I'm working on a project controlling a DC Motor with an ESP32 module. I'm rather new to mosfets and have tried a few things. But I always run into the problem of the mosfet heating up to a level I feel is not right.
Right now I'm using a module with a LR7843 mosfet and a PC817 optocoupler (LR7843 MOSFET Control Module - ProtoSupplies). I've connected power supply and motor according to the link.
I'm using a PWM pin to control the module. My power supply runs at 12 Volts. The motor is rated 12-24 Volts
My motor draws about 1 amp concurrent (at 100/255 PWM) and the module specs led me to believe that there shouldn't really be any heat uncomfortable to the touch. But when running the motor over one to two minutes, the mosfet gets hot. Not scalding hot, but still to a degree I feel like I'm missing something.
I'm hoping for some insight and whether this should work the way I'm intending. I can provide wiring details if necessary.
Not scalding is OK. Boiling is well within spec. The module's schematic, (in particular the 4.7K pullup on the optoisolator, and the relation of the pulldown resistors) looks a bit suspicious to me, but I'm no expert.
If you were switching a lower voltage, like 6V, the gate voltage on the MOSFET would be around 6*(4.7/(4.7+4.7+0.1))=2.96V which might not get you low-resistance switching. At 12V it looks OK to this non-expert.
You are on the border line of it working. Look at the data sheet: https://www.infineon.com/dgdl/irlr7843pbf.pdf?fileId=5546d462533600a40153566de53526d8, @DaveX did the math for you. If you have the capability to solder change the top resistor to 1K, this will give you a better turn on. I have used these modules and I generally modify them for this reason. Looking at the data sheet: https://www.farnell.com/datasheets/73758.pdf you will see that it drops about 1.4V across the LED, this leaves you (3.3 - 1.4) 1.9V through 1K equals 0.0014 mA through the LED.
The pulldown resistor should be increased to 47k. If the supply voltage is set higher than the max. Vgs of the MOSFET then a voltage regulator for the circuit around the MOSFET is the only sound solution.
Hey, thanks for your answer. With top resistor I assume you mean the 4.7k between the led and mosfet? I don't own these small on-chip resistor but I can try to solder a regular 1k resistor there.
Thanks for your answer. What got me suspicious are the temperatures measured in the link. They get 34 degrees C at 5 Amps and I'm drawing well below that.
Rather than risking damaging the board removing the 4.7K resistor, just solder a 1K resistor on top of the 4.7K. That will make it an 825 ohm resistor.
Sounds good, @2112 has good advice, do the same with the 1K on the LED side.
Several years ago I purchased a inexpensive assortment of 0805 resistors, they come in handy. I now purchase them by the reel when doing a project, I get several thousand for not very much.
