I purchased a repair kit for a TV SMPS. The kit contained 3 identical N channel MOSFETs (K8A50D), among other components. I tested all 3 with a DMM following a number of identical instruction videos. The MOSFETs passed all the test for shorts. All 3 appeared to fail to take a charge from the positive DMM lead to gate and allow drain to source continuity. I find it hard to believe all 3 were bad components but can’t see what I am doing wrong. I assume the DMM is supplying sufficient voltage to meet the Gate to Source threshold. Any suggestions or comments?
An extension of Dave Tweeds comment would be to use a 9 volt battery with an LED (your choice of color) in series with a 1K ohm resistor going from battery + to the drain pin of the MOSFET under test. Gate to source voltage should range from zero to 9 volts, almost enough to saturate the MOSFET ON 100%. The LED should get brighter with increasing Vgs. That verifies you have a good MOSFET.
If your DVM shows a voltage on the gate (about the same as the drain voltage) that you did not put there (it should read zero volts) then the MOSFET has shorted internally. A resistance reading of a few ohm from gate to drain or source is also a sign of a shorted MOSFET.
I cannot stress how sensitive to static they are when exposed. Walking up to a MOSFET laying on a table and touching the gate pin first is a good way to short it out. One should touch the metal tab or drain pin first, then put fingers across all 3 leads until your connections are done.