Ever need to put 120 VAC into a 24VDC Input?

[interd0g]

Yes, I thought about smoothing the rectified opto output to give a decent 24vdc at the plc input. I'm sure it would work.
In my application it is not too important if there is a little lag between the ac appearing and the 24dc plc input going true.
However it is really bad if the 24 vdc ' ac OK'signal persists much after the ac dissapears, hence too much capacitor would be a problem.
I think a better solution might be in holding the input DC when it rises to the 'on' level by means of a firmware timer which will hold over at least one cycle. The signal should go back to off one cycle or so after the ac fails.
Actually in this application it would be better if the ac OK signal dissapeared a little BEFORE the ac failed, but that would require a crystal ball instruction.

 

[interd0g]

I wanted to mention that in case like my current pet project, the inputs are sometimes from cables strung outside and in a lightning prone area. Likewise the utility company input is infected with horrible spikes and noise.
So putting an opto in there even for the 24 to 24 case is not a bad idea to give some protection against bad induced spikes and potential ground faults coming from outside.
Ideally the 24vdc source for the outside of the opto should be separate from the plc 24vdc.
The isolation is good to 7000 vrms. Maybe a relay is as good at least for the first 1 million goes round.

 

[interd0g]

If anyone is still interested in how to use an opto coupler like the rnastronics opto120, for bringing 120vac to a 24vdc plc input, I have now got an answer working to cope with the raw half wave rectified dc you get out of these things.
Its is posted in the thread pulse stretching?

 

[interd0g]

I thought i had this sussed, but not quite so. I have an opto coupler designed for 120 vac in and controlling 24vdc to the plc input.
The opto delivers a raw unsmoothed 1/2 wave rectified output to the plc input.
Because the plc would probably try to follow the 60hz cycling at the input I put an off timer with 25 mS duration on that input to bridge between cycles, but come off pretty smartly when the ac input ceases.
It worked........... most of the time. But once in a while the inout would become not valid for up to 100mS at a time,and the system would drop out on a falses utility input failure.

I am not sure why this is , but the Toshiba T1 scan time is set to its fastest (10mS) which is not that fast when you are sampling 60Hz with a period of 15mS.
Plus of all they say the T1 scan time can exceed the set value any time depending on the activities going on in the scan at the time. I see mine sometimes extends to 42mS which is a bit hit and miss trying to see what is going on with a 60 hz input.
So for reliability I need to think again.
Maybe the hardware method ( smoothing capacitor) is the way to go but it seems a little clunky.

 

[Bob A.]

Thanks for all the thoughtful replys. There were more than I expected.

I do a lot of very small one out systems using the smallest equipment that I can find and I frequently find when I'm done that there is one more thing that I would like to jam in there, but it is always something besides 24 VDC.

It would be really handy to have a bag of converters to connect inline to make the change. I have plenty of electronics engineering experience and am quite aware of the discrete components that can be bent and shaped into the necessary configuration, but that is not exactly what I'm wishing for!

Best Regards and Thanks Again,

Bob A