Need more help with high side switching.

[Rexlan]

Resolved - Need more help with high side switching.

I have it working correctley now ... thanks.

I have built this circuit and it does not work. It immediately burn out R3 which is a 4 ohm 1/2 watt resistor. I replaced it with a 5 watt and it gets hot but the circuit still does not work. Where am I going wrong?

What I am trying to do is provide +12volts to a motorcycle horn that draws about 6 amps. The FET is a P-channel FQU11P06TU and it is rated at 30v - 10 amps.

Thanks ... if I have it wrong please draw it for me!

Schematic is updated

**broken link removed**

 

[MikeMl]

Your source and drain are reversed. The little diode inside the FET symbol must be reversed biased, not forward biased as you have it connected. Connected your way, the FET always conducts; the gate has no control of the current flow.

A 6A 12V horn dissipates 72 W! if you are substituting a test resistor, it should be a 12/6 = 2Ω@100W.

 

[MrAl]

Hi,

Yes i agree with Mike. Also, what is that 4 ohm resistor for? You probably dont need that at all. Perhaps a 1k resistor if that. I am assuming that the horn connects from the output to ground.
You also want to double check to make sure that your MOSFET can take 12v from gate to source without a problem.

 

[Rexlan]

OK I found the proper symbol. This is how it is connected but it does not work. I am only turning the horn on for 1 second just to beep it.

The FET is a Fairchild FQU11P06TU


Actually it is connected correctly. That is the only symbol I had in my lib file. The diode is connected to the source (not the drain) and it is reversed biased ... still does not work and instantly fries R3 ... I am stumped.

 

[Rexlan]

Got it working ... thanks again Mike.

I had it correctly wired but with my bread board hookup it was marginal. I cleaned that up and it started working!