I am having an issue with my circuit. Specifically, with the FQP30N06L MOSFETs failing after only a couple of runs (a run being powering the circuit).

I'm using a 24V DC power supply and originally had a 3V Arduino running the program, but I switched it to a 5V in the hopes that this would solve the issue (it did not). Each MOSFET is connected to a KVLA105-5P 1" x 4" heater element, that's supposed to turn off once it reaches a set temperature.

What tends to happen is either it won't stop or it won't heat up at all (I've had both happen a number of times, right now it's not heating up). I checked the pins on the Arduino and they're behaving normally (high when heating, low otherwise). If it's heating but not stopping, the voltage across the heater is around 24V and if it's not heating at all, the voltage is zero. It does not get hot in either scenario.

I've replaced the MOSFETs a dozen times at this point and it always solves the problem, but I don't understand why they keep failing. Why do they keep failing?

I've updated the schematic as well as added images of the board. In the original schematic I had the gate grounded and the source hooked up to the Arduino. This configuration did not work as in there was no measurable difference between having the transistor or removing it entirely. Troubleshooting-wise I've replaced the 3.3V Arduino with a 5V one and added a level shifter (the screen runs on 3.3V logic), I've tried switching the power supply (have used both a wall connection via power jack and a standard external DC power supply) from 12V to 24V. I have tried adding each component one at a time and everything worked until I moved it to the system (I was testing using a different set of heaters but even when I move it back to those it no longer works) Right now, it is not heating at all and both of the leads to the heater are measuring 24V.


board bottom

board top [board with components][6] board zoomed in

  • 2
    \$\begingroup\$ You might be burning out your transistors especially if 1: they're not being turned on enough by 5V or 2: you aren't getting rid of the waste heat. The 32A rating probably assumes Tj of 25C which is impossible unless you have it on some sort of liquid-cooled cold plate. Edit: what's the 100k resistor between the emitter and ground? \$\endgroup\$
    – vir
    Jul 13, 2021 at 17:30
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    \$\begingroup\$ Why are the gates grounded? What are R5 and R7 for? \$\endgroup\$ Jul 13, 2021 at 17:34
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    \$\begingroup\$ Wait, I see it now. You need to connect the Arduino to the gate and the source to ground. \$\endgroup\$
    – vir
    Jul 13, 2021 at 17:35
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    \$\begingroup\$ @LilMamiChula are you saying that the schematic you've posted in your question is not actually what you've built? \$\endgroup\$
    – brhans
    Jul 13, 2021 at 18:57
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    \$\begingroup\$ @LilMamiChula - Hi, (a) Can you Edit your question to include a few photos of your actual PCB / protoboard / whatever, with at least 1 close-up of the MOSFETs and at least 1 wider photo of the whole board, please? (b) You said that "it didn't work at all in the original configuration". What does that mean? Knowing more of your history with troubleshooting could be useful. (c) Can you replace the now "known to be wrong" schematic with a "known to be the same as your hardware" version of the schematic? It's tough on readers to expect them to always fix wrong schematics in their heads. Thanks. \$\endgroup\$
    – SamGibson
    Jul 13, 2021 at 19:21

1 Answer 1


What is the purpose of R7 and R5? I think they are causing your problem.

MOSFETs will dissipate more energy if they are only partially turned on. And the amount they are turned on/off is determined by the gate-source voltage.

By placing resistors between the source and ground you make it harder for your gate signal to fully turn on the MOSFET. Current that flows through the MOSFET will lift the voltage of the source and thus decrease V_GS. The smaller V_GS will cause the MOSFET to dissipate more heat and can lead to failure of the device.

Usually this is the way you use a MOSFET:


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ R5 and R7 are 100k resistors should I still include them in the circuit just elsewhere? \$\endgroup\$ Jul 13, 2021 at 17:59
  • \$\begingroup\$ What are they used for? In any case the MOSFET source pins should be grounded. \$\endgroup\$
    – ErikR
    Jul 13, 2021 at 18:01
  • \$\begingroup\$ just a current limiting resistor to protect the arduino \$\endgroup\$ Jul 13, 2021 at 18:08
  • \$\begingroup\$ Where are you proposing to put the resistors? A small resistor between the GPIO pin and the gate (like 100 ohms) is often recommended but often omitted without any ill-effects -- as is the pulldown indicated in my answer. \$\endgroup\$
    – ErikR
    Jul 13, 2021 at 19:38
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    \$\begingroup\$ You should have the pull-down resistor - it prevents the MOSFET from turning on when the MCU is initializing or not turned on itself. Note that the pull-down is between the gate and ground, not the source and ground. \$\endgroup\$
    – ErikR
    Jul 13, 2021 at 20:28

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