Hamon divider needs to be calibrated. This is done by shorting out certain resistors and trimming others until you have a simple 2:1 division. When the applied voltage is halved, then, on removing the shorting links, you have a perfect 10:1 divider. Easier said than done!The next step is to build a Wheatstone Bridge incorporating the Hamon and a pair of external precision resistors.

author: enut11
author_rank: Super Contributor
timestamp: November 03, 2022, 10:28:41 pm
content: So far everything has been done by measuring resistors using a Keysight 34461A DMM. Even with statistical averaging using the Maths function, this is not very precise when you want to resolve a few ohms in hundreds of thousands. This is where I hope a Wheatstone Bridge will help.Equipment I have on hand:KS34461A DMMFluke 845 Null MeterSeveral LM399 10v referencesOne LTZ1000 7v referenceA home-made 5 dial KVD salvaged from a Fluke 803 voltmeter.Several precision resistors removed from a Fluke 803 voltmeter

author: enut11
author_rank: Super Contributor
timestamp: November 03, 2022, 10:52:38 pm
content: I selected four precision resistors from the Fluke 803, each with a nominal value of around 112K, and paired them to form 2 compound 56K nominal resistors.KS34461A statistical averaging (~22 deg C)Y 112365.9 ohmsParallel=56187.17 ohmsV 112382.8 ohmsW 112362.8 ohmsParallel=56189.67 ohmsX 112395.9 ohmsSo, the Y-V pair ended up about 2.5 ohms shy of the W-X pair.I say 'about' because I am not convinced the ohms function on my DMM is good enough to conclude that by simply adding 2.5 ohms in series with the Y-V pair will give me 2 equal resistors.

author: enut11
author_rank: Super Contributor
timestamp: November 03, 2022, 11:01:30 pm
content: My options for moving forward now are:1) Do further tests using the ohms/maths function on the DMM.2) Wire the two 56K resistors in series, apply 10.0v and trim one of the resistors for half the applied voltage output.3) As for (2) but use the Ratio Function on the 34461A to trim the values.4) Your suggestions...

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 04, 2022, 12:51:06 am
content: Reversal! Nothing is ever going to be perfect, but you can reverse the resistor pair and split the difference in null reading to find exactly 1/2 the voltage. If you have an infinite resolution adjustment, use the reversal technique to get it perfect.

author: enut11
author_rank: Super Contributor
timestamp: November 04, 2022, 01:20:53 am
content: Thanks Conrad. So you are proposing Option 2. I will set up a reversing switch for the comparison.I was wondering if the input impedance of the 34461A might affect the voltage reading.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 04, 2022, 08:33:33 am
content: Conrads suggestion is not in the list of options. It is about using a weatstone bridge and swapping the sides of an 1:1 divider. So there would be another auxilary branch with 2 about equal resistors.In a bridge circuit the meter would read close to zero. So the classical impedance of the meter is not the problem. The issue is more with the isolation of the meter from ground, so the leakage current towards ground. With some meters this can be reduced by using a seprate guard terminal and branch at the bridge. As an alternative (still not perfect) one can also isolate the bridge drive voltage (especially when using batteries - the bridge drive does not have to be super stable, so just the raw voltage from a set of batteries would be good enough).The analog meter look like it is not really sensitive (e.g. there is a parallel resistor). A usefull null-meter should be able so resolve at least some 1 µV or so, ideally better. The simple meter movement would likely need some extra amplification, e.g. with a Zero drift OP-amp like max4238 or AD8628. The null meter part would be a somewhat seprate project though. The KS34461 could read such a small voltage, but the leakage current to ground can be a problem. The main reason for a separate null-meter is to get good isolation from ground, usually by battery operation. The calssical null meter was a mirror galvanometer. These are also not that bad, just a bit slow to use.The setup looks a bit strange. Normally with a hammon divider the 6 resistors for the core part should be equal types and relatively close togehter to get good thermal tracking. From the picture is looks more like difference combinations from different types.Just for the 1:10 step one can also do the bridge part once for one set of 3 and once for the other set of 3. So one would get 2 versions of the 1:10 divider, with the tendency to have one too high and one to low. So the difference between the 2 configurtions would give an indication how good the dividers are.I think it may be a good idea to have a look at the Fluke 752 related informations that are available.

