N-Channel MOSFET Testing with Analog Multimeter

The right way of testing an N-Channel MOSFET transistor is to use an Analog Multimeter. First, find out the Gate, Drain and Source from semiconductor replacement book or search its datasheet from search engine.

Once you have the cross reference or diagram for each pin of the MOSFET and your Analogue Multimeter, follow the below instructions:-

1. Set to times 10K ohm range to check it.
2.  Put the Black Probe to the Drain pin.
3.   Touch the Gate pin with the Red Probe to discharge any internal capacitance in the MOSFET.
4. Now move the Red Probe to Source pin while the Black Probe still touching the Drain pin.
5. Use your finger to touch the Gate and Drain pin together. You will notice the Analogue Multimeter pointer will move forward of the meter's scale.

 How to cross-check?

Lifting the Red Probe from the Source pin and putting it back again to the Source pin and the pointer will still remain at the middle of the meter's scale. To discharge it you have to lift the Red Probe and touch just one time on the Gate pin. This will eventually discharge the internal capacitance again.

At this time, use the Red Probe to touch on the Source pin again, the pointer would not kick at all because you have already discharge it by touching the gate pin. 

These are the good MOSFET characteristic. 

If you notice that all the result that you measured kicked towards zero ohms and will not discharge, then the FET is considered shorted and need replacement. Testing the P channel MOSFET is just the same way as when you check N channel MOSFET. What you do is to switch the probe polarity when checking the P channel. Some Analog Multimeter have the times 100k Ohm range, this type of meter can’t really test FET due to the absent of 9 Volt battery inside the Multimeter. This type of meter will not have enough power to trigger the MOSFET. Make sure you use a meter that have the times 10k ohm range selector.