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I got a panel where the PLC output has 2 - 5 VDC and the relay connected to it says 24 VDC, it will not work below 16 VDC. What's the deal here, aren't PLCs usually 5 VDC? Was the PLC supposed to be programmed to send 24 VDC so it can activate the relay? I googled PLC relays but all I see are 24 VDC rated ones.

Also, on the output of the relay, the wire goes to the COM terminal of the relay... so the solenoid switch is staying on 24/7 regardless what the PLC is saying. Because otherwise, if it's connected to "NO", the relay will never activate because the voltage from the PLC is too low. Is this an oversight from the manufacturer and then a quick fix? What is this?

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  • \$\begingroup\$ I would say buy a 5 volt to 25 volt DC converter, available from many vendors, but buying the right voltage relay might be cheaper. \$\endgroup\$
    – Sparky256
    Commented Mar 17 at 0:22
  • \$\begingroup\$ 1. Please add the brand and model of PLC. 2. Please add a link to the datasheet for the output module. 3. Please show a wiring diagram. \$\endgroup\$
    – Transistor
    Commented Mar 17 at 0:35
  • \$\begingroup\$ When you say “the PLC output has 2 - 5 VDC” do you mean it’s rated at that or is that what you are measuring? I suspect it’s the latter and you may have a bad output. \$\endgroup\$
    – GodJihyo
    Commented Mar 17 at 1:30

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I got a panel where the PLC output has 2 - 5 VDC ...

5 V logic is not common in PLCs but various vendors offered it. 2 - 5 V is unusual and makes me think that you are misunderstanding a specification somewhere.

... and the relay connected to it says 24 VDC, it will not work below 16 VDC.

16 V would be a typical pick-up voltage for a 24 V relay.

What's the deal here, aren't PLCs usually 5 VDC?

No, 24 VDC is the most popular control voltage. 120 VAC might be next.

Was the PLC supposed to be programmed to send 24 VDC so it can activate the relay?

The output voltage is not controlled by program (software). It is controlled by hardware design determined during the design specification.

Also, on the output of the relay, the wire goes to the COM terminal of the relay...

Sorry, we need a wiring diagram to answer this and a link to the PLC output module datasheet.

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