How low can I go--measured in microamps?

Thread Starter

whitehaired novice

Joined Jul 15, 2017
281
Many of us have seen, some of us have bought the 5 digit 1 volt meter currently available from China at a reasonable price. I have one.

If I add a 1 ohm shunt I have a 1 amp ammeter. Can I add a 100k ohm resistor and get a 10 microamp meter? Or will the internal resistance of the meter load it? If it does, is the internal resistance stable so I can use a higher resistance and trim it to order?

This seems like an incredibly low current to me and I find it hard to believe that such a meter would be stable and accurate. But if it is it could be used for an inexpensive very high range ohmmeter--many meg-ohms.

Is it worth my time to try it? Thanks.
 

wayneh

Joined Sep 9, 2010
16,102
My cheap Centech meter has an input impedance of 1M ohm if I recall, for voltage measurement. Good meters are more like 10M or more. So yeah, I think you could get decent microamp measurements as you described.
 

MrSoftware

Joined Oct 29, 2013
1,511
There are existing meters that measure in the femto amp (10^-15) range, if you have the budget, for example this one. It would be interesting to see how they pull it off. At that range I'm guessing it's all inside an IC, but if you can find out their general method then maybe that will give you some clues as to how to make what you're trying to make.
 

ElectricSpidey

Joined Dec 2, 2017
683
From my personal experience with any of those cheap panel mount voltmeters from China is this...

They are pretty good at measuring voltages from sources such as power supplies or batteries, but the input impedance is very low and that makes them pretty much useless for electronic usage. (they tend to load the circuit)

I have never tried to shunt them and use them as ammeters, so I can't really speak to that. Just be aware that they have low input impedance as compared to a regular DMM.

Can you provide a link to the actual meter?
 
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Since I've set up systems capable of measuring about 2 pA, I can comment. Those low currents are best done with a feedback ammeter. This is a regime where all sorts of weird stuff happens. Cable motion, cable flexing generate currents. To avoid leakage, Triax cables are used. these have an inner and outer shield. To avoid current generation by bending the cable sometimes graphite is used when the cable is made.

it's probably a little easer now than when I did it.
 

Janis59

Joined Aug 21, 2017
742
For sure exist a IC allowing to measure about 3 electron movement per second. However I cant tell the 3 fA opamps, expecially the infamous LT7721, be stable or trustable. But anyway they exist and cost only a dollar.
Bit better stays with a range of 20-60 fA where are bunch of good candidates like LTC6268 or ADA4530, yet expensiver a half of order, but stable and trustable.
So, if IC are existant, why to think that China designers havent heard about it?
 

DickCappels

Joined Aug 21, 2008
5,759
I have used a 10 Meg input resistance DVM on the 200 millivolt scale to measure junction leakage current in the low nanoamps range. The input voltage divider acts as the shunt.
 

Thread Starter

whitehaired novice

Joined Jul 15, 2017
281
From my personal experience with any of those cheap panel mount voltmeters from China is this...

They are pretty good at measuring voltages from sources such as power supplies or batteries, but the input impedance is very low and that makes them pretty much useless for electronic usage. (they tend to load the circuit)

I have never tried to shunt them and use them as ammeters, so I can't really speak to that. Just be aware that they have low input impedance as compared to a regular DMM.

Can you provide a link to the actual meter?
Well I'm glad you asked for the link. I spent all morning looking to no avail and finally stumbled on a link which says the item is no longer available but--it provided specs which I'll attach. Looks good to go to me. I'll update when I get to work on it but I don't work as fast as most people here.voltmeterspecs.JPG
 
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ElectricSpidey

Joined Dec 2, 2017
683
1 megohm is certainly much higher than all of the meters I have used. (cheap panel mount)

Give it a try...what the heck...
 

DickCappels

Joined Aug 21, 2008
5,759
(some text removed for clarity)
Can I add a 100k ohm resistor and get a 10 microamp meter? Or will the internal resistance of the meter load it? If it does, is the internal resistance stable so I can use a higher resistance and trim it to order?
The 1 meg is 10% of your desired shunt value which means that yes, you can trim the shunt. You will need approximately 111k. With any luck the tolerance of the meter's input resistance will be a minor consideration compared to your much lower value shunt.
 
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