author: magic
author_rank: Super Contributor
timestamp: November 04, 2022, 09:34:49 am
content: A potential alternative 50% divider solution suggested by mawyatt is to build an ordinary, untrimmed 50% divider and swap its polarity frequently with an H-bridge running at 50% duty cycle. In theory it can be calculated that as long as the ratio and duty cycle are close to 50%, the output will be even closer to 50% than either of them.Practically, output resistance of the H-bridge must be negligible or matched on both sides (also the case with manual calibration by reversing) and I suppose filtering may take some thought (low frequency: capacitor leakage, high frequency: parasitic capacitances).The traditional configuration is ten identical resistors, not six. One at ground, and three strings of three on the switched section. This minimizes self-heating drift. OTOH, three switched plus three permanently paralleled resistors has lower cost and better statistical rejection of long term drift, for less frequent tweaking when self-heating is negligible.Old thread about those things:https://www.eevblog.com/forum/metrology/anyone-else-built-a-hamon-divider/

author: Gyro
author_rank: Super Contributor
timestamp: November 04, 2022, 09:37:42 am
content: Quote from: Conrad Hoffman on November 04, 2022, 12:51:06 amWhen I was balancing my divider, I ended up resorting to a pair of 50k Vishay foil resistors with a 5 ohm 10 turn pot at their junction as the centre tap for fine trimming. This gave a 5ppm per turn resolution for balancing. By repeatedly reversing the connections it was possible null both the Hamon divider and the 2:1 divider to 1ppm using a 1V energising source and 1uV resolution DMM.https://www.eevblog.com/forum/metrology/anyone-else-built-a-hamon-divider/If you do a forum search, you will find quite a few Hamon divider references - Dr Frank's is particularly impressive!

author: miro123
author_rank: Frequent Contributor
timestamp: November 04, 2022, 05:31:27 pm
content: My intention is not to hijack this interesting thread.Just one side question.I understand most steps in Hamon divider except the last one - How can I use the 1/10 in real life? Buffering the 1/10 output with any amplifier introduce an error.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 04, 2022, 06:10:14 pm
content: Quote from: miro123 on November 04, 2022, 05:31:27 pmIt depends on the amplifier: there are ways to build an amplifier with very high output impedance, e.g. by bootstrapping it's supply. There are also ways to correct the loading effect.Another option is to measure with a differential voltmeter - I know these are no longer common, more like thing of the past, but can still be very good. Similar a voltage comparison can be done with a nullmeter.Under normal conditions the Hammon divider is not a practical instrument, more like a way to check metrological instruments (e.g. a KV divider) or DMM ranges in some calibrations, if you don't have a all in one calibrator.One can still use the general idea also at less than high end grades (e.g to build a x10 amplifier by using 6 euqual resisrtors (e.g. from an array) for the gain setting resistors. This can still be easier than looking for accurate resistors sets with an 1:9 ratio (AFAIK available in the E24 series - maybe with E96).With less extreme targets the loading from a 10 Gohm impedance DMM may be acceptable or at least the correction good enough. This could be done with an extra measurement with additional series resistance, but otherwise the same voltage.

author: mawyatt
author_rank: Super Contributor
timestamp: November 04, 2022, 06:57:07 pm
content: Quote from: magic on November 04, 2022, 09:34:49 amIf one uses CMOS for the H-Bridge, then returning the filter Cap to the Input Voltage rather than ground will allow the Cap leakage to be less detrimental. This is because most CMOS has the P Channel pull up output device weaker (higher Res On) than the N Channel pull down device, so the leakage will help in this case if tied to the Input Voltage. Of course a better solution is to attempt to equalize the P and NMOS Rds On, and minimize the Cap leakage, but also returning the Cap to the input should help some.Best,

author: Kleinstein
author_rank: Super Contributor
timestamp: November 04, 2022, 07:03:05 pm
content: The revesal for the calibration would normally be with mechanical switches / relays. Just as a warning: some of the cheap DPDT switches can be a bit troublesome with uneven forces to both poles.

