What is status light on B810 module telling me?


Thread Starter

Andy Zillins

Have Modicon 984 system with several isolated AC output cards (B810). They have a Neon light that indicates when channel is energized. It will also indicate when there is no load. Most spare inputs with nothing wired to them do not glow, but a few do. Any idea why? Don't want to take this system out of service if I can avoid it. Is something failing?
Send me your fax number and I'll fax you a data sheet. The sheet does state "Output status are provided for each putput circuit. These indicators will be on when a load is connected and the output is on, or when there is no connected load!"

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A neon lamp indicator can glow at sub mA current level (a few tens of microamps will do - as in a neon tester).

Neon is ionized at approx. 90 V and then keeps glowing at approx 50-60 volts (it was 40 years ago when I learned that stuff).

So, bottom line, a lit neon lamp may indicate stray currents caused by stray voltages (induced into an open ended wire runnin in parallel with other, current-carying wires, over a long distance)


Andy Zillins

Thanks for replying. I think I have same data sheet. I'm trying to figure out why some outputs glow and other don't when they are all have nothing wired to them. I mean nothing wired to the PLC connector at all.

I'm mostly worried whether this might indicate a more significant problem with the module and thereby affect the outputs that are in use.