# MOSFET checks if still usable

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#### rgb83

##### Junior Member level 1
Hi guys,

I've this n channel MOSFET (P3NA80FI) that I desoldered from another board.

I've already done the following checks divided by continuity and diode mode of the multimeter.

- Continuity mode

1) G-D =97.6$\Omega$
2) G-S = shorted
3) D-S = 97.6$\Omega$

- Diode mode

1) D(+) S(-) =0.060V
2) S(+) D(-)=0.062V
3) G(+) D(-) = 0.062V

What it seems strange is that I don't receive an open load in the case 1) of diode mode due to the intrinsic body diode. Perhaps the MOS has the Cgs still charged, but I also short circuited these pins!

#### Vbase

##### Full Member level 6
G-S are shorted because the voltage at the grid exceeded the max (normally 20V) and damaged the device. It could also be high voltage at the drain, that normally cracks the plastic part of the device.

#### rgb83

##### Junior Member level 1
So, are you sure the MOS is broken? I heard main cause of failure is a short between G and D but with this I read 97.6 Ohm.

#### Vbase

##### Full Member level 6
The main cause is lightening or transient on the mains. The high voltage at the drain transfers the spike to the gate via the drain gate internal capacitance. The gate insulation breaks at voltage not much higher than 20V and the device is destroyed.

#### SunnySkyguy

This looks consistent with a fused G-S with diffusion metal oxide now bridging the D-S with ~100 Ω

I would guess over stress on Vgs or Igs.

0.6mA CC in diode test mode yields IR= 60mV

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