About Optoisolator with MOSFETs for DC Motor PWM Control

Hello everyone, I hope this is the right place for the topic.

I need your support on one issue, I have no background in electronics tbh.

I want to speed control 24V 5A dc motor(inrush current 10A). As far as I could see, I can simple make a Power MOSFET design. But I have seen some topics, in which optoisolators are added to the circuitry.

Does it mean, if I don't use optoisolator, Arduino, or MOSFET won't be safe?

The opto-isolator will help if the MOSFET burns-up and you get a drain-to-gate short, putting 24V on the Arduino, potentially frying the Arduino. If the MOSFET is not overheating and doesn't fail, you don't need the opto-isolator.

Do you have a schematic for the circuit you're building?

Note that most opto-isolators will invert (depending on how the input/LED side is wired) so you may have to invert the logic in your software.

[u]Here is a MOSFET driver[/u] circuit without the opto-isolator.

Safety will determined by the 24V power supply and the Arduino power supply.

If your Arduino power is derived from the 24V power supply then a optoisolator is not required. The MosFet can be protected by other means.

If they are from separate power sources and it is unwise to connect the power supply grounds together then an optoisolator is the ideal solution.

The next question is what PWM frequency do you plan. If it is relative low then a simple resistor from the Mosfet gate to the Arduino will protect the Arduino. Low being maybe 5k or maybe 10k Hz.

If you are in the 18K Hz and up range then you will likely need a driver of some sort to insure the Mosfet switches fast enough to not overheat. Many controllers are in the 18K+ range simply to insure there is no audible noise. If this is not a problem in your case a lower PWM frequency is likely fine.


I've attached my basic circuit, only IRL540 is the actual element here,which seems to fit my purposes I guess. I still don't know how to decide the resistance valus of R1, R2 and the type of Optoisolator.

@JohnRob ,

Arduino will have a dedicated power supply 9V. 24V is to power motor only. You can also check my schematic and see if it's viable.
I thought Arduino's PWM pin has a fixed frequency of 490hz. How would that work in this schematic?

Thank you very much for your answers. I am still trying to understand very very basic things :slight_smile:

This will be better, keep the load out of the gate - source circuit.

Tom.. :slight_smile:

Hi Tom,

Thanks a lot for help.

I ordered the parts. I have something like this at hand in the link below, for the same purpose, would I still need a schottky diode between poles?


If you are going to use that controller, you will not need the diode across the motor.

In fact remove it, as the controller will have the necessary protection, and as it can reverse your motor, the diode has to be removed.

Tom... :slight_smile:

Re the circuit in reply #4..

1/ 24 volts can be applied to the gate of the MOSFET - will it stand it?

2/ there is no pulldown resistor on the gate , so the MOSFET will turn off slowly, if at all.


15V zener gate to source and 10K gate to source.

Good to see someone is awake.

Tom.. :slight_smile:

This will be better, keep the load out of the gate - source circuit.

Tom.. :slight_smile:

That circuit clearly cannot work with a MOSFET, which need push-pull gate drive.

That circuit clearly cannot work with a MOSFET, which need push-pull gate drive.

This particular part doesn't meet OP's voltage and current requirements, but what do you think of something like this for this application?

"IXYS Integrated Circuits Division (ICD) announces the immediate availability of the IX3180 2.5 A Gate Drive Optocoupler....The optocoupler input is a light emitting diode which is then optically coupled to an integrated circuit containing a photo diode and high-speed, high current, push-pull power output."

Looks like a handy device... and 100nS is quick enough for most applications.

I'll bear it in mind.


That circuit clearly cannot work with a MOSFET, which need push-pull gate drive.

Hello Mark,
Been a long time since last reply. I just got myself a driver from aliexpress.
But I have been wondering, why that circuit would not work without a push pull gate driver?