How To Protect 120VAC PLC Input?

[Stationmaster]

Keithkill,

Small point of clarification..... There are no solenoids. There are two motors in parallel.

Stationmaster

 

[keithkyll]

Thanks Stationmaster. That's a big point!
Connecting caps or MOV's across motors may affect their operation. I expect the spike is during turn-off, so an MOV may be okay.
The best choice is still a relay - either mechanical or SSR.
Because they are just sensing if the motor is on, a current sensor would also be a good suggestion.

 

[rsdoran]

I think the point Keith was making is both are inductive devices; therefore can cause the same type issues.

 

[Stationmaster]

Yeah, Agreed.

I think we can all agree that a motor power lead should be isolated from a PLC input.

Stationmaster

 

[keithkyll]

Agreed. I see kittydog42's problem. How do you call it a design problem if it worked for 10 years? We all agree there is a problem with the design.
The Nano manual listed the AC input as 'Capacitive'. I wonder if they have a series capacitor inside. You only need 10 mA to turn on the input. A series cap would also work, and limit current. Anyone care to calculate the value? My guess is .22 MFD, 400V.

 

[allscott]

IMHO

Install a relay and be done with it, problem solved.

 

[CroCop]

Soild state removable IMO.

 

[kittydog42]

Following up six years later. I ditched the Automation Direct DL05 PLCs and switched to GE VersaMax Micro. No issues with direct 120V, just like the Nano. Eventually switched the GE for the Micrologix 1100, also no issues. The Automaton Direct units were just not very robust. The irony is that I wasn't buying it due to the low price, it was the one of the only 120V PLC with 120V inputs I could source to replace the Nano when Schneider discontinued it.