AC or DC PLC

I_Automation said:

Yes to mainly DC inputs, and all DC outputs, but most of my rack PLC projects have one 120 AC input card to monitor fused power, and if the PLC runs on a UPS to know if the incoming power has failed.

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I would always use an interposing myself for this kind of thing. But i generally would only be monitoring 1 or 2 such things in a panel. For our installations typically have a "mains fail" indicator which is either a 3 phase monitoring relay or a standard 230V coil relay for single phase installations.

I do mostly water utilities work, and all our control systems are 24V powered, with a minimum of 12 hours battery backup.

Guess it's different for most other industries.

Saffa said:

I would always use an interposing myself for this kind of thing. But i generally would only be monitoring 1 or 2 such things in a panel.

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If there was only 1 or 2 then I would use interposing relays also.


But a line with 10 to 16 fused powers for solenoid valves and DC motor controls, as many I do have, it is easier to change one input to AC then add a bank of 16 relays and sockets and the extra wiring.

I_Automation said:

If there was only 1 or 2 then I would use interposing relays also.


But a line with 10 to 16 fused powers for solenoid valves and DC motor controls, as many I do have, it is easier to change one input to AC then add a bank of 16 relays and sockets and the extra wiring.

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If it is easier it does not mean it is better solution or even good solution.

My panel UPS are 24V output.

Normally closed contact on the 230V power supply, if the RTU/PLC is telling me that's gone low, I know I'm on UPS.

20 minutes run time at 5A load.

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DC and probably the safest is Sourcing inputs and Sinking Outputs but they tend to confuse installers and electricians.

JaxGTO said:

DC and probably the safest is Sourcing inputs and Sinking Outputs but they tend to confuse installers and electricians.

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For all the European machines I work on this is their standard. But they always say it is because Sinking devices and outputs respond faster.


But to me the input of the PLC is not going to respond until the next scan after it turns on, unless it it wired to a Fast-DC or a HSCE input that doesn't seem to matter. And the 0.002 second faster output energizing would not make any difference in an industrial application using a Mitsubishi FX PLC.