PLC AC Voltage Detector 1

[roro36]

Using an analogue input card with a plc, i want to know if you can sense the polarity of the AC mains? With a 240V phase to neutral voltage connected through a 15KOhm resitor, the current is limited to 16mA.Can the PLC then use this to determine when the Voltage value is in the positive/negative half cycle?

 

[Skogsgurra]

Yes. It is possible with a fast enough PLC. But you have to make sure that there´s time to input the information, process it and react to it in the 8.3 or 10 milliseconds available. if you need any precision, I would look at a fast I/O board. It doesnt need to be analogue if you provide protection for the input pin. Your 15 kohms may need a zener as well. And probably a little higher resistance.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[IRstuff]

A potentially more reliable approach is to use a PLL to lock the phase detection directly. Since a PLL essentially does a prediction/correction automatically, it could avoid the problems of threshold, noise immunity, etc.

TTFN
faq731-376

 

[Skogsgurra]

A PLL may need ten or twenty cycles to lock and stabilize. That may be a problem if you need to react quickly to presence of an AC voltage.
I do not know of any COTS PLL modules that can be installed in a PLC rack. Are there any?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

If you have a fast enough input card then a simple half-wave rectfier would allow you to sense whether the voltage was present or not during a half cycle.


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