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> I have a simple question regarding MOSFETs

No, you don't :-).
But a simple answer will suffice for now.

>what is an appropriate MOSFET driver for controlling it with
> a PIC? I want to use an IRF510 to PWM some (relatively) small loads
> (fans and similar stuff). I was thinking about a simple BC847 
> driving
> the gate to +12V, but I'm not sure what is really needed (the 
> +5V-ish
> that the PIC spits out is too low for gate voltage I presume, since 
> it
> is quite close to the threshold and will probably increase the
> resistance). Do I need a resitor on the gate? How should I protect 
> the
> PIC from MOSFET failures?

For low FET loads or low speed PWM a simple driver is OK.
Using a "logic FET" which is fully on (enhanced) at Vg=5V will allow 
direct PIC drive.
Use a small resistor (say 100r PIC to gate and add a zener gate to 
ground. MAJOR blowups may destroy your PIC.

For crisp FET performance with PWM or other signals above say a few 
kHz you MUST insert and remove the FET gate charge rapidly. This 
requires current peaks of hundreds of milliamps to a few amps. Overall 
current is low (under 10 mA) but peak current is far beyond what a PIC 
pin will provide. Result is very slow switching.

The attached circuit is cheap, simple and works superbly.
It is one of the circuits which you will use forever once you discover 
it.
Add a capacitor across R5 to improve high speed performance (try 1 nF 
to start).

The circuit can be "improved" by replacing D1 with a PNP transistor 
base to base and emitter to emitter with Q8 and collector grounded BUT 
this is seldom needed.

Funny part numbers are due to this being an extract from another 
circuit.
I use Q8 = BC337 but any NPN with good beta (current gain) and at 
least a 500 mA rating will do.
R6 1k - 10k typically - use lower values in range for lower drive 
voltages.
Z2 just greater than gate drive voltage - do not leave it out.
R4 say 10r. Affects drive current peaks so don't make too high. 
Observe effect on switching edges on scope.
D1 SHOULD be high current, high enough speed but 1N4148 often does 
remarkably well.

This circuit is very ancient and extremely good at what it does for 
its low cost and complexity.
As a bonus it provides some protection for the PIC against major 
blowups. If you are driving a high voltage then you may wish to 
analyse possible failure modes. A reasonable zener at Z2 will usually 
see a FET fail drain-source short with gate open under catastrophic 
failure, which is good for the PIC.

Note: Remember that an inductive load will require a flyback diode OR 
some other way of controlling inductive spike at turn off.




        Russell McMahon




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