Mosfet Logic Control

Hello,

I'm working with logic control which consists of 2 PCBs. The left side of the circuit is on PCB1 and right side on PCB2. The problem I have is D31 is always on no matter what the input to Q15 is thus the output from pin 16 on U8-1 is always low. I've checked that the voltage @ gate of Q25 does change from 5V to 0V when the gate from Q15 is logic high and it goes back to 5V when gate of Q15 is high. I'm not sure why it's not switching as I expected. Any ideas?

Tie the pull up at the output of U40-3 to 24 volts so the gate of Q25 can then go to 24 volts for it to turn off. If you can't do that because 24 is not available you will need to change Q25 to a NFET and take the output of U40-3 from the emitter.

ronv said:

Tie the pull up at the output of U40-3 to 24 volts so the gate of Q25 can then go to 24 volts for it to turn off. If you can't do that because 24 is not available you will need to change Q25 to a NFET and take the output of U40-3 from the emitter.

Click to expand...

Yes you need an N-MOSFET.
And move R208 to the emitter and ground of U40-3 with its collector connected directly to +5V_ISO when connecting the emitter to the gate of Q25.

For the FETs, leakage current can turn them on. You have to swamp out the gate leakage current with a resistor. i.e.Q25 could turn on because of the leakage of Q40.
Then your 10 M ohm meter might swamp it out, so you can't measure it either, but the circuit may work with the meter attached,

Part of the problem with mixing bipolar and MOS devices.

KeepItSimpleStupid said:

For the FETs, leakage current can turn them on. You have to swamp out the gate leakage current with a resistor. i.e.Q25 could turn on because of the leakage of Q40.
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Click to expand...

R208 will swamp any leakage current.

If Q25 was N-channel it will work! (reverse S and D)!! Use a BSS123.
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The BSS84 needs its gate voltage applied from G to S. You have it applied from G to D.
The way you have things wired:
>If Q25-G is at 0 volts then the Q25-S=2V. The D31= on.
>If Q25-G is at 5 volts then the Q25-S=7V. The D31=on.
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