Precision differential voltmeter / DC supply VR2 Farnell

[loulou31]

Hi,
I have bought an old differential voltmeter and I am looking for any documentation on this test gear. I think that I can operate it without any manual, but it will help mainly with the schematics to put it in operation and calibrate it. I cannot find any information on the net of this model.
Thanks
Regards
Jean-Louis

[Craig Sawyers]

I had no idea that Farnell made a differential voltmeter. The company most associated with differential voltmeters are Fluke.

Can you post a photograph of yours?

Craig

[loulou31]

Please find photo of front panel of my test gear :



I can post photos or rear or inside if can help.

Thanks

[Craig Sawyers]

Well that is very interesting - and I would say rare. I have not seen a Farnell differential voltmeter! The differential voltmeters were the only show in town for precision measurements before decent DVM's.

The line of knobs along the bottom will connect to a Kelvin-Varley voltage divider, certainly four and possibly five digits. These are an unusual beast https://en.wikipedia.org/wiki/Kelvin...Varley_divider

And basically compare the voltage you are measuring with a voltage standard. With Fluke that is an aged Zener reference, but some used a standard cell. So it would be good to see what is inside.

The Fluke's had a voltmeter section, to see what the approximate voltage to be measured was. The instrument can then be set appropriately for the exact measurement.

The really big advantage of the differential voltmeter is that when nulled, zero current flows from the source, so the input resistance is infinite. Actually of course not quite because internal leakage paths will limit it to typically many giga-ohms and upwards.

Craig

[loulou31]

Hi,
I have checked some big electrolytic capacitors and switch on the unit.
It is working in precision voltage source, but only up to 20v instead of 100v.
I dont understand exactly how to operate the voltmeter. I have checked to test a 9V batterie, but I can do the zero with about 3V on X10 mode on thé multiplier.
I am asking on how operate mode "differential" with x0.1 or x1.
Fot the moment I will try to find the problem of limitation in source mode. There is internally a raw voltage that is about 25V more than the expected output voltage and opérationnel I think that the regulation is out of opération for voltage over 20V. I will try to redraw this part of the circuit.

Regards
Jean-Louis

[McMurdo]

You're going to have to get the precision source working properly before it'll work as a voltmeter.
We had one of these when I first started work but it was mainly used as a precision power supply. It was pretty accurate all the way to 99.999V. Plenty of current all the way up too, I can always remember my boss testing a 48V telecom relay or similar and slipping with the probes, resulting in a big flash and a bang.
The main problem with it was the space it took up on even a large bench and its weight.

[Jez1234]

Interesting bit of kit there. Not something I've ever used. Craig's post was most informative!

[wireman]

Keithley made at least one model as well. (660)

[loulou31]

I think that I have found the problem related to the limitation of thé output voltage. It is due to thé big capacitors AT thé output (1000uf 160v).
For me this instrument will be used as précision voltage source. I have to check the current capacity that should be around some hundreds of mA.The regulation voltage is done with 3 x 2n3054 in séries.
For the voltage setting over 5 decade it is done via a simple divider with precision resistors housed in a métal box containing oil I suppose. There are 4 resistors per decade : 1,2,3 and 6 that allows 1 to 9 with séries combination. It is not done with Kelvin Varley divider.

Jean-Louis

[loulou31]

The max current is 400mA
I have found the ajustements for the DC source : zero and full range.


Jean-Louis