24 VAC in 24 VDC input, Siemens S7-200

Hello All. New guy here. Good to meet you.

I am working on retofitting an existing system that uses multiple Logo modules to control 15 relay outputs from 14 24 VAC inputs. The system needs to be expanded to 30 stations, which would require more Logo modules than any one human should have to deal with.

I can't convert to straight 24 VDC because of the 3 wire configuration of the stations and the 24 VAC valves being controlled. I'd also like to avoid having to put in 30 interposing relays.

Consequently, I'm looking at an S7-200 system with 24 VDC inputs and relay outputs. This is convenient because I'm familiar with the interface and have all the required programming tools.

Here is the question: Can I simply half-wave rectify the AC input from the PB with a diode into the 24 VDC input and then use a positive-transition one-shot in the logic?

It seems like you should be able get away with a rectifier diode with a small capacitor across the input as long as you don't need really fast response times.

mendonsy said:

It seems like you should be able get away with a rectifier diode with a small capacitor across the input as long as you don't need really fast response times.

Click to expand...

A resistor of the proper value in parallel with the capacitor might also be needed to discharge the capacitor unless the plc physical input provides enough load to discharge the capacitor within the needed time frame. I'm not sure you would even need a positive-transition one-shot in the logic. It might work better without it. Only experimentation will decide what works best.

Why don't you use an S7-200 with AC inputs and relay outputs?

Lancie1 said:

Why don't you use an S7-200 with AC inputs and relay outputs?

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That's the problem; there ins't an S7-200 with 24 VAC inputs. S7-1200 doesn't have them, either.

There are a few PLCs with Inputs rated for 24 VAC. Try a GE Fanuc 90-30 with a IC693MDL241 24 volt AC/DC Input Module.

http://www.qualitrol.com/shop/p-14650-ic693mdl241.aspx

The inputs are sink/source. They have AC input style optos. Should probably work fine with AC. If inputs "flicker", then you may be able to do digital filters to stabilize. Otherwise, the diode/cap setup suggested may be the only solution. Try 1N4004 or UF4004, and 1uF/35V Tantulum. UF4004's are nice for snubbing DC outputs.