Voltage Levels in DO Cards of PLC

M

Thread Starter

Mihir Ramkrishna

Which DO cards are better from the Reliability perspective? Cards that give 24 VDC or cards that give 110 VAC?

Kindly note that the PLC system will be powered from UPS 110VAC. Also, we would be using interposing relays as the final voltages required for field are going to be 110 VDC. OEM offers both these options. I understand these options matter when we do not use an interposing relay. In our case it should not be a problem so why not use lower level voltages which in my opinion are more reliable.
 
C

Curt Wuollet

The most reliable will be 24VDC transistor outputs. Relays have a lifetime. Triacs are in the middle.

And none of them will be extremely reliable without suppression of inductive loads.

Regards
cww
 
S
My take is a bit different.

DC are the most reliable in most circumstances. Solid state outputs don't have an age limit per se, but will be sensitive to unusual conditions that cause an overload.

Properly used, relays are quite reliable. I've seen a few fail, but not many. If I need an AC output to drive a motor starter or something like that I'll either use a DC output with an interposing relay or a relay output (which is the same thing internally anyway).

AC solid state outputs (triacs) are the absolute worst. I never use them for any reason. Ever. They leak and cause unintended turn-ons, like alarm horns, or fail to turn off, like when driving relays with a low minimum coil voltage tolerance. I've seen them fail many times, on machines where one of my relay outputs would be clicking along just fine.
 
Top