I am using a Logo!24RC Basic Module with 24V Digital inputs. This has worked well for my application in the past, but I now need to integrate a sensing device that supplies 24VAC when activated.

I thought I might just connect this to a DI and put it through an off-delay, hoping that it might appear as random spikes on the line which an off-delay of a 1/4 second would clean up, but this doesn't seem to work. Any other suggestions?

I thought of adding a diode to dump the negative swing of the AC signal to ground so that the DI gets only the positive voltage part of the AC waveform, but of course the signal would still be fluttering at 60Hz. Should I just buy a rectifier?

Thanks

Thats what I was leaning toward... do you think it would work to connect like this:

AC24V ---- Diode->|----Cap----- Ground

With the Logo Digital in between the Diode and the Capacitor.

If I'm reading the product information correctly, it looks like inputs 3-6 will sink at least 1.5mA when the input is made, while 1-2 or 7-8 sink less since they can trigger with only 0.1mA. For a few mA, it looks like I'd be fine with a 10uF capacitor (drops only 2.5V in a half cycle at 60Hz while supplying 3mA) and any standard 1A diode with at least 2*sqrt(2)*24 ~ 75V reverse voltage rating. Am I thinking about this correctly?

You MUST have a connection between the - (minus) 24 VDC and the N (Null) of the 24 VAC.

Have you do that?  Else it never works.

Have you tryed the other way? This LOGO can also on 24VAC. Run the LOGO! on 24VAC and use the Digital inputs on 24 VDC.

Try it

Yes toeter I think I can have the contractor wire the 24VAC neutral back to the logo as well. I'm pretty nervous about this 28.8V limitation though, thanks ixo for providing that information.

Can anyone tell me more precisely what current the DI will draw close to 24V?

Perhaps someone knows what transistor is sitting behind the input so I could just look up its input current - voltage curve?

Thanks very much!