author: enut11
author_rank: Super Contributor
timestamp: November 04, 2022, 07:52:02 pm
content: Quote from: Kleinstein on November 04, 2022, 08:33:33 amThanks for the clarification on Conrads reply @Kleinstein. My intention is to balance the Hamon with 2 additional resistors in a bridge configuration. I now have some good suggestions on how to calibrate those 2 resistors.On the 10:1 divider (black resistors), I did skimp on how it was constructed.As @magic pointed out, I should have used more series resistors in the upper half instead of 3 in order to minimise self-heating drift. I will look into this.I can run my Null Meter from a battery and hopefully this will minimise leakage errors.The analog meter that I included is just a toy. I thought I might improve it's sensitivity using Conrad's null meter amplifier from the MML article.I did not have enough resistors of the same type to build a true integrated Hamon divider.The 10:1 section was built using Russian low TC 22K WW resistors.The 100:1 section consists of Fluke resistors for the upper half and of course the Russian resistors for the lower half.enut11

author: enut11
author_rank: Super Contributor
timestamp: November 04, 2022, 07:58:18 pm
content: Quote from: Gyro on November 04, 2022, 09:37:42 amThanks for this tip. I can build a variable low ohms trimmer using some salvaged Fluke parts.

author: enut11
author_rank: Super Contributor
timestamp: November 04, 2022, 08:02:21 pm
content: Quote from: mawyatt on November 04, 2022, 06:57:07 pmThanks @mawyatt. The CMOS H-bridge solution sounds a bit complicated but I will look into it. Might learn something new.

author: mawyatt
author_rank: Super Contributor
timestamp: November 04, 2022, 09:35:39 pm
content: Quote from: enut11 on November 04, 2022, 08:02:21 pmYou are welcome, here's a post on such. Hope you find it interesting!!https://www.eevblog.com/forum/projects/fun-circuit-to-play-with/msg3109636/#msg3109636Best,

author: Kleinstein
author_rank: Super Contributor
timestamp: November 04, 2022, 10:11:49 pm
content: When aiming at very high accuracy the CMOS logic chip outputs could be a problem: there is still a difference in the ouput resistance for the high and low signal and this difference effect the result. For higher voltages, like 10 V one would need the rather high resistance HEF4000 series, which are not ideal. Chances are one would need lower resistance, like gate driver chips (e.g. ICL7667) as extra drivers. This still adds load spikes to the "voltage source". So things are not that easy.

author: mawyatt
author_rank: Super Contributor
timestamp: November 04, 2022, 10:19:55 pm
content: One can use discrete N and PMOS FETs with somewhat matched Rds ON, or with CMOS matched by adding a small resistor in series with the NMOS by adding such resistor in the VSS lead!!Best,

author: mawyatt
author_rank: Super Contributor
timestamp: November 05, 2022, 09:44:53 pm
content: Just for fun decided to simulate this CMOS H Bridge concept again. Here's an old thread related, just found this.https://www.eevblog.com/forum/metrology/precision-voltage-divider-on-ltspice-group/msg3064598/#msg3064598https://www.eevblog.com/forum/projects/fun-circuit-to-play-with/msg3109636/#msg3109636Added a resistor in the VSS pin for the 4049B CMOS Inverter, which is a really weak inverter but can handle over 5V VDD if desired and don't have any good LTSpice model for a 74AC04 which would better for this application since it has a much lower effective output Z, but is only good for ~6V VDD. we paralleled up 3 4049 inverters to help lower the output Z and since 6 are in a package used one resistor to VSS compensate all 6 inverters. The inverters were toggled with a model FF which produces 0 to 1 and scaled by 5 to provide the prover 5V logic level to the 4049 inverters. Added a simple RC LPF and ran some simulations.Adjusted the PMOS compensation resistor to ~116 ohms and with a 5 V VDD produced ~2.4999940 Volts with identical 100K Q and Qbar resistors when toggled at 1KHz. Change one resistor to 101K (other is 100K), which is ~1% error and result was same ~2.4999940 Volts! Changed resistor to 102K for a 2% error, same ~2.4999940 Volt output, then 105K, or 5% error and we got ~2.4999938 Volts!! With a 74AC04 the VSS resistor would be much lower for the 5V VDD.Now for twice the VDD or 10 Volts. You need to adjust the VSS resistor because the Rds ON differential between P and NMOS is lower at higher VGS which comes with the higher VDD. This turns out to be 66 ohms and produces a 5.0000091V with equal 100K resistors, with a 101K this becomes 5.0000095V, and with 102K we get 5.0000095V, same result for 105K or 5% error!!Anyway, one can easily assemble a 74AC74 and couple 74AC04, a few resistors on a test board and have some fun with this circuit!!

