SLC 5/04 Output Card Stuck On

[chippieboy]

I have an strange thing happening with a SLC 5/04
On a 1746-OA16 output card I get 120 volts with or with out the output being true. The LED for that address is off and RX Logic online shows the output off but I still get 120 volts. I swapped it with another card from another machine and still have the same issue. Any ideas?

 

[Snyd1]

Its a triac output.

 

[ArcReactorKC]

Have you actually placed a load on the output? Or are these readings only with a digital meter?

 

[dmroeder]

What others are getting at is that triac outputs "leak". So if you don't have a sufficient load on the output, it will appear to be "on" even though the PLC is not actually turning it on. If you had an interposing relay between the output and the LED, you'd likely find out that the output is not actually on.

 

[plvlce]

All solid-state devices leak a very small amount of current even when off -- the 1746-OA16 spec is 2mA per point maximum.

Without a load to drain it, a modern high-impedance voltage tester will see voltage due to this. To get an accurate reading, you need an old-fashioned wiggy or a meter with a low-impedance mode.

If this is a concern you can install a suitable resistor as a load to dissipate the leakage.

See also this thread.

 

[Mark Snodgrass]

Ye old WIDOWMAKER.