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µA Meter Sensitivity Amp

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Quarlo Klobrigney:
I have to replace a meter to a piece of analog test equipment in schematic that does not specify what µA range the original was or very much else.
A bunch of resistor dividers, nothing much to see, most ranges have the negative side of the meter to GND, the positive side to the junction of a 2MΩ input resistor and a 120KΩ to GND, "totem pole style". Another one a 5MΩ over a 1MΩ, etc. The only thing it says is about the meter is "range 1 V, input impedance 2 MΩ"

I've never seen the test equipment or the meter, so what did I know. I can tell you it wasn't 50µA. I'm thinking it was somewhere in the 2-5µA range.
To make it worse, it's a center zero type.

The only thing I find at the local shop which is just a local "have it today" shop from China, is a new 50-0-50. The meter barely indicates any signal.

So the question is this ( I'm tired and the answer escapes me, and it's 03:43 and my runtime is 20+ hours ) can the 5µA signal be made larger to the 50µA meter with a simple Op-Amp circuit or other IC that will be a linear translation from the original to the new range?
Where could I find such an example or does someone here know how? The new meter has a DC resistance of 2KΩ.
I wish I could  :=\ Age related insomnia..........

5 µA would be very low for a meter movement, usually only found for very small ones and those with a a light pointer. 50 µA would be pefectly normal - that was common for analog multimeters.

Amplification needes extra supply, but would be possible. It may be easier to change the resistors, so that a more normal movement would work.

Old high value resistors may fail - especially carbon film in the MOhms range tend to drift up. So it may not be the meter movement that is broken, but some of the resistors.
A simple circuit plan around the meter may help.

You can measure what the current was with modern DMM that has uA range. Just connect DMM in uA range instead of moving coil meter and put DUT it in, for instance, 1 V range and apply 1 V to input. It won't be accurate (because MC meter has different resistance from your DMM in uA range) but will show you order of magnitude. If you have 10V range, even better, it will have less error..

But, if you take piece of paper and draw a schematic, i would say your estimate for meter being something less than 5uA would be correct (2 Mohm resistor), provided that is correct.

To measure such small currents, you need precision autozero opamp. For inspiration how to convert small current to DC voltage look at uCurrent schematics. https://www.eevblog.com/files/uCurrentRev5schematic.pdf .

Quarlo Klobrigney:
It's a German design for a stand alone test kit from wayyyyyy back in the hollow state days so it naturally has to be well designed (over complicated).
I don't want to have to utilize a separate digital meter and it has to have an analog meter for those needle movements the thickness of a bees dick.
It has a few ranges and configurations and is high impedance with the meter in a self contained box.

So an amplifier that is linear to upscale the fly farts to the new low impedance meter is what it will have to be.
Still awake :palm: so if I sound loopy, I am.


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