author: Kleinstein
author_rank: Super Contributor
timestamp: November 06, 2022, 07:23:57 am
content: The point with the Hammon divider and adjustment with bridge turn over is to have a first principles / no calibration needed divider. One would need a well floating null-meter anyway to really use the divider.Using the extra trimmed resistor at the supply side of the CMOS chip is kind of cheating: how do you know the right resistor. Changing that resistor from some 100 Ohms to 200 Ohms would resuclt in some 0.05% change in the output. So the low sensitivity to resistor tolerances is meaningless. Another problem is that the inverter chip itself consume power - that part needs to be included in choosing the right magic resistor. With the switches there is also trasient loading to the supply, which is the reference. The settling of that supply also effects the result.When it comes to low resistance and good isolation mechanical switches are still quite good compared to logic chips.

author: miro123
author_rank: Frequent Contributor
timestamp: November 06, 2022, 09:08:41 am
content: Quote from: Kleinstein on November 06, 2022, 07:23:57 amThanks for valuable inputs Kleinstein.My impression is that Hammon divider was nice tool back in time when electromechanical meters was used. The modern DMMs has different input-properties, The use of this bridge is limited. I'm not say obsolete but limited. Am I correct?Lets put the question other way around - What can be done with Hammon bridge noways? - E.g. budget COT solution with 1ppm Vdc linearity are available.PS: Some minor Hammon bridge ideas applies nowadays - e.g. in statistical arrays.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 06, 2022, 11:02:20 am
content: The Hammon divider was never meant for normal practical use, more like a tool in the cal lab or to check a meter in the old days.One could check the effect of loading by doing an additional measurment with an added series resistor (e.g. 10 x the divider impedance). The effect of output impedance is largely linear. So 1/10 the difference would be the correction to apply. This principle still applies to modern meters.If you have a meter that has known good linearity over a large range, there is no need for the precision divider. However one still may have to check this. 1 ppm for the divided output would be 0.1 ppm of the full scale and at that level there are not that many meters or calibrators to offer that degree of accuracy - especially not a low price. The commercial Hammon bridge divider Fluke 752 is still in use today - though it needs care in choosing a suitable null meter. Much of the use is with higher voltages, like going up from the usual 10 V reference. Here the linearity of commercial meters is a bit more limited.With modern meters, there is still the alternative of using a circular divider: e.g. a ring of 20 nominally equal resistors and lots of switches to go though many configurations and than average over these. I am not aware of a commercial solution to work this way, though the principle is long known and moden latching relays and digital read out should make it quite practical: no adjustment phase and just averaging over some 10 or 20 configurations / readings. The up-side is that at least for the 10:1 step the circuilar divider can largely avoid the error from self heating, that limits the accuracy of the Hammon divider.

