I have a bunch of 1mA meters, but no 50-0-50 or 1mA-0-1mA meters...
I'd like some hints as to how I might modify, or create a circuit
that centers the needle in the middle of the scale. Like if I wanted
to make a meter for measuring positive and negative current flow,
like you might use for a vehical alternater/gen charging system. I'm
guessing I'll need some kind of active circuitry...

 

Did you try using the 0-adjust screw?

Bob

 

 

https://en.wikipedia.org/wiki/Galvanometer
https://www.google.com/search?q=1mA+galvanometer&tbm=isch

 

Basically you need to be able to handle polarity reversal. A differential amplifier
properly biased could do this. Note this would be for external shunt use. If shunt
is inside meter then it would have to be disabled, or removed and used with the
circuitry.

Current you trying to measure ground referenced or isolated ? If former you
will need split supplies to run the diff amp, otherwise you can do this single
supply.

Useful IA handbook - https://www.analog.com/media/en/tra.../designers-guide-instrument-amps-complete.pdf


Regards, Dana.

 

I have done this a number of times quite successfully. You need a very delicate touch and a spotlessly clean work area.
Carefully remove the movement from the case. There is a spiral spring on the front and back of the movement. It is simple to move the front adjustment to make the pointer move to 1/4 scale. You then need to adjust the back spring in the same way to move the pointer to 1/2 scale. Be very careful to not bend or distort either of the springs.
When you have finished the adjustment, carefully re-mount the movement back in its case. Make sure it is adjusted accurately to half scale before you replace the front because the external zero adjust may now be out of the range of the internal one.

Good luck,
Keith

 

Attached is the circuit I built to drive the ammeter on my power system - converted a 6" 270 degree 16mA FSD to an 80A-0-80A battery meter. Also I adjusted a couple of those meters (nice and big internals to work on) using Keith's method above, but a problem I encountered was that driving the meter to the old zero pos (ie full -ve) caused the spring to foul the mount, as it was extended past it's designed intent. The active circuit is better, and has run successfully for nearly 10 years now; it only needed re-zeroing once, about 3 years in. To use it you'd need to recalculate resistor values to suit your shunt and meter impedances.

 

Thank you Keith Walker! I was lucky, in that I only had to remove the face, and manually adjust the
front trimming sprong a little further to the left than the bezel trim screw would normally allow.
Works like a treat!

 

Attached is the circuit I built to drive the ammeter on my power system - converted a 6" 270 degree 16mA FSD to an 80A-0-80A battery meter. Also I adjusted a couple of those meters (nice and big internals to work on) using Keith's method above, but a problem I encountered was that driving the meter to the old zero pos (ie full -ve) caused the spring to foul the mount, as it was extended past it's designed intent. The active circuit is better, and has run successfully for nearly 10 years now; it only needed re-zeroing once, about 3 years in. To use it you'd need to recalculate resistor values to suit your shunt and meter impedances.

That useful to know, I am printing you off and putting a copy with my meter stash. Thank you.
Is the ca3140n op-amp critical, or will any single supply substitute?

 

I used it because it was to hand and had adjustable offset. It's not critical, by any means. My installation was 24V, and your intended operating voltage will have a bearing on what is suitable.