I'm working on my capstone project for school, and I'm at a loss as to what to do. The PLCs we have only have AC inputs and outputs, but due to time and budget constraints, I am unable to source some relays in time to drive everything. I have the outputs wired through relays, but I'm stuck at the inputs. I understand the input modals rectify the AC signal before putting the result to and optical isolator. If so, then could it be possible to power the inputs directly with just a DC current?

I don't have the part numbers for the input modules, but the PLCs are Allen Bradly PLC-5/2.


Oh, and if it matters, the project parameters was to created something using a PLC and I decided to make a drag tree using LEDs and IR triggers.

 

It turns out that if the voltage is high enough the AC inputs will work fine with a DC input if the opto-isolator comes after the bridge rectifier. In fact bridge rectifiers are sometime used in DC circuits for protection against polarity reversal. Try it first to see if your particular AC inputs will work with a DC voltage. Be very careful with anything over 30VDC; it is quite possible to receive a lethal shock.

 

Thank you. I will give it a try when I can get into the lab. Luckily I over built the output stage of my sensor array, and the output mosfets can handle rectified AC voltage.