I have a microcontroller that I want to use to switch on/off a heavy load (a car horn) with a 30N06L N-channel mosfet. Everything works when running a load of 4Kohms (with the car battery at 12v), but when I move to the heavy load of the horn, then things go awry. I can hear an initial honk, but then the horn stops. I am guessing that the high current powering the horn is causing interference with the rest of the circuit somehow. Does anyone have experience with this?

Also, the reason I am going with this particular mosfet is because it is what I had laying around. If it makes more sense to go to a similar P-Channel mosfet, I can order a few.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ There's a button in the editor that looks like a circuit diagram with a pencil on it. Try clicking that to draw a better circuit diagram. The one you have might be good enough, though, I'm not sure. \$\endgroup\$ – Hearth Nov 3 '18 at 18:31

Your testing suggests that the larger current of the horn is causing problems - perhaps involving the microcontroller. The circuit might benefit by adding a flyback diode across the horn (cathode to +12V).

Since MOSfet source pin is common with microcontroller ground pin, some grounding problems can occur here:modified (crude) schematic

  • \$\begingroup\$ +1 - all the main issues nailed down. \$\endgroup\$ – Andy aka Nov 3 '18 at 18:44
  • \$\begingroup\$ So how do I isolate the source pin from the rest of the ground? \$\endgroup\$ – Chase Roberts Nov 3 '18 at 18:45
  • \$\begingroup\$ that blue wire, \$\endgroup\$ – Jasen Nov 3 '18 at 18:52
  • \$\begingroup\$ Adding the diode doesn't seem to change anything. Also, all the grounds are already connected. \$\endgroup\$ – Chase Roberts Nov 4 '18 at 1:09
  • 1
    \$\begingroup\$ @ChaseRoberts Green wire carries relatively small currents, and is involved with turning the MOSfet's gate ON/OFF. It is probably best that its route to battery "-" be only via the MOSfet source. Green should carry no horn current. Blue carries large horn current plus the smaller green current. Connecting the voltage regulator end of green directly to battery "-" can cause problems. I would try to keep the length of green short and direct. \$\endgroup\$ – glen_geek Nov 4 '18 at 3:50

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