The load is permanently switched on… I have obviously done something wrong. Or the mosfet is faulty. Can anyone please verify.
Some info on the problem…
The load is permanently switched on.
What I have diagnosed:
The optoisolator will allow current to flow when it’s switched on Ok.
The voltage across the zener diode is 0V when the optoisolator is not powered. It reads 10V when the optoisolator is powered.
Check to make sure you don’t have the fet source and drain swapped. The drain should be the negative of your bed and source should go to supply negative. If you swap them it will basically be on as long as voltage is present.
Another thing, be mindful of heat, you don’t have much in the way oh heatsinking. Take the RDson of the fet and multiple that by expected current to get power losses in watts. Then find in the Datasheet the junction to case value and multiply that by the power losses and add the ambient temperature to that. That will tell you how hot your chips internal temp will be. If it’s over 120 then expect fire/failure. A heatsink can be added, but you will have to do a little math to figure out how big it needs to be.
The Rds is 0.0017 @10V
I was confused on how to mount a heatsink. The fet doesnt have a hole to mount it to the drain tab. Does it get mounted on the top of the fet instead?
Whoops, multiply the RDson times the current squared. The voltage should not matter in this case, only the current passing from drain to source.
You can run solder traces around unused parts of the board. Solder copper foil/sheet to the tab. Those small stick on heatsink may also work.
It works perfectly!
The mosfet is even cold to the touch…