3HC Pulldown Resistor Size

[p8blr]

I'm using some 24V pressure sensors on my printer that require pulldown resistors. I'm using 10K resistors, so the IO_IN pins are sinking 0.0024A, is that ok? They seem to be working but intermittently they sometimes read incorrectly. I don't see anywhere in the documentation how much current the IOs can tolerate, other than a voltage spec.

Thanks.

[dc42]

@p8blr you didn't specify what Duet you are using. Most pullup resistors on Duet 3 boards are 27K to +3.3V, so a 10K pulldown would be marginal.

Our recommendation:

• When using 12V or 24V sensors, use NPN-output sensors in preference to PNP-output sensors. NPN-output sensors do not need any resistors added.
• If you must use a PNP-output sensor, use 2 resistors so as to both reduce the voltage and to provide a low-value pulldown to the Duet input without excessive power dissipation. See https://docs.duet3d.com/en/User_manual/Connecting_hardware/Z_probe_connecting#pnp-output-normally-open-inductive-or-capacitive-sensor.

[p8blr]

@dc42 I'm using a 3HC.

I can try using 2 27K resistors like you mention and see if that helps.

Thank you.

[dc42]

@p8blr the link I posted suggests a 2K2 pulldown resistor and a 10K to 15K resistor to the output of the sensor.