Cropico ESC1 Electronic Standard Cell

A few days ago I managed to buy one of these for £5.50 on Ebay, it was was one the first zener diode replacements for the Weston cell,  but when I powered it up from my bench power supply it was dead!   so,  I cracked it open and after a while checking through the components I discovered one of the tantalum caps had shorted,  so I replaced that with an electrolytic one of the same value and when I powered-it-up,  it was working once again!.

The bad cap which was replaced!














The unit powers-up with 27 volts (or 18 x 1.5 volt C cells! ) and consumes ~16mA

From some measurements I took with my 7150 it seems that my 7150 has drifted quite a bit from the cal I had done a few years back;   the good thing is I can reasonable confirm this drift because the 1.0000v reference from my Time 2003S reads exactly the same on the 7150 as the Standard Cell.

The 7150 readings were..

Standard Cell  @ 1 volt
0.999815 volts

Time 2003S @ 1 volt
0.999815 volts

IDM305 DMM 1
0.9989 volts

IDM305 DMM2
0.9986 volts

Ambient was approx ~19.5 degrees C.



Inside the cover of the unit was a historic record of the measured Standard Cell voltages.

An identical unit was taken apart over on one of the popular electronics forums eevblog, the linked page also contains some useful PDFs.

http://www.eevblog.com/forum/metrology/cropico-esc1-electronic-standard-cell-a-look-inside/


This is another useful link to a site called "Johns Virtual Museum of Interesting Gadgets" where gives a short bit of information about the unit.

https://sites.google.com/site/johnhurll/home/cropico/ecs1-electronic-standard-cell

TENMA 72-10495 2CH 0-30V 5A Power Supply

This is my new power supply,  I was tried of using my old one which was a complete pain to use as the voltage and current adjustment was far too course;  this TENMA one has a digital pot which allows for course and fine adjustment.  Initially when it arrived I let it warm up to ambient room temperature before I switched it on;  then I tested both channels with both light and moderate loads and it all seems to work really nice;  the fan speed is variable based on the load,  and when you turn the pot through the entire voltage range you can hear it using relays to switch through the various transformer taps.

A few weeks later...

Well now I've had chance to play around with it a bit I must say it's really quite nice.  my only niggle with the unit is in the way that when you set either a voltage or current directly it always defaults to voltage and always at the volt unit,  this is a pain if you need to periodically tweak the millivolt, say if you were simulating a 0 - 5 volt sensor.  the other niggle is you can only change the voltage and current while in edit mode,  and after a few seconds it auto exits edit mode;  and entering edit mode always takes you to voltage and the volt unit and not the last unit (millivolt) setting.

I haven't tested the power-on voltage curve under max current conditions as I don't have a dummy load, but I'm guessing there wouldn't be much overshoot to worry about.