Handy Tips - Logic Level FET's
(P-Channel)
For a more detailed guide on
MOSFET's in general, also have a read of the N-Channel FET page.
This guide is designed to
explain how to use P-Channel FET's, as there are a couple of tricks
to remember when using them. P-Channel FET's are useful for
switching the positive supply
of a target circuit on and off. Particular attention must be placed
to the target circuit if the supply
voltage is greater
then the micro controller's logic
voltages.
If for example, your target
device is being powered by 12 volts, and your logic high state from
the micro controller is 5 volts, then the MOSFET will never turn
off, as Vgs will either be
-12V or -7V (remember that this guide is designed for logic level
MOSFET's). Anything over -3
volts will generally drive the logic level MOSFET on (varies between
types of LL MOSFETs).
The solution is easy - if you
are driving a greater voltage then the micro controllers logic high
voltage, then use a
NPN (2N3904)
transistor to control the Gate of the MOSFET, tying it to
ground with a
logic high to create a (in
this case) -12 Vgs,
and 0 Vgs
when a logic low
drives the transistor to turn off.
The reverse biased diode in
parallel with the motor should be used when ever you are driving
inductive loads, but is not required with purely resistive loads.
If your target device is
being driven from the same voltage levels as the logic high output
of the micro controller, then no driving circuit is required (note
that the MOSFET's control is inverted);
Here's the Logic Level
MOSFET that I use;
Finally, keep in mind that
FET's are very sensitive to static, so handle with care. I am yet to
damage one while "hobby-handling" though.
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