AC or DC PLC

Provided it's extra-low voltage ripple free DC, i.e. 24V derived from a PELV or SELV rated power supply.

24V AC inputs used to be more common but have fallen out of favor in the last 20 years. If that 24VAC was derived from a isolated transformer that is.

Of course 110V / 230V AC input cards are just nasty, and at least in my industry are very rare to find these days except in ancient equipment.
 
Of course 110V / 230V AC input cards are just nasty, and at least in my industry are very rare to find these days except in ancient equipment.

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.
 
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.


This is a great idea, I'm sure it's more common than I think, but I've never seen or heard of it.


I personally run into more DC inputs as well, but I do still encounter some 120VAC inputs from time to time. Recently commissioned a system with a ML1100 that had a lot of 120VAC inputs on the base unit, and one of our customers had a large ControlLogix system with almost entirely 120VAC discrete inputs.
 
if the PLC runs on a UPS to know if the incoming power has failed.


Every UPS I've seen used in the automation world has had some kind of built in feedback showing it is active. Creative solution though, if it isn't.
 
Every UPS I've seen used in the automation world has had some kind of built in feedback showing it is active. Creative solution though, if it isn't.

Not a proud point, but some I have done for the customers budget used regular office UPS's that usually have a USB output because it was intended for a PC, not a control panel.

Plus J.I.C. **** happens I know for sure.

EDIT: Kept it clean and still got bleeped!
 
Since I started using PLCs in the late 80's I have never used AC inputs ... always DC

If the cabinet has UPS, I have usually installed a relay to detect the supply failure.
 
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.


You can allways have 120VAC or 230VAC interposing relay and 24VDC input card if power faillure can't be monitored from UPS outputs. Probably cheaper than adding extra AC input card.
 
24VDC on all

The 24VDC power supply is powered by the UPS.

The APC UPS I use has a remote IO module, including a "hardwire kit" (to replace the plugs/receptacles). Through the remote IO module, I can track battery health and/or if on battery with outputs to PLC. Also I can remotely turn it on and off, so the control cabinet does not have to be opened and it does not get activated on a normal panel shutdown..
 
Last edited:
Lare,

i the US, new construction using higher than 120 volt controls is not allowed. NFPA 79
i cannot cite book, chapter, and verse, but it has been in the rules for quite a few years. the items that have been grandfathered in is ok, i don't know the year when it stopped.
james
 
Lare,

i the US, new construction using higher than 120 volt controls is not allowed. NFPA 79
i cannot cite book, chapter, and verse, but it has been in the rules for quite a few years. the items that have been grandfathered in is ok, i don't know the year when it stopped.
james


Do you mean that it isn't allowed to wire 120AC first to 120VAC interposing relay and then from that relay contact side to 24VDC input card?


But instead it is allowed connect 120VAC directly to PLC 120VAC input card?
 
Do you mean that it isn't allowed to wire 120AC first to 120VAC interposing relay and then from that relay contact side to 24VDC input card?


But instead it is allowed connect 120VAC directly to PLC 120VAC input card?


120 VAC can go directly to a 120 input card.


I think when he said "over 120" he was referring to the old common practice of no transformer at all and the pusgbuttons, relay coils and indicator lights all 240 or 480V.


Plus, as far as not allowed anymore, I have a customer with an air compressor that is less than 1 year old and all the controls are 480V.
 
Lare,

i the US, new construction using higher than 120 volt controls is not allowed. NFPA 79
i cannot cite book, chapter, and verse, but it has been in the rules for quite a few years. the items that have been grandfathered in is ok, i don't know the year when it stopped.
james

If I had a nickel for every pushbutton and selector switch I have run across with 480vac on the contacts ... Yeah, I get rid of those every time it is possible.
 

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