The following is the power control part of the circuit I use in my car to control the wing mirror folding motors, the idea is that the whole circuit can power itself off after its done its job to prevent even minimum battery drain. (full circuit here)

enter image description here

The power goes through the P-channel power MOSFET and there's an N-channel signal MOSFET to drive the power MOSFET's gate low. The signal MOSFET's gate has a pull-down resistor to keep it off and may be driven on either from a 5V arduino signal or a 12V car signal, through diodes.

The N-channel MOSFET is failing every few days or weeks. I have tried using a 2n7000 small signal MOSFET and I have also tried the way overkill RFP50N06 power MOSFET in its place but both eventually failed. In both cases I believe I have stayed well within the maximum absolute ratings from their specs.

This is my first direct use of MOSFETs but as far as I've checked a MOSFET will burn mostly if the gate is kept on at a voltage below the Gate-Source Threshold voltage because part of the Drain-Source current dissipates at the MOSFET. In this case the threshold voltages are 2-4V and the gate is driven at 5V to 12V minus the diode's voltage drop, so it should be fine. In any case the Drain-Source current should be so small it shouldn't even matter.

What else could be the problem?

(I have also once tried a BJT NPN transistor in this place and I was surprised it didn't work, haven't investigated further)

  • 3
    \$\begingroup\$ are you aware of what voltages in-car equipment has to withstand? -12v and +24v indefinitely (wrongly connected jump-start) and +100v for a moment (alternator load dump transient), quite apart from any transients your mirror motor and any excess wiring is creating. \$\endgroup\$
    – Neil_UK
    Commented Apr 5, 2018 at 6:04
  • \$\begingroup\$ Thanks, I didn't know wrongly connected battery was considered survivable. I think in that case the N-channel FET's internal diode would only be letting a small current through, other than that both MOSFET should stay in the off state so it should be fine... In normal operation the circuit should withstand 18V, above that if it dies and doesn't cause a short circuit I'm happy. If the P-channel MOSFET dies and does short circuit then the 10A car fuse should blow. \$\endgroup\$
    – balrog
    Commented Apr 5, 2018 at 9:15
  • \$\begingroup\$ When I say 'has to survive', I mean mass market stuff has to survive. With millions of cars out there and 100,000s of garages staffed by, well, let's say not you and me, enough batteries will get wrongly connected to kill a lot of cars, and get their manufacturers a bad name for being 'fragile'. Negative rail is easily handled by a series supply diode anyway. It's the positive transients you need to watch. \$\endgroup\$
    – Neil_UK
    Commented Apr 5, 2018 at 11:21

1 Answer 1


You probably have long wires going to the Nfet Gate diodes or ground and Vgs is probably being exceeded. (use 15Vmax? (rated 20V) Put a clamp on that gate.

Inductive spike fault.

Assuming my theory is correct, better idea. add 0.01 uF cap across NchFET D-S, G-S; as a long wire inductive loop damper. If logic or dry contact switch rise time will be fast enough but dampen.

  • \$\begingroup\$ Well, let's see if it holds up now. I added a voltage divider so that all the signals connected to the mosfet gate should be about 5-6V and added a 12V zener diode across the 100k resistor (didn't have 15V ones.) But I'm not sure a simple 15V zener diode would be a good idea. As far as I understand if the battery voltage was 16V continuously, the diode would be continously shorting ACC to ground for example... and I did run a previous car at 16V for a few minutes when starting with an RC hobby 4-cell LiPo battery. \$\endgroup\$
    – balrog
    Commented Apr 6, 2018 at 13:15
  • \$\begingroup\$ or if my theory is correct about long wires. put snubber cap 10~100 nF cap across 100K to GND \$\endgroup\$ Commented Apr 6, 2018 at 13:53

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