How to test MOSFET with multimeter
Multimeter use how to test MOSFET with Digital Multimeter and Analog Multimeter
MOSFET TESTING Mosfet working principle is input voltage control output current , we can use basic bias to test whether it is a bad or good mosfet . Mosfet has Gate Drain and source legs , take part number to find leg function from datasheet then connect test leads as photo. Note analog multimeter on this testing when set to resistor test function or Ohmmeter , the black test lead supply V+ and the red test lead supply V- ; the reason is the black test lead is connected with positive polarity of internal battery and the red test lead is connected with negative polarity of internal battery. If you are unsure about polarity of your multimeter test leads , you can check it with multimer manual or ask the multimeter expert.
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| HOW TO TEST MOSFET |
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| Connection diagram to test MOSFET with Analog Multimeter. |
Step to check MOSFET with Analog Multimeter.
1. Set analog multimeter to Rx10K as this range the multimeter will supply 9VDC from internal battery. 9VDC is high enough to bias gate leg so mosfet is working as ON state.
2. Discharge mosfet by shorting or connecting 3 leg together by using resistor's leg. then do not touch the legs.
3. Connect test leads with mosfet leg as photo above
4. Use resistor's leg to temporarily connect between G leg and D leg ; the pointer of multimeter move to indicate some resistance on ohm's scale as photo.
5. After removing resistor's leg from mosfet's leg the pointer can hold the same position.
as mosfet is still working ON state , good mosfet will work like this. but the bad mosfet can not , the pointer of multimeter can not hold the same position ( it move back to ∞ position ) after removing resistor's leg.
6. Test OFF state , use resistor's leg to temporarily connect between G leg and S leg.
the pointer of multimeter that hold the ON stage will not be ON sate anymore because - Voltage from S leg will bias mosfet to off state.
7. Summary , good mosfet can bias to ON and OFF state but bad mosfet can not bias ON and OFF state.
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| Bias Mosfet to ON state , temporarily connect G and D leg |
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| Good mosfet hold the ON state after removing resistor's leg , the pointer stay the same position but bad mosfet can not. |
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| Bias Mosfet to OFF , temporarily connect G and S leg The pointer that present the on state will move back to ∞ position ( OFF state). |
Step to check MOSFET with Digital Multimeter.
1. Take mosfet part number to find leg function from it's datasheet.
2. Discharge mosfet by shorting or connecting 3 leg together then do not touch the legs.
3. If there is a diode at D and S mosfet leg , turn rotary switch of Digital Multimeter to diode test function , some mosfet has internal diode connected between D and S leg.
Good mosfet has good diode it will display 0.5-0.8V across it when forward bias testing and display " OL " when reverse bias testing.
Bad mosfet ( shorted ) will show 000V.
Bad moset ( open ) will show OL 2 times.
4. If there is not any diode at D and S legs of mosfet
Turn rotary switch of digital multimeter to resistor test function.
testing has 2 results .
Good mosfet will show low resistance 1 time and show "OL" 1 time ( E MOSFET )
Good mosfet will show low resistance 1 time and very high resistance 1 time ( D MOSFET)
Bad mosfet will show results as
Bad mosfet (short) show 0 ohm 2 time.
Bad mosfet (Open) show OL 2 time.
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| Some part number of mosfet has diode connected between D and S leg. |
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| Example MOSFET has diode at D and S leg so the testing is as same as Diode testing. |
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| Example MOSFET has diode at D and S leg so the testing is as same as diode testing. |
then switch lead tests to do testing again.
Good mosfet will show OL 2 time because there is insulator at gate junction so it is very very high resistance that multimeter can not measure . Bad mosfet will show some resistance.
6. Check G and S leg , set digital multimeter to resistance test function.
conduct testing ; then switch leads and test again.
Good mosfet will show OL 2 time because there is insulator at gate junction so it is very very high resistance that multimeter can not measure . Bad mosfet will show some resistance.