[volt-nuts] Cropico DO4A Digital Ohmmeter

Bob Albert bob91343 at yahoo.com
Sun Apr 29 19:47:24 EDT 2018

 You didn't mention the power rating of these resistors.  Do they have a brand name?  They are likely wire wound; how much inductance, self resonant frequency?
I could use them for various purposes depending on the answers to the above.
The thing that makes me curious is that your 'average' is outside the individual values; did you make a typo?
Looking in old texts, you will find a four-resistor method of making a decade box.  Since your resistors are 20 Ohms, you perhaps would want to use a parallel pair for each value, resulting in a box that goes in 10 Ohm steps to 100 Ohms.  If these are very accurate resistors, you will have to be careful how you connect or switch them so as to add minimal error.
How to use such an instrument remains a good question.
    On Sunday, April 29, 2018, 4:10:54 PM PDT, <geoelectronics at rallstech.net> wrote:  
Fractional and even Ohm resistors-

Once, in a package from Russia, with some items totally not related to this group discussion, the seller included 2 boxes of 1/2% 200 ohm resistors with a note of thanks for the purchase. Each box contained several cardboard carriers with 25 resistors lined up in idividual precise slots.

At that time I checked them with the bench Simpson 260 and sure enough they were 200 Ohms. Checked several and they were all 200 Ohms. On the Simpson VOM all tested were remarkably the same, so next they were checked on the bench Simpson 463, a 3 1/2 digit .1% (I think) digital meter. There they were again, 200.0.

That's when I decided to put them away for a future project, aside from the usual resistor stash (I almost said TRASH!).

Reading this thread today prompted a second look at them, this time with updated equipment

Each sample tested read between 200.2 and 200.7 Ohms. A random sample of 10 tied in parallel read 200.0529. Interesting.

Next step was to figure out what combinations 10 or 20 of those could created if hooked together with clip leads, in various clusters of series/ parallel groups.

Simple calculated that from 1 to 10 in parallel would yield 10 different resistances, same # for series. So 20 different combinations divided over 2 groups of 10 resistors.

Next I tackled the possible combinations that utilized one set of series plus one set of parallel possibilities, in series. This yielded 100 combinations

I didn't even begin the possible combinations in parallel.

There are some very interesting numbers on that list, which include 22.22222, 33.33333, 66.66666 266.66666 along with many the normal whole numbers starting at 10.

This is where my research stalled (read: mind=boggled) and decided to ask the computer experts here if they can do a spreadsheet that would calculate all the possibilities of 20 ea. 200 Ohm resistors.

I can envision two smallish boards with two rows of 10 terminal each, one row on each side and the resistors soldered across the terminals ladder style or maybe criss-crossed so they would all be in series already.

Clip leads to do the dirty work, and a spread sheet for "programming" information.

If this or some similar would be of interest to the traveling calibration folks, I will donate 100 resistors to the cause, enough for 5 pairs.

George Dowell


On 2018/04/29 04:35 PM, Bob Albert via volt-nuts wrote:

 I have a few very low resistance resistors (around 50 milliohms), four terminal of course, rated at .05% or maybe better.  I built a low resistance Ohmmeter but don't use it since I got my HP3456A with very good resolution.  I can measure the resistance of a screwdriver shaft with it, although there can be issues with contact potential.  It has a function to eliminate that error but I am not too confident.  Measuring using AC would be a good solution.
 I bought some cheap Kelvin test clips but they don't hold on too well, and I don't know how to modify them for more solid clamping.  (They are similar to ordinary clips but each jaw is insulated separately so you can use them for 4 terminal measurements.  The DIY meter simply uses four clips.)
     On Sunday, April 29, 2018, 2:18:50 PM PDT, Dr. David Kirkby <drkirkby at kirkbymicrowave.co.uk> wrote:  
  On 29 April 2018 at 21:57, Nigel Clarke via volt-nuts <volt-nuts at febo.com>

 Sorry, I realise now I could have explained better, I do know what
 transistor it is but that's not the problem, this looks to be quite a
 complex power suply/charger circuit, with at least three unmarked
 adjustment pots so if I change the device, even for the same part number,
 it's quite possible it will need readjustment and that's what I don't have
 any information on.

 Nigel, GM8PZR

 I see this is a low-resistance (10 u ohm resolution) ohmmeter. Have you any
 idea how you are going to check the calibration? I just bought a Simpson
 444, which has 1 u ohm resolution. I have not got it yet, but are wondering
 how I am going to check the calibration.

 0.005% resistors are available from Farnell in a fairly limited number of
 values, but certainty not in the range of values needed for a milli/micro
 ohm meter.

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