Hi all,
I just wanted to share my Hamon Divider build (aka Poor man's Fluke 752A Reference Divider) and was wondering if anyone else had constructed one?
In case you haven't heard of it, the Hamon divider (invented by B.V. Hamon in 1954) is a way of creating a precision Divide by 10, 100 etc that is effectively self-calibrating. Fluke used this principle in their 752A Reference Divider and they still use in some form with Josephson Junctions.
My build was inspired by Conrad Hoffman's article, which I found whilst browsing Joe Geller's excellent site....
http://conradhoffman.com/HamonResistor.htmlhttp://www.gellerlabs.com/752AJunior.htmI happened to have a matched set 10, 1970's vintage, 1k precision wirewound resistors which fitted the bill perfectly for a 10:1 divide. Basically for calibration the upper 9 resistors are arranged in 3 parallel groups of 3 resistors to create a compound 1k resistor which can then be compared with the bottom 1k resistor as a 2:1 divide - as long as the voltage across the two 1k resistors is exactly equal the when the resistors are all placed in series the divide ratio will be exactly 10.
The main photo shows the finished divider (together with 100:1 extender) and my dodgy adjustable 2:1 divider which I used, Wheatsone Bridge style to measure the 2:1 ratio with my 1uV resolution Datron 1045 DVM.
![](https://www.eevblog.com/forum/testgear/anyone-else-built-a-hamon-divider/?action=dlattach;attach=175684;image)
The 10:1 divider mode selection is done with a barrier strip and links - at these resistance levels switch contact resistance would be an unacceptably high error source (Fluke managed it by using higher resistance chains but this gives other challenges with leakage and guarding).
The internal view shows the internal resistor mounting and wiring, the resistors are mounted on an internal insulated sub plate, connected to Input Lo in an attempt to provide some screening and hopefully reduce thermal gradients. The resistors are mounted on PTFE barb standoffs - but that's because I have a big bag of them,
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/xsmiley.gif.pagespeed.ic.R8GFI-pF6f.png)
, not needed at this resistance level.
![](https://www.eevblog.com/forum/testgear/anyone-else-built-a-hamon-divider/?action=dlattach;attach=175686;image)
Things have evolved since I built the 10:1 divide, I found a batch of 10k Vishay MPR24 5ppm 0.01% resistors going really cheap on ebay so decided to add a 100k 100:1 divide. This uses the same arrangement (as Conrad Hoffman's article), but this time I was able to use a sealed toggle switch without worrying about contact resistance (leakage on a large barrier strip would probably be more of a concern). Having a 100:1 divide is useful for 'leapfrogging' ranges when meter calibrating.
![](https://www.eevblog.com/forum/testgear/anyone-else-built-a-hamon-divider/?action=dlattach;attach=175688;image)
I still need to box the adjustable 2:1 checking / calibration divider and add a reversing switch (it's fiddly having to keep reversing leads). Basically it consists of a pair of matched 50k Vishay foil resistors in series with a 5 ohm 10 turn pot (all 1970s vintage again), I added a couple of low value (10 and 13 ohm) resistors to balance the center of the pot's span.With 1V input this gives a span of 50uV (5uV per turn).
For testing, the Hamon divider (in 2:1 mode and adjustable divider are put in parallel across a 1V (approx. non critical but low enough to avoid resistor heating as one of the 1k resistors is made up of 9 resistors in series parallel in this mode) and with the DVM on 10mV range (1uV resolution) connected between the taps. The pot is then adjusted to null as close as possible. Reversing the adjustable arm then gives an offset which you adjust the pot to halve. After a few iterations reversing the adjustable arm causes no change in offset (ie it is exactly 2:1) - any remaining offset is then due to error in the Hoffman divider. This is the way it's done in the 752A too.
I calibrated the divider using fixed resistors rather than pots, high value shunt resistors on the 10k 10:1 and low value series on the 100:1 (the resistor in the 100k 100:1 is 1.3 ohms
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/xsmiley.gif.pagespeed.ic.R8GFI-pF6f.png)
). I've had the whole thing built for about a year now and it really hasn't drifted significantly - in 2:1 test mode at 1V input it's about 2uV off on the 10:1 (2ppm?) and 5uV on the 100:1 (5ppm?). That seems a pretty good result to me, it's about a tenth of the stated accuracy of the 752A.
![Cool 8)](https://www.eevblog.com/forum/Smileys/default/xcool.gif.pagespeed.ic.cOxV6sPNRd.png)
Probably just luck and aged resistors though.
If I was doing it again I might be tempted to put it all in one box (the 100:1 add-on is a bit messy), but it does make the it smaller this way and the adjustable divider (when I eventually box it) will be useful for resistor matching.
Sorry it's a bit long folks.
![Cheesy :D](https://www.eevblog.com/forum/Smileys/default/xcheesy.gif.pagespeed.ic.KPzuH4J-TG.png)
Edit: added 'Voltage' to the subject line, added photos