author: donlisms
author_rank: Frequent Contributor
timestamp: November 06, 2022, 12:08:21 pm
content: Late to the party!It seems like the main point gets lost at times, which is that if you put a set of N nominally equal resistors in series, and then in parallel, the ratio between them is pretty much exactly N^2, with small additional error terms of 1/N*(average error)^2, and cubed, and so on. In other words, if the error of each resistor is within 0.1% of their average, for N=10, the error in the 100:1 ratio is better than 1ppm; and I think actually 0.1ppm because there's a 1/N factor. No additional tuning is really needed; just careful design and implementation. (Tetrajunctions help.) If you want better than 1ppm, you can use resistors better than 0.1%, but they don't have to be alotbetter.This, by the way, was not Hamon's invention; it was a known concept; you just do the math. I think calling the series/parallel concept "Hamon" is a bit of a misnomer, but he used it to good effect in his well-known 100:1 build up resistor of N = 10, which became a Leeds & Northrup product. There were lots of other bits of design detail that went into trying to make it as precise as possible.ESI called their version "SR-1010" (and 1030 and 1050), and I think maybe there is some Fluke thing. The SR-1010 manual is recommended; lots of details.Conrad's popular configuration (for which we are eternally grateful!) uses N = 3, giving a 9:1 ratio, and instead of just one set, it has two sets. "Calibration" is a simple matter of comparing the two sets with each other, using any technique that will get you there, but if you can get all the original 6 resistors within the square root of your target, you shouldn't really need to worry about calibration much. Maybe a little. If the balance comes out bad, it seems likely that something else is going wrong; not the resistor configurations. Kleinstein mentioned the kind of "ring" method, where you measure each resistor in turn against all the others in the set; this was a common high-precision calibration technique back in the day.Tuning with trimmers is entirely optional, and would depend on some other objective besides the pure resistor combinations. (Perhaps instead of a group of 3, you chose a single resistor for one of the "sets".) The idea is that with a set giving 3R, and a set giving 1/3R, you should be able to make a 1:10 divider that is quite accurate, without a lot of fussing with calibration and trimmers and that sort of thing.As an alternative to Conrad's "extension" for 100:1 ratios, one might consider making an N = 10 divider using the same techniques. But now you need 20 resistors in the range, rather than just 6!As to how to build a null meter using little doo-dads, I don't know, but I know that simple is good!

author: KT88
author_rank: Frequent Contributor
timestamp: November 06, 2022, 12:46:42 pm
content: Something to keep in mind is that opamps also use a divider to set it's gain. Usin a Hamon divider would give a (very) well defined gain. In order to get the best results a composite amplier is recommended to mitigate any considerable gain error from the limited open-loop gain of a single amp.CheersAndreas

author: mawyatt
author_rank: Super Contributor
timestamp: November 06, 2022, 01:24:20 pm
content: Quote from: Kleinstein on November 06, 2022, 07:23:57 amIf one was to use this, then a bunch of parallel standard CMOS buffers, or a very low Z CMOS buffer, or just a simple matched P and NMOS discrete FET inverter works. This gets Rds ON into the sub-ohm region so the the effective output Z is low enough to not be a major contributor to the output error.Adding the VSS resistor was more about playing around when using a plug-in breadboard, that's why we mentioned:"Anyway, one can easily assemble a 74AC74 and couple 74AC04, a few resistors on a test board and have some fun with this circuit!!"For an actual use case the matched discrete P and NMOS would be the better choice, we've done this before and it works very well!!Edit: Replaced the 4049B with discrete inverter with 120 & 130 mohm N and PMOS (20N06 & 20P05) and removed the "Magic Resistor", resulting in 4.9999976 V output with VDD of 10V. Also note with the discrete inverter one only needs to connect the PMOS source terminal to the Reference Voltage, the rest of the circuitry can be connected to a regular VDD supply.Best,

