PMHD2024 (Poor Man's Hamon Divider) - Page 1
Forum section: electronics / general

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 18, 2024, 04:07:46 pm
content: Some years ago I've bought a lot of about 50 precision resistors, Vishay S102 bulk foil, quite cheap probably due to a strange value - 87.15K +/-0.01% . As far as I know, these resistors were used for a sensor calibration, each supplied in a separate nice case as on the photo below. For a long time I was trying to find a way to use these and now I've built a Hamon divider. More accurately, not a complete divider but only a base for it, a chain of 12 selected resistors in a box. I've called it PMHD2024, as per the thread title. I've used a cheap Hammond box and a cheap D15 socket.The first unit is hand-soldered however I think now about using a PCB to simplify the build. In some way it is a very basic equivalent of the SR1010 unit, much smaller but providing for similar options. The D15 plug is also inexpensive and I will build plugs for various applications, for now only 10:1 divider plug is built (as on the photo) however a Hamon calibration plug, 1:2 divider and even a plug with a DIL switch for connecting to various points of the array is easy to build. This base unit can be used for INL testing as well, I suppose, with the right switching arrangement.My first tests show less than 10ppm error for 10:1 division and very stable performance. The idea of using a D-type connector is to get rid of expensive 4mm terminals and switches, the performance of D-type in my experience is good enough for this application down to ppm levels. I will try to run some stability tests in the next few days.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 18, 2024, 09:30:47 pm
content: Here is my first attempt in using myreptile boxtemperature chamber with my newHIOKI DM7275 voltmeter, to measure the tempco of the 10:1 divider made with PMHD2024.The input was 10V from Fluke 731B (the actual value measured by Hioki is 9.99994V, so the divider error is about 10ppm to 14ppm). The tempco appears linear between 20C and 28C, about -0.6ppm/C.CheersAlexP.S. the "notch" on the graph is me opening the chamber door for few seconds

author: dietert1
author_rank: Super Contributor
timestamp: May 19, 2024, 10:22:21 am
content: Your setup seems to be good for 0.1 ppm noisewise, except at the end of the sweep after more than two days (!) there seems to be a difference of 1 ppm. The cause could be some temperature lag. One could ground the aluminium enclosure from the outside and use the two pins for a thermistor inside the enclosure.I'm curious to see two sweeps with the Hamon plug (division ratio 1:1, two directions).Regards, Dieter

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 19, 2024, 10:35:53 am
content: The Hioki DM7275 is a very nice voltmeter, it almost needs the 8th number, however for my needs 7.5 digits is probably enough. I am slowly building a number of shunts for it, the Hioki can store 30 setup "screens", including accurate calibration constants for shunts, for example and with the HP3245A it makes a very decent Ohmmeter. If you don't need AC capabilities, the DM7275 is a very good value for money, you get almost HP3458A noise and stability levels with no fan noise and a better connectivity. And for about 1/10 of the price for a new unit with warranty. There are some things I don't quite like about the Hioki meter but nothing that I would consider a show-stopper.The whole cycle was about 2 hours (not two days!) and the difference is most likely just a thermal lag. The temperature sensor from Hioki meter is fixed to the divider case with a copper tape and actually works better than expected. Unfortunately, while the reptile box controls the temperature quite accurately, the rate of temperature change is not controlled and it warms up much faster than cools down. Still a useful tool and with the new meter is easy to use (Hioki takes both measurements, voltage and temperature, simultaneously).CheersAlex

author: doktor pyta
author_rank: Frequent Contributor
timestamp: May 20, 2024, 08:47:29 pm
content: Hi Alex,have You checked the insulation resistance between two nearby pins of the DB connector /K617/?Are there any D-subs with PTFE insulation ?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 20, 2024, 09:34:04 pm
content: In a good make D-type, the contact and isolation quality is sufficient. D-type connectors I've used (Amphenol) measure over 1 Pohm at 100V between two adjacent contacts or between a contact and the shell. The isolator is "thermoplastic", most likely glass filled PBT, though it is not PTFE, it has resistivity only 15-20 times lower, sufficient for resistor values up to 10M in this application, I think.CheersAlex

author: CalibrationGuy
author_rank: Unknown
timestamp: May 20, 2024, 09:44:20 pm
content: I wonder if the performance would change if you used thermal grease between the resistors and mounted them to a small aluminum block.TomG.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 18, 2024, 10:49:08 pm
content: Selected 10K Vishay foil, about +40ppm average deviation with values from +80ppm to -40ppm and resistors on both ends close to the average deviation. I plan to put it in a box and measure over the weekend. The pcb is made with a double copper thickness and seems to be working well.CheersAlex

author: iMo
author_rank: Super Contributor
timestamp: June 19, 2024, 06:32:26 am
content: Would not be better to place the D15 connector at the shorter edge of your box and have longer distance (with some slots in the pcb) between the resistors and the connector?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 19, 2024, 08:31:33 pm
content: Quote from: iMo on June 19, 2024, 06:32:26 amWhy? The resistors will not dissipate much in a metrological setup and it makes sense to keep the track resistance to a minimum.CheersAlex

author: iMo
author_rank: Super Contributor
timestamp: June 20, 2024, 07:13:07 am
content: Because of the thermal flows from outside through the large connector mounted close to the bunch of resistors.Quote from: Alex Nikitin on June 19, 2024, 08:31:33 pm

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 20, 2024, 01:03:32 pm
content: Quote from: iMo on June 20, 2024, 07:13:07 amI don't see that as a problem as long as the temperature gradients kept to a minimum, and for that the smaller the overall size, the better. IMHO the shortest distance to the socket pins is more important for keeping potential errors small. For a better temperature stability (especially if resistors used have a noticeable tempco) it would probably be sensible to provide a better thermal coupling between resistors, with an epoxy filler or perhaps just with a quality thermal grease.CheersAlex