One Digital input of a PLC is 110V DC. If Somehow AC voltage superimpose in this input what will happen to CPU? Whether CPU correctly execute the program as usual or there will be a delay to execute the program? Can anyone explain this matter?

arocon

 

I think in order for an AC voltage to be superemposed on the DC line, the AC voltage would need to grounded. I would assume that your DC voltage in grounded also. If this was the case your voltages may add together to create a cycle of a voltage between 240volts and 0 volts. This may cause damage to the PLC.

If your DC system is not grounded this may happen: I have crossed wired a 120vac hot line to the 24vdc + line, since the DC system was not grounded the input to the PLC was OK and worked fine. I only found the the mistake when I got zapped by touching the + terminal on the 24vdc power supply, ouch!! Hope this helps, my electrical burns should be put to good use!

 

It all depends on the amplitude of the AC voltage. Look at the specs for your PLC input. There should be four values:

1. Absolute maximum voltage.
If the sum of AC and DC superates this value, your input may be or is likely to be destroyed.

2. Absolute minimum voltage
If the sum of AC and DC goes below this value, your input is more likely to be destroyed (because also current sense is reversed).

3. Minimum voltage to recognize logical 1
If the sum of AC minimum and your DC high level goes below this value, your PLC will falsely detect logical 0.

4. Maximum voltage to recognize logical 0
If the sum of AC maximum and your DC low level goes above this value, your PLC will falsely detect logical 1.

HTH

 

Too vague and outlandish to honor with an answer.
Meir

 

arocon
First of all many PLC digital inputs if speced for a high DC voltage input such as 110vdc, may tolerate a 120 vac input without blowing up. The problem is that the ac voltage will make the DI go on and off at the ac rate. I saw this on a Motorola PLC, someone tied 24vac to a DI and the DI was flashing on and off, it did not get damaged because the applied ac voltage was the same as the typical DC voltage it expected (24 vdc). Now, if they had put 110Vac onto that input it would have fired the board in a heart beat (well a few milliseconds actually). I would recommend that you do not or try to avoid having ac voltages imposed upon DI inputs, some plc's will not like it at all. It is not good practice and can lead to false triggers, or other problems.
As far as the CPU goes ,it should not be affected, but may just see that the input is going on and off. Many PLC DI inputs are isolated so that vooltages from the outside world do not touch internal buss signals.

Matt Hyatt
technical consultants
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