50uA current standard

Having come into possession of a relatively new (~2008) Simpson 260 series 8 VOM, I decided to make a 50uA current source to help measure this meter's full scale deflection, and calibrate the VOM if necessary.
In this video, I discuss my experience of making W2AEW Alan Wolke's circuit - he is a resource that I highly recommend for anything electronic and amateur radio related!
I include below links to the documents I refer to into my video and share my learning process in how this "Wilson Current Mirror" works.
"Zero-temperature coefficient reference diodes" - Microsemi Micronotes series 205 summer 1997:
www.microsemi....
Microsemi 1N4577 diode current/temperature charts:
www.microsemi.....
W2AEW, Alan Wolke's excellent video on Simpson 260 calibration and the circuit I used:
• #324: How to "calibrat...

[@ivolol]

There's a 6.4V drop from the top of the circuit to the bottom of the zener. At the "input" to the wilson mirror, that is is also taken up by two diode junctions, say ~1.3V, leaving 5.1V. The resistors down the input line to that point are (10k + 2.5k + 91k). Following ohm's law, we can divide 5.1 by that to arrive at around 49uA of current coming down that line, which is mirrored on the output. For "normal" zeners, they usually hold a 0 tempco closer to 5.1V or 5.6V. BTW for the ...next time you are building such a current mirror, I suggest to weld the BJTs right next to each other so you can bend them to literally touch their bodies together. Even dab a tiny bit of thermal compound between and then superglue to hold it in place if you wanna go ham. The diodes themselves aren't temperature stable but their shared temperature keeps the mirrored current stable.

[@queuerious]

@ivolol, great analysis 👍🏻. Thanks for watching! I appreciate your good observations. I had thought of physically fixing the transistors together, but thought the ambient air would be essentially uniform enough… but perhaps I could do better.

[@ivolol]

@@queuerious to be fair, yes, I have not measured the empirical effect that doing so or not has on the circuit tempco either. Just a 'nice to do'.