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The average hobbyist should not rely on this tester because of the deficiencies listed below.  Instead he or she should buy a transistor tester that can analyze the device and tell what the pin out is and polarity, N-channel or P-channel.  Also it can tell if the device is a MOSFET or a regular power transistor – BJT.  Or maybe something else like a voltage regulator.

Deficiencies I see.  The LED current limiting resistor should be much higher than 22 ohms – a better value would be 470 ohms.

The 100 ohm gate resistor should be much higher.  I would use a 220k resistor.

This tester assumes that the MOSFETs all have the same pin out with the base and emitter in the same locations.  It can be reasonably assumed that the center pin is the collector.  But not all MOSFETs have the same emitter and base locations.  So the test should also be performed with the base and emitter pins reversed.

Also, this tester assumes that all MOSFETs are N-channel.  But there are also P-channel MOSFETs which will not test correctly with this tester.  Also depletion mode MOSFETs will not test correctly with this tester, they will test like they are shorted.

The MOSFET’s gate turns on at a voltage that depends on the type of MOSFET.  Standard MOSFETs require more than 5 volts to turn on.  Logic level mosfets require in the 2 to 4 volt range to turn on.  It would be better if the gate was connected to the center pin of a 1 megohm potentiometer instead of a switch.