author: enut11
author_rank: Super Contributor
timestamp: November 06, 2022, 08:47:33 pm
content: All the discussion on calibration using active voltage comparison is interesting but I will stick to the traditional passive method for now.Quote from: Conrad Hoffman on November 04, 2022, 12:51:06 amConrad mentioned "Reversal!" and, with a bit of a nudge from other members, I think I now understand what he was talking about.Once the Hamon circuit is configured for 2:1 division using 2 jumpers, it is then connected to two other equal external resistors in a bridge circuit.In my case, the 2 Hamon "resistors" are 7.3K and the 2 external ones are 56K as per the diagram below. There will be a small trimmer R in each leg. In addition, the 2 legs of the bridge are connected via a DPDT reversing switch.Using a Null Meter, the external (56K) resistor leg is trimmed until successive reversals result in a stable offset. This happens when the two 56K resistors are identical.Continuing with this method now on the Hamon pair, a trimmer is adjusted so that the Null Meter now reads zero offset, resulting in two equal 7.3K resistances.Once the external resistors and the Hamon jumpers are removed, the result is a perfect 10:1 divider.Next step for me is to set up small stable trimmers to make this possible.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 06, 2022, 09:04:35 pm
content: If the 56 K resistors are prettey close to start with, it may not be clear which one is smaller. In this case it may be better to have the trimmer in the center. A resistor parallel can reduce the trim range.For adjusting the Hammon part one would likely need a ratgher small variable resistor (e.g. a few ohms only). In this case the wiper resistance can be a problem. One can redue the effect of the wiper resistance by not using simple series trim, but using more a parallel trim and more like a 5K-50K trimmer. There are several possible combinations of parallel and series connection to reduce the effect enough to do the fine trim. The coarse part may want us a selected resistor to start with (should only be needed once).

author: donlisms
author_rank: Frequent Contributor
timestamp: November 06, 2022, 10:13:32 pm
content: I suspect Conrad explained what he meant by "reversal" in his article on building a Hamon Divider. His suggestion was to put two volts into the divider (with the resistors configured for 1:1), tune for 1v across the bottom resistor, then confirm by measuring the voltage across the top resistor. Tweak until they are both the same. Seems simple and effective to me.

author: enut11
author_rank: Super Contributor
timestamp: November 06, 2022, 10:38:07 pm
content: Thanks @donlisms. I tried this and it can give a good result but DMM input resistance loading can affect the result as can DMM reversal errors.In any case, it is always good to confirm the outcome using another method. My Fluke null meter has a 1uV FSD sensitivity which I doubt Keysight 6.5D DMM can match.enut11

author: enut11
author_rank: Super Contributor
timestamp: November 07, 2022, 12:07:02 am
content: Meanwhile, using the resistors from Reply #2 and @Gyro center-pot trimmer idea from Reply #8, with a 50 ohm pot I was able to test the Wheatstone bridge. However, balancing was very difficult with a single turn pot.I have ordered a couple of 100 ohm 10T pots, the lowest value I could see available on eBay.

author: Gyro
author_rank: Super Contributor
timestamp: November 07, 2022, 10:17:28 am
content: Quote from: enut11 on November 07, 2022, 12:07:02 amYou can always put a lower value resistor across the pot to improve its resolution. Just pick a value that gives you the resolution you want, while still being able to achieve a null with your two fixed resistors.You may notice in the photo of my divider that I added a pair of ordinary low value metal film resistors between the 5R pot and the Vishay 50k Resistors. I did this to tweak the the pot adjustment range to give a centered +/-25ppm. I used two resistors (irrc, they were something like 10 and 15R) to try to avoid introducing any additional Tc or thermocouple effects. This is all optional of course - I intend to use it (when I get around to boxing it) as a precision standalone resistor matching unit, where I can measure their imbalance by reading off the 10 turn dial.

