I was trying to help my friend fix an issue with his car not getting a signal to the coil (obviously this isn't the correct fix, but it has had the car running previously).

The circuit is driving the trigger of a car's coil pack, is driven by the distributor with around 12VDC.

enter image description here

R1 is 3.3k

R2 is also 3.3k

PC817 optoisolator



simulate this circuit – Schematic created using CircuitLab

12v is enough for more than 30-50ma that the trigger pulls from datasheet

MOSFET failed with 1300 ohms gate to drain, 1100 gate to source.

Bench testing with 11.6v without the mosfet when opto is off, gate has 11.6v, on, 0.24v, seems to be working fine.

I'll spare you the details of when the mosfet is installed since it's failed.

He doesn't have an oscilloscope to test for voltages and claims the diode in the mosfet should be sufficient, what other options do we have to protect the mosfet and improve the circuit?

  • 1
    \$\begingroup\$ yes, R2 was 3.3k. \$\endgroup\$ Commented Aug 31, 2020 at 21:26
  • 1
    \$\begingroup\$ Ad a schematic. There's a drawing tool button right there in the question editor. \$\endgroup\$
    – user16324
    Commented Aug 31, 2020 at 21:39
  • \$\begingroup\$ What brand/model is the car? \$\endgroup\$ Commented Aug 31, 2020 at 21:52
  • \$\begingroup\$ Schematic added there was no optoisolator so those are just the led and transistor next to each other. Car is a 1993 Audi 80. \$\endgroup\$ Commented Aug 31, 2020 at 21:55
  • \$\begingroup\$ In schematic, where are 1300 ohms gate to drain, 1100 gate to source? \$\endgroup\$ Commented Aug 31, 2020 at 22:22

2 Answers 2


M1 could blow up if the Gate Source voltage is too much. Autoelectric systems are full of spikes. Place a zener between gate source of M1 to clamp the gate below the gate source rating. For example if the Vgs rating is 20VDC then a 18Volt zener would suffice. Load inductance is always present. This causes drain voltage spikes at turnoff which could exceed the avalanche rating of M1. Use a freewheel diode or snubber or clamp to adress this. If your load had a short M1 would blow up. The high prospective fault currents would exceed the current rating of M1. Normal fuses are too slow to protect mosfets. Consider some form of current limit to protect M1.


Check on the secondary side for loose connections, corrosion, bad plug wire, etc. The secondary voltage driving the spark plug is reflected back to the primary when the MOSFET turns off, and a poor connection or open circuit on the secondary side could cause a voltage spike exceeding the rating of the MOSFET.


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