[volt-nuts] Cropico DO4A Digital Ohmmeter

[Bob Albert]

 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.)
Bob
    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>
wrote:

>
> 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.

Dave
_______________________________________________
volt-nuts mailing list -- volt-nuts at febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.