I'm designing an output with a MOSFET transistor to control loads such as relays, motors or incandescent lamps, similar to the outputs of a PLC.

Obviously, there is no problem as long as the load doesn't exceed the maximum current that the CMOS can handle. But there is the possibility that the user connects a load that the current to be circulated is greater than the allowed and that is where the fuse would act as in the following schematic:


simulate this circuit – Schematic created using CircuitLab

I think there is no problem with this design (obviously the fuse must be of some special character for inductive loads) but it is possible that something is not considering or it is possible that there is a better solution.

Any comments or suggestions are welcome

  • 2
    \$\begingroup\$ There is always a race between your fuse blowing and your load (or FET) blowing. If the load and FET are robust the fuse is likely to go first if not (e.g. a speaker or a small FET) the race is on.... Electronic over current detection and shutdown is much faster and safer. \$\endgroup\$
    – Oldfart
    Nov 3 '19 at 14:31
  • 3
    \$\begingroup\$ Fuses don't protect components. Fuses protect wiring and prevent fires. \$\endgroup\$
    – Dave Tweed
    Nov 3 '19 at 15:15

Your scheme will work for limiting the overall power dissipated by the FET. The problem is that the current can rise very quickly and damage the FET.

I have successfully used a small value resistor in series with the FET to limit the fast spike of current during a fault until the fuse (or breaker in my case) has time to blow. This could also be used as the basis for detection for an over-current shutdown circuit.

The resistor should be sized to limit the short circuit current to the peak non-repetitive surge current specified for your transistor.


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