Hello, thanks for taking time and possibly helping me understand some of the components in this circuit.

I am looking at adapting this circuit for 24V (which really means close to 30V).

IC1 is a PIC817 and according to the datasheet (PC817 Datasheet) it can handle up to 80V (V_{CEO}= max 80V) so that shouldn’t be a problem.

The MOSFET is IR which is robust. (IRL40B209-ND datasheet) It can switch sufficient voltage and amperage for a lot of requirements.

I believe R2 is only there to pull the gate of the MOSFET down so that the MOSFET can be switched off. The higher the ohm, the slower the MOSFET will be turned off and the less power lost when the switch is on. If speed isn’t a concern, the higher the better. If the switch is to be PWM capable, then R2 needs to be balanced to the desired switching speed. I have no idea how to calculate this.

I have no idea what R3 is there for. It looks to me that R2 and R3 form a voltage divider which effectively (since they are equal ohms) divides the voltage to be switched by half and applies that as the Maximum voltage to the gate of the MOSFET. The circuit was originally for another MOSFET and that may have been necessary. Since this MOSFET has V_{DSS} of 40V minimum, what’s the point? Can R3 simply be left out? The higher the ohm on R3, the slower the MOSFET can be turned on.

D1 is required if switching inductive loads so that back EMF doesn’t kill anything. This should be a fairly robust diode with sufficient Amp and Volt to handle the back EMF. The schematic calls for RL204 which appears to be rated at 400V and 2A. Is this sufficient for what can be expected for back EMF? (My specific requirement is non-inductive so I could leave this out but I want a circuit which can be used for future Projects which may be inductive.)

I also have no idea what D2 is there for or what it is doing. What’s worse, the schematic calls for a 1N524 which Digikey has listed at various voltages between 10V and 18V. What is the purpose for this Diode? What voltage should it be rated at if switching 12V? What voltage if switching 24V?.

I hope I have provided sufficient info that maybe someone can answer some of the questions I still have.

Thanks for whatever assistance you may be able to provide.