I have a solid state relay an AQZ207 to be exact. I want to be able to measure the off state leakage current of the FET which in the datasheet seems to around 10uA Max. I had some ideas, but not sure what to do. I know the multimeter I have a fluke has a burden voltage and everything that will affect the results.
I remember seeing some kind of resistor placed in parallel with the meter to measure the voltage drop of the resistor that some how scales to current? Anybody know of this method? I'd love to read a white paper or detailed explanation. I'm assuming the resistor acts like a current sensor since if you know the resistance and applied voltage you can calculate current.
However, is this to offset the multimeter's high impedance from the measurement also?
To actually test if the FET is leaking, I'm not sure how to test that in operation there would be a voltage drop across the resistor if the FET was leaking since its being loss in the gate?
Best Answer
Assumed the current you want to measure is indeed about 10uA and you feel OK with a burden voltage of about 1V and your multimeter has a voltage range of about 1V, then all you have to do is put a resistor is series with the FET you want to test. Most multimeters have a voltage range where 1V is somewhat in the middle of the range. In this case:
$$R_{series} = \dfrac{U_{burden}}{I_{expected}} = \dfrac{1\text{V}}{10\mu\text{A}}=100\text{k}\Omega$$
and measure the voltage across the resistor. The voltage measured is a pretty good indication of the current through the resistor, but it is important to check your meter's impedance. For good results it should be at least 10 times higher than the resistor. In this case the error will be better than 10% (\$\frac{100\text{k}\cdot 100\%}{1\text{M}}=10\%\$).
The leakage current will flow through the resistor and Ohm says:
$$I=\dfrac{U}{R}$$
So the voltage across the resistor is proportional with the current through it. R is known, U is measured, so you can calculate current I.