How to connect multiple sensors to single PLC input?

Question

Hello. I have a scenario where I have multiple PNP "error signal" from several sensors (let's assume 3 for now). If the sensor output is HIGH, then there is error. If the sensor output is LOW, then there is no error. The design is that if any of the sensor detect error then the PLC input will be HIGH. My question is, suppose if all three sensors detect error, it means all three will send voltage to the same port in PLC. Will it cause over current and damage the PLC input? Is the current in PLC port = current sensor 1 + current sensor 2 + current sensor 3?

Answer 1

^{simulate this circuit – Schematic created using CircuitLab}

Yo do only have to add three diodes D1-D3 to your PLC input. The currents don't sum, right the opposite. If only one sensor is HIGH, then the current through sensor and PLC input is the same, if more than one sensor is HIGH, then current through PLC input is the same, but current through sensors are divided.

You may also omit the diodes and connect them parallel if they are industrial sensors. I do connect, for example photocells, parallel to increase the detection zone.

Answer 2

You more have to make sure 1 sensor does not damage another,

E.g. if 1 outputs "HIGH" and 1 outputs "LOW" if its a driven output, its essentially a short circuit on the sensor outputs.

If its "Open Collector" outputs, or "Open Emitter" outputs, then they are safe to have in parallel so long as they are all the same type. as you say its a PNP output, I am going to assume Open Emitter,

I am going to assume the PLC input is measuring voltage, if so you should not have to worry about damaging the PLC input, as it should be a very high resistance, so the current from the outputs would be very small. This is assuming that the PNP output voltage is not higher than the maximum voltage rating for the PLC input.