Plant manager, but I do my own small electronics at home, which is how I ended up here.
Our filler PLC has a spare 24 V digital input that we want as a «remote hold request». Plan: an operator presses a button on a Hermetry board, the command reaches a Raspberry Pi in the cabinet, the Pi drives the input, the PLC finishes the current cycle and holds. The PLC input is the usual type, it switches on when +24 V is applied against the cabinet 0 V.
What I built: GPIO17 drives a PC817 LED through 330 ohm. On the 24 V side the opto’s collector goes to +24 V through 10k, the emitter to the gate of an IRFZ44N with a 10k pulldown to 0 V. MOSFET drain to +24 V, source to the PLC input.
With the opto on I measure about 9 V on the PLC input against 0 V. The input does not switch, and the MOSFET gets warm. With the opto off it reads 0 V as expected.
Is the Pi’s ground tied to the 24 V supply’s 0 V somewhere? If so the opto isn’t isolating anything.
No, kept them separate on purpose, that’s why the opto is there.
An N-channel on the high side needs its gate several volts above its source to turn on fully. From the same 24 V rail you can’t get the gate above 24 V, so the source can only rise to whatever the gate gets minus the threshold.
Probably the least qualified person in this thread, but our electrician sorted almost the same thing in our boiler room, and Emily’s point is why yours sits at 9 V. Your 10k pull-up and 10k pulldown make the gate about 12 V, minus the threshold gives the source a bit under 10 V. The MOSFET is half on, so it gets warm.
His fix was to throw the MOSFET out. A PLC input like that only draws a few milliamps (ours was about 5 mA at 24 V, the manual says), and a PC817 transistor can switch that by itself:
Pi side: GPIO17 -- 330R -- PC817 anode (pin 1), cathode (pin 2) -- Pi GND
24 V side: +24 V -- PC817 collector (pin 4)
PC817 emitter (pin 3) -- PLC input I0.5
PLC input common -- 0 VOpto on, the input sees close to 24 V. Opto off, nothing. Isolation stays. If you want something with a DIN rail clip and a label on it, a ready-made 24 V opto or relay module does the same.
On the Hermetry side he set the hold command as High impact so whoever sends it has to confirm. A board control won’t send a high-impact command though, so it goes from the device’s Commands tab. For a hold request that stops a line, we found that was fine.
MOSFET is out and the input switches cleanly at 23.4 V, measured. Felt silly after reading David’s sketch, three parts fewer. We kept line.hold as a Basic command on a board button with Ask before running on, because the PLC itself decides when to hold, so the board is only asking. Thanks all.
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