Turn me on

[AvionicsMaster1]

I do indeed have the MOSFET in the low side of the horn circuit. Off the same power supply using it's own MOSFET I've got a rotating beacon pulling about 4 amps using an N Channel MOSFET(NTE 2376) to switch it. Neither are on at the same time and the power supply is capable of driving the horn because if the leads of the horn are touched directly to the power supply the horn makes noise. When I send 5 volts out to the MOSFET it turns the rotating beacon on for the time I have it programmed. When I use this arrangement to power the horn, as soon as 5 volts is put on the gate the horn thunks then after 3 or 4 seconds begins to turn on. After a total of 5 seconds it is fully on but the MOSFET and the 12F683 are extremely hot. In fact, this arrangement has already burned up one PIC. I have a 1k resistor going from the PIC to the gate. If I remove the horn load with a new PIC and use an LED on the IO pin to see if I get the rate I want, 1 second on 1 second off, the LED works fine. I just can't get the horn to come on without burning up something.

I then tried to use a transistor as a silicon switch and that's when I found the current trying to draw through the base.

I know this isn't really an electronics bulletin board but what I am trying to build at least uses a PIC. Thanks for any thoughts or ideas.

[mackrackit]

I do not see a fly back diode mentioned in your description.
Do you have one?

[HenrikOlsson]

Hi,
What's the powersupply voltage and what's the current draw of the horn?

I still think it's a Vg-s problem, ie the Gate to Source voltage is too low to fully turn on the MOSFET, why the PIC gets warm I don't know though - perhaps you get a dip in th epower supply voltage and the whole thing starts to oscillate or something (?).

If the powersupply is 12V then I'd try something like in the attached schematic. It has the drawback of inverting the signal but that can be taken care of if it works. Also, if the PIC is powered from the same supply as the horn (thru a regulator of course) make sure you have ample decoupling right at the powersupply pins of PIC.

The intrinsic body diode of the MOSFET is most likely enough in protection in this case.

/Henrik.