@c0d3z3r0 I meant in the situation where you use
SELV in the picture you made with grounded tip but without the earth connection on heater negative, and then you try to measure the heater waveform with an earth-referenced scope, which then turns that
SELV into
PELV via the alligator clip!
I don't think that works because the MOSFET and gate driver need to have common GND and the MOSFET needs GND on the 24V rail as well. Since that currently used gate driver can't be driven with 24V VCC, you have the common ground problem again. Am I missing something?
You're right, it might need a 7805 just for the VCC pin of the gate driver. I imagine the gate drive IC, the mosfet, and optocoupler transistor would all live in the 24V domain and use the heater ground for their return paths. The controller PCB pretends they don't even exist, all the main PCB sees is an LED.
it looks like the tc (and current) measurement as it is done currently, won't work in the SELV szenario because it breaks isolation as well... Isn't the OpAmp comparing the two voltages +/-, that are both referenced to GND? So we have + -> GND (RED -> BLUE), - -> GND (GREEN -> BLUE), out -> GND with common GND again...
I'm not understanding this bit. Maybe you can draw it?
Why would the diodes get hot at all? They are not inside the handle but on the control board.
I suppose if some component on the pcb pisses away heat, that would heat up the whole board, and because these are smd diodes, they all start heating up. I do agree that the BAT54S's should be replaced with a low-leakage ESD-specific diode.
I want tot also point out the in the reverse engineering of CD-2BC station, the heater AC winding negative and board ground are both labeled GND. I still don't fully understand what's the best way to ground this station with a DC supply. Let's wait to see what solution Mr Axx comes up with.