Short pilot light

[patm72]

Has anyone encountered problem due to a short pilot light? What could be the cause of this?

The application is the following: 24 VDC pilot light (ABB model CBK-KLF8G) turned on by proximity switch Turck BI3U-M12-AP6X-H1141 (3-wire device). Wired in parallel with this pilot light, I have a PLC input. Lately, when the prox would turn on, the PLC wouldn't get the input from the prox, which LED, in turn, was not ON.

After some troubleshooting from maintenance personel, I learned that apparently, the pilot light was short, thus sucking the current that would normally have turned ON my input. By replacing it, it worked. Is this a problem that could have been avoided at first by an LED instead of the pilot light? by putting a resistor in series with the light?

Thx

 

[VEBill]

Having your PLC get the same information as the humans (via the pilot light) might be better than having the PLC get different information than presented to the humans. It sounds like this failure was fairly easy to troubleshoot.

Light bulbs more often fail open circuit. LEDs and resistors would fail much less often than would a light bulb.

These are the sorts of decisions that can give safety engineers grey hair.

 

[OperaHouse]

Lamps can fail shorted, but it is fairly rare with low voltage lamps because the filiments are shorter and thicker. After years of being in this business, I hardly believe anything anyone is telling me. I would more likely believe this is a wiring or socket problem that fixed itself when the lamp was changed. I like resistors in series with lamps because it extends lamp life because of reduced voltage and gentler turn on. Lamps have easily an 8 times increase in resistance from off to on. That can be hard on some solid state components allready at their limits. You could go to a 12V bulb and dissipated another 12-14V in a series resistor. The PLC opto input would likely work with as little as 6V even if the prox switch couldn't supply all the current of the increased load. Just remember that a lamp half the voltage will require twice the current for the same brightness. An LED may just overall work better.

 

[weh3]

Why not use a PLC output for the pilot light, and turn it on when the prox is on? The impedance of the pilot light is most likely low anyway compared to the PLC input, so the prox will drive the pilot light in favor of the PLC input.

There are also prox's that have an LED built-in, if you absolutely need a visual for troubleshooting.

Wm

 

[SolderingGunslinger]

There are LED Pilot lamps available mounted in standard incandescent pilot lamp bases. If there is some reason that you must use a fixture with the pilot lamp socket, one of these replacements might be just the ticket.

The LED replacements aren't inexpensice, but I have used them to replace indicator lamps on transmitters, specially those in remote sites which get only a quarterly check-up. It is good to walk into the transmitter building and see the proper indicators glowing (or the fault indicators glowing...you at least know where the problem lies).

I remain,

The Old Soldering Gunslinger