@VolksTrieb It’s an overkill because it’s unnecessary and from an engineering point of view, a waste of resources.
didnt look up the specs for that specific fet btw…
According to this datasheet you need to connect B via pullup resistor to 5V and use the collector (maybe with pullup on the gate of the mosfet). So its inverting. think about that.
@VolksTrieb If the controller board is sending an inverted signal (high should turn bed off), then, yes, the circuit should be changed, so the optocoupler pulls down the gate, rather than up.
@Gato1 yeah thats not the prob. most firmwares can be configured for inverted outputs… He just did no connect the base according to his pics
@VolksTrieb The base can be left open, it’s a BJT, not a FET.
This is the circuit I built for my printer (I didn’t design it, I just followed the schematic) http://forum.velleman.eu/viewtopic.php?f=53&t=9598&start=15#p38706 with the addition of 2 LED’s, one to tell me when the 24v incoming power was on, and another to tell me when the outgoing 24v was on (K8200 beds don’t have built in LED’s like most reprap boards)
It was the drain and source wired up wrongly…
It works perfectly now!
The Mosfet is even cold to the touch!
I highly recommend this Vishay SQM120N04 Mosfet if anyone else needs to run a 20A heated bed.
Thank you all for your help.
@Alex_Hayden , the zener is connected in reverse bias, as to have it’s zener voltage pass through. I get a stable reading of 10.1V on the zener terminals…
@Panayiotis_Savva I’m glad to know it worked