author: TimFox
author_rank: Super Contributor
timestamp: November 07, 2022, 07:50:29 pm
content: For similar purposes, I wanted to use a Wheatstone bridge, with two legs made from equal resistors, to adjust resistors in another divider for equality.As eBay treasures, I had some four-terminal Vishay resistor networks (comprising S102 foil elements) of different values.The trimpots are low-resistance cermet 1.25" 22-turn pots across discrete RNC90 Vishay foil resistors.I exploited the concept mentioned by enut11, where two voltage dividers, of different resistance values, can be made to contain equal resistances by adjusting the bridge for null, reversing connections to one side, and re-adjusting untill the bridge is at null for both connections between the left divider and the right divider.To adjust each pair, I excited two pairs through the rotary switches from one or more 9V alkaline batteries for null voltage on a Fluke 845AB meter (depending on resistance).After much tweaking, I verified that comparing each divider to the other two gave a null result.Attached are the wiring diagram and a photo of the top panel of the result.The circuit was built on a panel of 2-oz Cu single-sided FR4, screwed to a Bud AC-401 aluminum chassis box.The first application of this set of three "half bridges" was to calibrate a series-resistor voltage divider (for ratios, not absolute resistance), built on a similar panel and identical chassis.Again, I started with eBay finds of precision wirewounds, and added a small trimpot across a smaller fixed resistor in series with the main resistor to cover the manufacturer's tolerance, so each resistance is slightly larger than the nominal value.Unfortunately, I had to start adjusting from the low resistance end, made from 0.1% resistors, while the 10k and 50k resistors had 0.01% tolerance, so the adjustment range was larger than I desired.Nominal resistors: 5 each of 2k (2 x 1k in series), 4 each of 10k (single 10k), 3 each of 50 k (single 50k), and 4 each of 100k (2 x 50 k in series).Each resistor branch connected to two binding posts (no switches).Nominal resistances (from common): 2k, 4k, 6k, 8k, 10k, 20k, 30k, 40k, 50k, 100k, 150k, 200k, 300k, 400k, 500k, and 600 k ohms.The voltage division ratios, from the common terminal, range from 2:1 to 300:1.First, each pair of 2k resistors sharing a terminal were adjusted for equality, then the first 50k resistor was adjusted to equal the five 10k resistors below it, etc.

author: enut11
author_rank: Super Contributor
timestamp: November 09, 2022, 06:59:38 pm
content: Hi @TimFox. That's a very nice, versatile series of half bridges that you have made. Thanks for the construction hints. One problem I have yet to solve is hand-capacitance when nulling the bridge. Any tips on that?It is getting harder to find those "eBay treasures" you mentioned and new prices for high quality components are way over budget for hobby projects. Inevitability, I end up creating compound resistors which may look ugly but do the job.Thanks for the inspiration and design.enut11

author: TimFox
author_rank: Super Contributor
timestamp: November 09, 2022, 08:54:33 pm
content: Quote from: enut11 on November 09, 2022, 06:59:38 pmI also had problems with hand capacitance. I made up some shielded cables using normal Pomona banana plug test leads and shield braid, with a short green banana plug wire at one end.The green wires went to the chassis ground connections on each box (cable shields not shown).I found it easier to float the 9V battery(ies), so I could ground the low end of the null meter to the Guard and Ground terminals on the front of the Fluke 845 (following an example in the 845 manual).In the attachment, the low terminal of the null meter is the connection of "Common", "Guard", and "Low" on the front panel.In the attached drawing, each shielded cable has its shield braid connected at one end to one of the boxes, and each box has a test lead to connect its chassis to the terminals on the 845.

author: enut11
author_rank: Super Contributor
timestamp: November 11, 2022, 03:37:28 am
content: Quote from: Gyro on November 07, 2022, 10:17:28 amThanks @Gyro. While waiting for parts to arrive I have been tidying up the Hamon setup and working on an external Half Bridge. The latter will be useful in its own right for checking equal value resistors and is a simplified version of @TimFox design above.A very rough prototype of a Half Bridge incorporating @Gyro mods to allow better balancing appears below. Red, blue and black banana sockets on the left are inputs from the Hamon Divider. The green screw terminals allow individual external resistors to be calibrated. The reversing switch (centre) and red-green-black banana sockets and grey knob comprise the fixed components of the 1/2 bridge . The 470 ohm pot will be replaced with a 10T 100 ohm pot when it arrives.

author: TimFox
author_rank: Super Contributor
timestamp: November 11, 2022, 04:29:36 am
content: When calculating the available adjustment from low-value multi-turn pots, check the manufacturer's data on resolution.It is dictated by the diameter of the wire used to wind the resistive element, as the slider goes click-click-click across the element.With low-resistance pots, the wire diameter is relatively large.Bourns (among others) tabulates this resolution for their good pots.

