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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?

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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.

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  • \$\begingroup\$ Thanks Jippie! I have a question about my method in determining leakage when the relay is off and on.. When the Relay is OFF To Test for minimal current leakage of the FET I would put a voltage through a resistor and supplying the contact with a small amount of current while the relay is off. A voltage reading is taken across the resistor, if any voltage is across the resistor it means there is leakage in the FET.. But if the relay is turned on, the entire applied voltage should across the load, if it isnt it would mean that there is loss across the FET gate? \$\endgroup\$ May 31, 2013 at 22:05
  • \$\begingroup\$ Correct. You want different series resistors in the case where the FET is conducting and where the FET is not conducting. Depending on your situation you'll have to decide which burden voltage is acceptable. If the burden voltage is much lower (say 10 times or more) than the total voltage, then you are probably fine. Remember that most generic components have at least 5% tolerance anyway. \$\endgroup\$
    – jippie
    May 31, 2013 at 22:09
  • \$\begingroup\$ You'll need a DC supply (maybe 10V) across the relay-resistor pair in series or you won't measure anything. \$\endgroup\$
    – Andy aka
    Jun 1, 2013 at 9:12
  • \$\begingroup\$ @jippie I am using an Aglient 34410A Bench top multimeter now. In the pdf file it seems like the Burden voltage I should be using for my calculations is around 300mV on the 100uA scale? Page 125 cp.literature.agilent.com/litweb/pdf/34410-90001.pdf \$\endgroup\$ Jun 3, 2013 at 14:22
  • \$\begingroup\$ 30mV if I'm correct. This is the full scale burden voltage, it will be proportional with the current. \$\endgroup\$
    – jippie
    Jun 3, 2013 at 17:42

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