[SOLVED] P-Channel Mosfet woes. Can someone tell me what's wrong with this?

[MD68]

No matter what the signal from the microcontroller is, the device is always being supplied with power (always on). The "4V ??" is what I measured with a meter when the signal from the microcontroller was off and the device had been removed. Surely the voltage there should have been 8.4 not 4???
Can't use an N-Channel, device has to be on the ground rail.
Microcontroller signal is 5V

Any help appreciated.

 

[FvM]

Surely the voltage there should have been 8.4 not 4?

Yes, can't be understood from the schematic, which is basically fine. Probably defective parts or a wiring error.

As a first step, you should verify that the Q1 base-emitter voltage is actually zero, respectively < 0.3 V.

 

[MD68]

I completely removed Q1 and just left the gate pulled up to +, the device was still on. So looks like the Mosfet is defective.

 

[FvM]

So looks like the Mosfet is defective.

It might be blown if the "device" is inductive with a stored energy exceeding the MOSFET's avalanche energy capability.

 

[MD68]

The device is just a heating coil, it shouldn't have any inductance. Besides, it shouldn't have come on at all the very first time I connected the power anyway. Just tried another p-mosfet, this time with just an led and a resistor as the device. Same, led on all the time. Oddly tho' it's not as bright as it is if I connect the led+resistor directly to the power.

- - - Updated - - -

This is how I have it breadboarded to test the mosfet, now unless I'm going mad, that LED shouldn't be on, but it is. With all 4 p-mosfets I have.

Just out of curiosity, does a power mosfet like the 5305 need a particular voltage before it works properly? All the examples I have seen they use 12V or more, I've never seen a power mosfet example with less than 12.

EDIT: Source and Drain mixed up, DOH!! Sorry. Problem Solved.