Controlling DC motors with heater outputs



  • Hello All,

    I recently burned the mosfet of the bed heater while using it to power a bigger DC motor with PWM (1.5 A current at 12v).

    We will probably replace the mosfet or use other outputs but it would be nice to protect these ones properly.

    What would be aquadate protection for the mosfets when driving coils like motors or solinoids? Would a Diode do the trick?

    It is clear that protection is needed for PWM out put is is also needed for simple on and off outputs (like solinoids)?

    Thanks in advance for the answers!



  • @Eric Diode (or possibly TVS) is a must, as I belive heaters outputs are not protected,
    Also small cap in parallel to motor is often used to lower a little bit voltage spikes.


  • administrators

    Yes you must always use a flyback diode when connecting a brushed DC motor, solenoid or other inductive load to a heater output. Use a fast recovery silicon diode, or a Schottky diode. The rated current must be at least half the maximum current drawn by the motor. The starting current is likely to be much higher than the running current.



  • Thanks for the fast responses. So with A flyback diode the mosfet should survive?


  • administrators

    @Eric said in Controlling DC motors with heater outputs:

    Thanks for the fast responses. So with A flyback diode the mosfet should survive?

    Yes, What kills the mosfet is avalanche current caused by the the inductive back EMF from the inductive load exceeding the rated voltage of the mosfet. The mosfet can tolerate a small amount of avalanche energy, but your DC motor is likely to generate more than that.


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