The fast diode has nothing at all to do with this problem.
As I said before, since it's installed backwards (reversed bias), it's invisible to the normal
operation of the circuit. It only works if you were to have back-EMF from a motor/inductor.
Guess #1.
It sounds like your comparator isn't switching. Check Pin 2 (Pot tap line) to make sure
that it's voltage is going up and down when you turn the pot CW & CCW.
If Pin voltage isn't changing, you are using the wrong pin on the pot.
If it is changing, then check the exact voltage change.
It MUST change above and below the reference voltage (6.2v) on pin 3.
If it's not changing above and below 6.2, it's not going to switch the FET.
Guess #2.
If it wasn't guess #1, then check your output wiring. The 15V zener on pin 7
is to protect the gate from high voltage (from the 1k5 pullup resistor).
If the wiring is all correct, change the chip. The LM311 might have been
zapped by static, or was bad from the box..
The grounds on Pins 1 & 4 are switched to the output pin(7) to pull down
the voltage. (Supplied by the 1k5 pullup resistor).
That 'Low' turns off the FET.
I'll be around this afternoon, let me know what you find.
Happy Holidays,
Rich
Linuxfromscratch.dk
