Opamp Transistor Driver Assistance

The LM324 opamp will probably be plenty stable in this application. The big problem to solve is, if you want speed control, you want to measure speed, not something only approximately related to speed, like motor voltage, or even motor voltage compensated for by adding a motor current factor to it. Do you have a linear, analog speed measurement signal available for the opamp to force into a match with a speed setpoint value you provide, in a closed loop motor voltage controller? If so, the DAC will be needed only to produce the setpoint value.

I just tried a really simple circuit of just "DAC" to the gate of the IRF 510. As long as I can get 5 stable "settings" I think I can be ahppy with that.

Perhaps speeed control was too strong of a term.

This circuit is going to be in airconditioned cabinet so the IRF510 is going to heat up to whatever it heats up in controlled ambient enviroment. I guess I will use pots for each setting so they can be "calibrated" when needed.

Thanks!

Ed V.

It sounds like most of your problem of stability can be solved with a large enough heat sink.

What is it? Every doofus thinks that simple voltage or current control is applicable to DC "brushless" motors. It just does not even apply (unless there has been interesting changes in the underlying technology that i have not heard of). The self adjusting computer fans have a relatively complex IC to drive them. If you just gotta have a easy to control variable speed motor or fan get a RC hobbyist variable speed motor and controller. they can even be purchased as ducted fans (see "jet" engines).