Electronics > Beginners

FET circuit not working as expected.

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Clear as mud:
I have an on-off switch in a recent circuit design.  The actual electro-mechanical switch is only rated for 100 mA, so I used a P-channel MOSFET to switch a higher amount of load current.  I'm attaching a schematic of the switching arrangement.  +12V is supplied from a "wall wart" type of supply, VCC goes to the input of a 5V and 3.3 volt regulator (L78S05CV and LD1086 linear regulator ICs, respectively).  The switch applies either +12V or GND to the gate of the P-channel MOSFET, a IRF9388PbF.  Data sheet: http://www.irf.com/product-info/datasheets/data/irf9388pbf.pdf

The problem is that the MOSFET remains always on.  I have not connected any load to the circuit except for a power indicator LED that is connected to the output of the 5V regulator.  The LED remains lit regardless of the position of the switch.  I measured voltages and everything else seems fine.  The gate voltage on pin 4 of the IRF9388PbF changes as expected, but the drain and source remain connected.  Someone told me Drain-Source shorted is a common failure mode, so I tried taking the MOSFET off the board and installing another one, but it did the same thing.  Am I not being careful enough to avoid damaging the FETs, or is there an error in my design?

kxenos:
I think you have to switch source with drain for it to work as you want it

ConKbot:
I'm sure the FET is switching just fine, but your body diode isnt ;)  swap your drain and source, and it should all be fine.

EDIT:
if you add a diode symbol to your schematic symbol for the mosfet, it makes it a lot easier to double check for mosfet polarity correctness.

Clear as mud:
Well, darn it.  I guess that's why I posted this question in the Beginners' section.  I know I checked the polarity before I laid out the board, but I must have looked at a wrong example or gotten it confused somehow.

sleemanj:
Standard mosfet symbols that don't show the body diode should be eradicated if you ask me.  I ALWAYS get the polarity of Source and Drain mixed up, if the body diode is on the symbol though it's plainly obvious "which way is up"!

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