Hi, I’m really new and asking for help in wiring up the big MOSFET for the first time. I’ve connected 36VDC power through VIN and GND, but I’m not getting basic functionality with the switch. The LED light stays on no matter whether the onboard switch is ON or OFF, and I’m also reading constant 36V through VOUT no matter what state I put it in either. No other pins are connected, but I noticed that the SW pulls high to 36V when the switch is off.
My ultimate goal is to do PWM control to vary a 36V load.
Is the board fried?
If the LED is staying on regardless of the switch position, then that does make it seem like the board has somehow been damaged. If you post some pictures of your board showing all of your connections along with more specific details about how you were using it, then I might be able to help you identify what happened.
What specific power supply are you using with the switch? What load was connected to VOUT, and did you try to actuate the switch with anything other than the built-in switch?
By the way, while I’m happy to help you troubleshoot what happened to your board, please keep in mind that the Pololu Power Switches might not be good parts for your ultimate application since they are not specifically intended to accept PWM input. What PWM frequency do you plan to use?
- Patrick
Thanks for the quick feedback! I think the most likely case is that it’s damaged - there was a small short between Vin and Vout and I was hoping it didn’t damage the devices. I’ve ordered another one anyways as a backup.
Just to follow through with this anyways, I’m using 36VDC in to power a heating element (~400 Ohm). I’ve been having the onboard switch set to off and sending a signal (50 Hz, 24 VDC) through the ON pin. I believe this is the intended usage in order to control the board on and off. The PWM signal was chosen in order to vary the effective voltage supplied to the heater and consequently try to control the power output of the heater as well.
I’d still like to try to make this power switch work for my application, but are there other schemes that you might recommend to do variable power output on a simple circuit like this?
Thanks for your help!
For a low frequency PWM like that I think the power switch should be okay.
A motor driver could also work for this application, though its bidirectional capability would be overkill for your requirements. A setup with a single MOSFET would also work, though you’d probably need to figure out a few other parts (e.g. flyback diode, pull-up resistor) to make that work. This MOSFET as a Switch tutorial might be helpful if you want to go that route.
- Patrick
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Thank you for the kind support. I replaced the MOSFET and I wired up the above schematic with a 50% duty cycle control input through the ON pin. I expected to see a 50% duty 36VDC output signal from the VOUT pin - a square wave similar to the control signal I input. Instead, I’m seeing closer to 96% duty through an oscilloscope. Is this possibly due to internal capacitance in my board, and is this just the wrong design approach? Thanks for any help
We looked into this more carefully today testing conditions similar to yours with a unit here and saw similar behavior (though not quite as extreme). A 50 Hz PWM frequency is definitely too high for these switches, and any frequency over a few Hz is probably not going to be practical for anyone trying to use the switch for something like this. (For reference, in our tests a 20 Hz signal at 50% duty cycle resulted in a output voltage duty cycle around 60%, and it continues to increase further with the frequency from there.)
So, unfortunately the power switch is just not a good part choice for this. I recommend one of the alternative approaches from my previous post instead.
- Patrick
