I'm having problems with this annoyingly simple P-Chan FET circuit ....
With the correct input voltage it seems to work fine, but when the input polarity is reversed the FET doesn't turn off for some reason.
With the input polarity _reversed_ the FET terminal voltages are:
Drain = -24V Source = -24V Gate = -12V
So the gate is at +12V wrt the Drain/Source. Since Vgs(th) for this FET is about -2 volts it really should be OFF.
At first I thought I was measuring leakage currents on the output so I loaded the output with a few Ohms and they got hot - so the FET really is conducting.
With power removed the FET switches off.
I replaced the FET with another the same and got the same result.
The body diode is the right way around.
All the zenners are for static protection. With reverse input polarity (and the FET conducting when it should be off) D3 is toast and has been removed until I can get the FET to turn off.
The potential divider on the gate is to reduce Vgs to 12V. Max Vgs for the part is +/- 20V.
Any ideas?
Colin