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I'm trying to control a 130dB siren with a pic microcontroller. I figured the best way to do this was with this MOSFET since the siren pulls about 1A. I had some problems when connecting this to the Pic, so I created a simple test circuit to troubleshoot it:

enter image description here

I couldn't find a voltage regulator in the schematics program, so please assume that the +5 source is a voltage regulator connected to the same 12V source. Anyway, the problem is that when I apply power to the circuit (without applying power to the gate), the siren sounds, but not as loud as it should be. When I connect +5V to the gate, the siren turns off and then even after opening the switch it stays off (i.e. it won't make any noise). My initial thought was that I had a bad MOSFET, so I tried a different one with the same effect.

FYI, when I connect the siren directly to 12V it works fine.

Any idea why I'm seeing this behavior?

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  • \$\begingroup\$ What happens if you connect a resistor (say 10k) from gate to ground? \$\endgroup\$
    – jippie
    Commented Jul 14, 2013 at 12:23
  • \$\begingroup\$ Which alarm system are you using? I'm trying to implement an alarm system as well with a microcontroller. \$\endgroup\$
    – Pototo
    Commented Sep 4, 2014 at 4:49
  • \$\begingroup\$ @Pototo This is for a custom made alarm system. \$\endgroup\$
    – Matt Ruwe
    Commented Sep 10, 2014 at 22:13

2 Answers 2

Reset to default
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The issue is the floating gate on the MOSFET.

Add a 10 kOhm to 100 kOhm resistor between gate and ground, and the issue should be resolved.

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  • \$\begingroup\$ Perfect! That's exactly what the problem was. Thanks. \$\endgroup\$
    – Matt Ruwe
    Commented Jul 14, 2013 at 12:34
  • \$\begingroup\$ @Matt: Remember that the gate of a MOSFET acts as a capacitor; if you don't give the charge somewhere else to go then its behavior won't change. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Jul 14, 2013 at 18:00
  • \$\begingroup\$ @IgnacioVazquez-Abrams: Makes perfect sense - I didn't know that a MOSFET's gate acted that way. Thank you. \$\endgroup\$
    – Matt Ruwe
    Commented Jul 15, 2013 at 12:37
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There are a few reasons why it may not work but as already mentioned you do need a gate to source resistor to discharge the gate and turn the device off when the switch opens: -

  1. Vgs(threshold) might be a little high with the specified mosfet causing it not to turn on properly. What you can do is try the circuit with the switch but use the 12V supply for powering the gate when the switch closes. Don't forget the gate resistor though.
  2. The mosfet is wired incorrectly - check and re-check
  3. The mosfet is damaged - maybe some back-emf from the siren has broken both mosfets - you'll need to put a diode across the siren and use a fresh mosfet.
  4. You are using a different mosfet (easily done so double check)
  5. The 12V power source has become discharged.

My favourite is number 1 so check this out with the switch connected to 12V and don't forget the resistor. However, I'd also put a diode across the siren as mentioned in (3) - anode to mosfet drain, cathode to 12V and make sure it can handle the current of the siren (at least 1A but preferably 3A)

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