help needed in selecting bias resistor for power mosfet

• What miswiring are you attempting to protect against?
• How is this circuit supposed to do it?
• I'm lazy: is that a P-channel or an N-channel MOSFET?
• What's the source of the 24V?

If your intent is to limit the O/P current, this really won't work. Drain current varies radically with gate voltage, temperature, and between parts, so the short-circuit current won't be predictable. For a one-off, you might adjust Vg with a pot, but that's still nasty.

The best thing might be to put a small resistor in the source driving a comparator. When the voltage drop across the resistor exceeds some target value, turn off the fet.

I confess that I once made a heated microphone, using a PNP power transistor as the heater, and limited heater current with a selected base resistor, thus committing the ultimate sin of beta-based design.

John

What is a 'miswiring protection circuit' ?

Dream on !

Graham

That's a p-channel FET, and the circuit will work properly, acting as a low-resistance passthrough for the correct input polarity and as an open circuit for the wrong polarity. Provided, as you say, the gate voltage is correctly applied. Given that your 24V source may sag, and it certainly isn't 24V during startup, it's preferable to employ a zener in the gate-drive circuit, IMHO.

We prefer to think of power and current flow from left to right, source to load, like this.

.. P-channel POLARITY-PROTECTION SWITCH .. mosfet .. + (O)------- D S ---+---+---> + The FET comes on early, .. G _|_ | shorting out the substrate .. 24V power | /_ 22k diode that otherwise .. source |______|___| carries the current in .. | 12V to the desired direction. .. 3.3k zener LOAD .. | .. - (O)----------+--------------> -

The 22k resistor insures 0V on the gate during power off.

Normally a p-type FET conducts in the other direction, but this circuit works because of the principle that a power MOSFET can carry current in either direction when it's turned on.