PLC hardware question....

Another quick question. What PLC module or configuration can be used to detect the presence or absence of a 12 Vdc signal.

Can you be more specific ? My experience is mostly 24 VDC I/O. Are you looking for an entire input sensor system of 12 VDC ? Or do you have only one 12 VDC signal ? And are you using active high or active low logic ?

um. an input module?

Use a control relay with the 12vdc on the coil side and 24vdc on the relay side. When 12vdc is present on the coil the 24vdc will make your input high. When 12vdc drops from the coil, it will open the relay and the input will go low.

I'm sure there is a more advanced way, but this is easy and cheap.

I have this equipment that send a 12 Vdc signal out that represents that the equipment is operating normally. I want to interlock this signal with the machine that its preceding it.

Look at your 24 VDC input module, some are rated for 10-30 VDC.

Look at your 24 VDC input module, some are rated for 10-30 VDC.

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I wouldn't want to mix up different voltages, especially from different systems. I'd use an optocoupler or a relay like doyletuttle says.

Thank you all for your replies. I think that I'm going to go with using an optocoupler from ABB to do the job. Since the PLC rack that I want to communicate has no 24V dc I/O modules, everything is 120Vac, I can switch the 120V ON/OFF using the optocoupler and feed it to the PLC.

I'm looking at this model 1SNA 645 035 R2400 from ABB.

JoseM said:

...the PLC rack that I want to communicate has no 24V dc I/O modules, everything is 120Vac

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All the more reason to use an interface relay...

JoseM said:

I'm looking at this model 1SNA 645 035 R2400 from ABB.

Click to expand...

FYI, that part number is a relay, not an optocoupler. A relay will work fine, unless your signal will be turning on and off a lot. A relay is mechanical, so it will eventually wear out. If that is an issue, you want to use an actual optocoupler (solid-state, with no moving parts). A quick look at the catalog finds the ABB 1SNA 608 019 R1000 will take a 12VDC input and switch a 120VAC load. Note that it requires a minimum 25ma load, which may be more than the PLC input draws. Might need to add a resistor in parallel with the input to create enough load for the optocoupler to work.



-Eric

Thank you Eric for the clarification, I mixed the words. This signal would not be switching ON/OFF constantly. I would only switch if the system goes down for any reason. So this relay should be a good approach.