enter image description here


N-channel mosfet (Q4) work fine for couple of time and suddenly turn on constantly.

I changed Q4 multiple of times and each time it work for couple of time and suddenly turn on constantly.

the output of mosfet is connected to a 2.5 amper 12 volt car windshield washer pump and the reason of C11 and R33 is because of water pump noise that cause microcontroller to dont work normal.

What could be the reason of this problem ?

  • \$\begingroup\$ Is the MOSFET getting hot? What is the voltage on the gate? Slow/incomplete turn-on may be frying the device. \$\endgroup\$ Nov 17, 2020 at 16:28

1 Answer 1


You need to prevent back emfs from the washer motor destroying your MOSFET when it deactivates. The normal method is a "reverse" diode in parallel with the motor such as this (equally needed for a BJT as a MOSFET): -

enter image description here

This suggestion is made assuming that the motor is never operated in reverse - if it is then a slightly different arrangement is needed.

  • 4
    \$\begingroup\$ The internal MOSFET diode is not going to work in this configuration. \$\endgroup\$
    – Andy aka
    Nov 17, 2020 at 15:20
  • 3
    \$\begingroup\$ Well the information in my answer is beyond suspicion i.e. it is the recognized thing to do when protecting output transistors from inductive load flyback voltages so, it then throws doubt on how you have done the "fix" i.e. I am suspicious about how you implemented it. So, please take a photograph and provide a viewing link via IMGUR or a similar site. \$\endgroup\$
    – Andy aka
    Nov 17, 2020 at 16:11
  • 2
    \$\begingroup\$ Because you brought up the internal body diode, I want to point out that the diode for protection should be across the load, not the FET. (Andy drew it correctly here, I just want to make sure you notice the difference) \$\endgroup\$ Nov 17, 2020 at 18:23
  • 1
    \$\begingroup\$ @DerStrom8 it will be - the reverse recovery time causes it to remain conducting when going from rapid forward to reverse bias and that is inconsequential here. \$\endgroup\$
    – Andy aka
    Nov 17, 2020 at 18:34
  • 1
    \$\begingroup\$ Your circuit indicates no reason for Q5 to fail that I can see. Double check what you built and remember what I said about a photograph. You still need the diode by the way. \$\endgroup\$
    – Andy aka
    Nov 17, 2020 at 19:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.