I'm working on a circuit to turn on and off a set of lights on a vehicle.

I have to do the switching on the high side of the lights and need to use a positive voltage to trigger them on. So, I've got a little NPN transistor that grounds the gate of my P-Channel FET when it gets voltage and sends current from the battery to the light.

enter image description here

It works great breadboarded out but I am concerned about protecting the MOSFET and (maybe the BJT) when it's on a vehicle. Do I need a Zener or TVS diode somewhere? Where should it go? What does it need to do? What values do I need to look for when sourcing it? I am more than happy with overkill.

This is the FET I was planning on using: SQJ459EP. It has a diode built in between the drain and the source but it might need something to protect the gate?

Thank you guys for any help you can give me!

Edit: Fixed Schematic

  • \$\begingroup\$ That circuit won't turn off a load because of the MOSFETs body diode conducting. \$\endgroup\$
    – Andy aka
    Jan 15, 2018 at 20:09
  • \$\begingroup\$ WOW my brain is having a seizure trying to understand if that works drawn that way up... \$\endgroup\$
    – Trevor_G
    Jan 15, 2018 at 20:10
  • \$\begingroup\$ Where does INPUT come from? \$\endgroup\$
    – Trevor_G
    Jan 15, 2018 at 20:11
  • 1
    \$\begingroup\$ I drew it up a little quick and I guess I flipped the Source and the Drain. For the input, it will probably be tapped into the low beam or an aux circuit that comes on with the key. \$\endgroup\$
    – AndyC
    Jan 15, 2018 at 20:17
  • \$\begingroup\$ I would use something in a DPAK or D2PAK. I don't like those fet's due to the small clearance between drain and source, hidden under the package. \$\endgroup\$
    – user57037
    Jan 16, 2018 at 0:58

2 Answers 2


For auto stuff you need TVS diodes pretty much on every input and output.


simulate this circuit – Schematic created using CircuitLab

I'd also add D1 to protect you from kickback in the lines to the lamps.

  • \$\begingroup\$ Might as well put a Zener across Vgs of M1 as well, no? Unless the part has protection built in. \$\endgroup\$
    – user57037
    Jan 15, 2018 at 22:48
  • \$\begingroup\$ @mkeith yes why not... \$\endgroup\$
    – Trevor_G
    Jan 16, 2018 at 0:08

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