author: enut11
author_rank: Super Contributor
timestamp: November 11, 2022, 05:02:17 am
content: Good point @TimFox. How do I determine what is a good 10T pot resolution for this application?Does this also dictate the practical size of the low-ohms shunt resistor?Specs for the 100 ohm pot I purchased from eBay below:

author: Gyro
author_rank: Super Contributor
timestamp: November 11, 2022, 09:53:39 am
content: Yes, the resolution and span of the pot are determined by the shunt resistor across it.In my case, I have 2 x 50k Vishay S102C resistors and a 5R 10 turn pot (the low value pot is nice as its resistive element is a simple spiral wire rather than wound, so effectively 'infinite' resolution). The span is given by 5R in 100k, so 50ppm, or 5ppm/turn.If I read correctly from reply #2,you have two 56k resistors, so a very similar situation. To achieve he same 50ppm span, you would need 5.6R, or taking the 100R pot into account, just under 6R (5.93) as a shunt resistor.I think you said that there was about 2.5R mismatch between the resistors, you might want to compensate ths by adding one or two low value series resistors at the ends of the pot to centre the span to +/- 25ppm. I just measured the ones on my lash-up and they are (coincedentally) 10R and 13R. As I mentioned previously, I used two resistors in an attempt to match TC and number of junctions (assuming that the TCs match, or are even important at this level). They affect (reduce) the spanveryslightly, but you can tweak this with the absolute shunt resistor value if you're really concerned. At 5ppm per turn, the contribution of the wiper bumping across the pot winding turns should be insignificant.Don't be afraid of using standard metal film resistors for these low value parts, they have minimal effect on stability. Remember that you only need to maintain absolute balance for a maximum of a few minutes at a time - the whole point of the reversing switch and pot.Whether 50ppm span is right for you, only you can say. It depends on how far out the resistors that you are comparing are likely to be. I have 50ppm because of that's what I got with the parts I had to hand, you could just as easily choose 100ppm or 10ppm depending on your needs.EDIT: Regarding the winding turn resolution steps, the spec you posted doesn't really help as it covers parts from 100R to 100k. These will have radically different wire diameters and winding density.

author: TimFox
author_rank: Super Contributor
timestamp: November 11, 2022, 02:57:33 pm
content: Quote from: enut11 on November 11, 2022, 05:02:17 amUnfortunately, that datasheet does not show the "resolution" of the device, just the "linearity" (which is important for other applications, but not yours).If you look up the Bourns pot that is similar to that one, you can get an estimate of the resolution, since they probably use almost the same wire diameter for a given resistance.An ideal low-resistance pot would use a single wire (as discussed by Gyro).When I used 10-turn pots in another application, I did a brute-force algebra calculation to determine (as a function of the resolution and the other resistors in the circuit) the change in voltage from one "bump" to the next, to calculate the smallest adjustment I could make in the voltage.I first intended to use 22-turn wire-wound trimpots in the boxes I discussed here, but quickly determined from their resolution spec that the jumps were too large.I changed the design to use cermet pots, which have infinite resolution, even though the tempco of the overall resistance was worse (100 ppm/C, IIRC).Vishay makes some great, yet expensive, 22-turn foil pots in the same form factor that have good tempco and infinite resolution.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 11, 2022, 03:35:06 pm
content: Just for the trim of the auxiliary bridge the TC is not important. The pot only covers a small range (like 100 ppm) and thus drift in the pot is attenuated by something like a faktor or 10000 or 1000 at least. There is also no need to use a very low resistance one, some 100 ohms - 10 K would be OK. If needed one can add selected fixed resitors to improve on the match of the 2 main resistors.

author: TimFox
author_rank: Super Contributor
timestamp: November 11, 2022, 04:46:15 pm
content: So long as we are talking about bridge circuits, where only the ratios are critical and the absolute values far less critical, the absolute tempco of a pot usually matters very little, since it does not affect directly the ratio of the two resistances.