I want to control it with the PIC16F877A PWM pins and this PIC pin connected to MOSFET with an opto-coupler (PC817)

Circuit Diagram

C1 is a 10uF electrolytic capacitor, and the motor is rated for 12V DC.

Can I use another MOSFET like the IRF3205?

  • \$\begingroup\$ If you don't bother to put any effort into writing and formatting your question, why should we bother to put any effort into answering it? Next time, please take the time to format it. \$\endgroup\$ Commented Aug 12, 2016 at 5:55
  • \$\begingroup\$ I want to help me (if this circuit is right or wrong)? \$\endgroup\$ Commented Aug 12, 2016 at 6:06
  • \$\begingroup\$ You probably need a resistor in series with the opto-isolator LED for a start, so no, the circuit is not "right" \$\endgroup\$
    – Icy
    Commented Aug 12, 2016 at 9:58
  • \$\begingroup\$ resistor in series with the opto-isolator is this the only wrong thing in this circuit \$\endgroup\$ Commented Aug 12, 2016 at 11:24
  • \$\begingroup\$ Why the opto-isolator...? I'd scrap it and drive the MOSFET direct. \$\endgroup\$
    – Majenko
    Commented Aug 12, 2016 at 12:07

1 Answer 1


Yes, substituting a IRF3205 for the original IRLZ34 should give almost similar results. You mention that you will use this driver to do PWM. There is a problem here, in that the switching of the MOSFet will likely be too slow. This is a result of the gate capacitance Ciss, and gate-to-drain capacitance Crss interacting with that large gate-to-ground resistor (10000 ohm).
This will especially be noticed where PWM repetition rates are high. Pulse rise-times will differ from pulse fall-times, usually not-what-you-want when PWM'ing. In addition, the slow rise & fall times will cause the MOSfet to run hotter than it should.
Note that IRF3205 is a bigger device than IRLZ34, and has roughly 2X Ciss than IRLZ34, so these rise & fall time problems will be worse. These problems can be helped by reducing the 10000 ohm resistor to a lower value - you'll likely have to increase opt-isolator current to compensate. MOSfet gate driver chips are designed to avoid these problems, like TC1411N, among many others.
Icy has pointed out a problem with opto-isolator IRLED drive circuit. In addition, this opto-isolator is a bit slow for PWM use, and its rise/fall times are asymmetrical: 4us/18us with a 100 ohm load resistor substituting for your 10000 ohm resistor. Normally, we expect the PWM pulse widths to be propagated to the MOSfet switch unadulterated by the pulse drive chain. I'd suggest a faster opto-isolator.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.