uA current measurements with normal voltagemeter

[Dknot--]

Hello guys,

I have voltagemeter, which has resolution of 1mA. I still would want to measure current of about 1-20uA with it.
I would need resistor in series with my load, and much amplification for making voltage measurements, at least 2000.
Is there any good, detailed instructions available how to make amplifier circuit etc. for this kind of measurements from basic components? I did not find any detailed descriptions of how to do this kind of measurements with google,

Thanks already,

-DKnot--

 

[crutschow]

If you put a 100k ohm resistor in series with the load and measure the voltage across the resistor, you can measure up to 20µA with the 2V range on your voltmeter (0.1V per µA).

 

[Dknot--]

Hello crutchow,

that sounds like a very unaccurate way to do measurement. I would need reliable and well documented way to do that.
Thanks for your answer,

-Dknot--

 

[Reloadron]

Option 1 is to just buy an analog DC current meter similar to one of these for current.

Option 2 is exactly as crutschow suggested placing a shunt resistance in series with your load and measuring the voltage drop. You don't mention the compliance voltage behind your current source? If you want to use a precision low resistance shunt resistance and add instrumentation amplification then consider using a good, low noise instrumentation amplifier like the AD524 chip from Analog Devices INC.

Crutschow's method will work fine assuming you have the compliance voltage available from your current source. Using a precision resistance and measuring the IR drop will produce very good and accurate results. Additionally this accepted method can be well documented with calibration if need be clearly demonstrating the uncertainty of the measurment plane.

Ron

 

[3v0]

Hello crutchow,

that sounds like a very unaccurate way to do measurement. I would need reliable and well documented way to do that.
Thanks for your answer,

-Dknot--

Time to brush up on ohms law, it is the documentation!

V=IR or I=V/R or Amps=Volts/Resistance.

We know the value of the resistor and we can read the voltage across the resistor with the meter it is easy to calculate the current using any resistor. But if we use a 100K resistor as suggested our meter will read 0.1V per µA.

Accuracy depends on tolerance of the resistor and the accuracy of the volt meter.

100K 1/2Watt 0.05% resistor UXEB100K00A1A Vishay/BC Components Thin Film Resistors for under $2 from Mouser in the US.

 

[MrAl]

Hi,

Yes you can often get away with using a somewhat high value resistor and measure the voltage across the resistor and use Ohm's Law, but the meter should be a digital one and have input impedance of 10 Megohms or better. You also have to calculate in the input impedance of the meter for more accurate results. The input impedance of the meter is in parallel with the high value resistor.
Also, some meters claim 10 Megohms but they might be 9 or 11 Megohms so you'll have to test your setup to see what you end up with. In a pinch though 10 Megohms might be a good enough estimate. 10 Megohms in parallel with 100k is close to 99k which is only about 1 percent off so it's not too bad. If you dont like that, use a 101k resistor and that will bring the total to 99.99k which is only 0.01 percent off.

 

[Ratchit]

Dknot--,

I have voltagemeter, which has resolution of 1mA. I still would want to measure current of about 1-20uA with it.

Voltmeter resolution is usually measured in units of volts, not millamps like you state. Current from 1-20uA is usually measured with a microampmeter.

I would need resistor in series with my load, and much amplification for making voltage measurements, at least 2000

2000 what? If 2000 volts, you would need no amplification at all to fry your meter, unless you have a high voltage probe.

I would need reliable and well documented way to do that.

For your perusal, I will show you a reliable and documented way my meter measures small currents. You will not get the same sensitivity, because you do not possess the same unit, but the principle is what counts. My meter is a Sencore FE 149 with a 15 Meg input impedance. Notice they suggest putting a 1.5 volt battery in series with the voltmeter, giving by the resistance formula, 1.5volt/15Meg = 100nanoamps for full scale deflection on the 1.5 voltage scale. So the higher the resistance of your voltmeter, the smaller current you can measure. See the attachment.