Mixing 24VDC and 24VAC in one single Controller-PLC

Hi everyone,

As I red in forum: 24VDC vs. 24VAC (1) 24VDC vs. 24VAC | Automation & Control Engineering Forum both have advantages and disadvantages.

Lately I have been building some large BMS control cabinets with 40 to 50 relays to control light systems and pumps, and many other things. And made them with 24 VAC due to price, but I always like to privilege 24 VDC.

Do you think I can supply the controller with 24VDC and make a separate 24VAC circuit for the relays? Probably yes for the digital outputs (because I have in CAREL NO/NC contacts), but what about the digital inputs?

I use CAREL controllers, and these require me to connect the digital input to the neutral of the 24VAC or the negative of the 24VDC to the digital input, to confirm the circuit is closed.

Any suggestions or comments?


Thanks oneeye14 for your reply.

Carel c.pCO controllers, the ones I use, yes, have been designed for 24 VAC, but they work in a very strange DC voltage. 28 VDC, but they accept a -20% range from the 28 VDC, so we can go down to 22.4 VDC. I'v had good experiences with 24 VDC WAGO power supplies, and I really want con continue working with them.

In Chile I am getting popular on retrofits, I have 10 years experience in HVAC, and I am starting to save some old Chillers or compact units where a good maintenance and maybe changing a compressor and installing a new controller, with a logic designed by me, saves the machine for another 5 to 7 years, what I didn't write in this forum, is:

I really would like to continue using the 24 VDC WAGO power supplies, and I could even adjust them to 28 VDC, but I would like to work the digital inputs in 24 VAC with the existing power supply of the machines I make retrofit. Do you think it will work?

PD: I would also do that on those large cabinets with more than 50 relays, to work them in AC and power the controller in DC.

Thanks again.
I can't see any reason why you cannot power the Carel with 24vDC and digital inputs with 24vAC. There is perhaps a degree of separation of the 2 supplies, maybe with distinct differences in wire colours.

Perhaps you could increase psu up to 26vDC as a compromise with the supply being a nominal 24dDC.