Yeah, you can (and should) lose the pot as long as R2 is adequate to limit current through the transistor.

I'd lose the pot and increase R2 to at least 2k2 - maybe 4k7.

 

Going to give it a try.

 

Great news. used an IRFP064n, replaced pot with 100 ohm (R1) just for the sake of protecting the opto from the high inrush to the mosfet capacitor (probably not necessary). R2 4.7k ohms.
Vour = Vin*(R2/(R2+R1) = 12v*.97 = 11.7 volts on the gate.
Kept the 660 ohms on the opto 24 input.
24/660=.036 amps should be enough to drive the opto led (4n25) completely on?
Circuit seems to be working like a charm. drawing 6.5 amps, held my finger on it for about 30 seconds, could hardly tell it was slightly warm. Switching seemed fine as well.
Many thanks again for sharing information.
Larry

 

Don't forget to check the power in the 10K pot.

 

Yes! That's one of the great features of a MOSFET, that no current is required to hold it on.

A downside is the current required to switch the MOSFET at very high frequencies. (They have an inductance as well as capacitance.) I don't know where the crossover occurs, but at a high enough frequency the current needed to switch a MOSFET will exceed the amount needed to switch a BJT.

That's one of the things I look for when testing MOSFETS among harvested components - Using the voltage present at the prods of a continuity tester; I charge the gate capacitance, then with the same continuity tester I check that the D/S channel is conducting - if the gate charge leaks away during this test, the MOSFET is regarded as suspect.

Next; I use the tester to charge the gate capacitance in the reverse direction and verify no leakage through the D/S channel.

 

The charge on the gate capacitor is enough to hold it open? Fully?
Not sure what was meant by the 10k pot, I removed it from the circuit.

 

The charge on the gate capacitor is enough to hold it open? Fully?

No, this is wrong. It will leak away slowly, never heard of anyone using Ian's method for checking mosfets. If your circuit is working as you now have it configured, I wouldn't mess with it. Just use it and enjoy it.

 

Ok. I'll go with that.
Thanks.

 

Next; I use the tester to charge the gate capacitance in the reverse direction and verify no leakage through the D/S channel.

MOSFETs often have a body diode that will conduct from S to D, so you need to be careful of polarity when you look for leakage that way.

 

No, this is wrong. It will leak away slowly, never heard of anyone using Ian's method for checking mosfets. If your circuit is working as you now have it configured, I wouldn't mess with it. Just use it and enjoy it.

If the open circuit gate charge leaks away - the gate oxide layer has most likely suffered ESD damage.

 

Now that I have a circuit that works I would like to ask a question regarding an efficient way to regulate the voltage to drive the IC (atmel attiny 26L) that will drive the circuit. The source is a 12-14 vdc battery. Is it ok to discuss this here or do I have to start another thread? Thanks.