outputs directly controlling indicators

Just wanted to know some thoughts/experiences on using PLC outputs to directly control panel indicators (lamps). I have a machine that I noticed used a icecube-style relay (24vdc I believe) to control a couple of indicators via a separate relay, just for lamp testing. A simple lamp test rung is within the program that does this. This particular machine is using a SLC502. But I've also seen this method used on a micrologic 1400.
For solenoids, contactors, and the like are self-explanatory. I just wasn't too sure if it was "good practice" to use relays for indicators as well, or just a waste of time and money.

If the outputs are solid state there leakage current can be enough to turn on a lamp or at least cause it to glow dimly.

Or PLC outputs are DC and the lamps are AC.

If the indicator has an incandescent lamp, then the inrush current can be high, depending on the lamp wattage. This inrush current can damage transistors in the output stage of an output module. This can be a problem when the indicator flashes on and off continuously for signalling purposes. PLC manufacturers normally state the maximum size of incandescent load to be connected directly to transistor output modules.

The other consideration is maintenance. Cycled outputs will eventually fail and the bigger the load the faster this will happen. Your typical “ice cube” relay pulls maybe 50mA if not less but can (again typically) carry a much bigger load than the average PLC discrete output. An incandescent lamp may very well be towards the upper end of the tolerance of a PLC’s discrete output thus they (the PLC outputs) will wear our sooner. With an interposing relay, when the relay wears out (which should take longer because they are designed for bigger loads), you’re not replacing a PLC or PLC card, just a relay.

Or yet another possibility: The load may well be withing the limits of the individual output, but the possible combined loads exceeds the max current limit of the card. For example, a 1769IB16 has a .5A per point rating, but all 16 points are on a single common that cannot exceed 4A.

The long and short of it is having an interposing relay adds reliability and flexibility (in my opinion).

Cool. I just wanted other peoples take on it. I have an output of the above mentioned PLC using an icecube relay just for lamp testing a couple indicators, some of the other indicators that are on a separate control panel (cooling/radiator panel)are being directly PLC driven indicating when the pump and fans are on. 24VDC indicators. So, I was just curious on possibly why the different methods were used on the same machine, and if I should "fix" this.

The only time I ever waste time, money, panelspace, and materials to isolate an output for a lamp, is to do a voltage translation. 24VDC output say to a mus-specefied 120VAC lamp.

Isolation relays have their place, but that place is not everywhere.

Liam Moran said:

If the indicator has an incandescent lamp, then the inrush current can be high, depending on the lamp wattage. This inrush current can damage transistors in the output stage of an output module. This can be a problem when the indicator flashes on and off continuously for signalling purposes. PLC manufacturers normally state the maximum size of incandescent load to be connected directly to transistor output modules.

Click to expand...

Hi Liam, could you please increase my knowledge by explain how a resistant load has in inrush current?.


radrast, thank you for supplying some commonsense to this discussion. Interposing relays for indication lamps?, sounds like l had done a good sales job when l used to be a sales rep. But l wouldn't have done that either.

I will correct my post by saying , if the number of lamps x current, is => than the output contact rating, you will of course need an interposing relay.

HI,
Your machine may have been wired this way for the sole purpose of lamp testing. Turning on several lamps with one output. The combined current for the lamps is likely too great for a single transistor output, or it is just a simple way of turning on some lamps that are normally powered from other means. Normally I don't worry about using relays on 24 volt lamps regardless if t is a stack light, on the door or console. The biggest lamp I've seen was 40 watts, 120 volts, still safely within the limits of an output module, unless there are too many on the same module.

I have seen shorted indicator bulbs cause fuses to blow, which stops the whole machine until repaired. In that case it wouldn't matter if there was a relay or not.

I like having the lamp test circuit on a control panel. I get preventative maintenance sheets weekly to check the lamps. Pain in the rear if there is no built in function.

BD

Duckman. - Resistance increases with temperature. When an incandescant lamp is energised the filament temperature increases from room temp to over 2000C in a few milliseconds. The inrush takes place because the filament is cold when the power is applied.

CeeDee said:

I have an output of the above mentioned PLC using an icecube relay just for lamp testing a couple indicators, some of the other indicators that are on a separate control panel ...

Click to expand...

Another possibility is that when the lamp test relay is OFF, normally-closed contacts on the relay allow the lamps to function operationally - else if the only way of turning the lamps on is by the lamp test relay how would they function?

I have seen shorted indicator bulbs cause fuses to blow, which stops the whole machine until repaired. In that case it wouldn't matter if there was a relay or not.

I can see your heart ack if an indicated lamp failure causes a fuse to blow and the machine stops, were is the ENGINEER/sparky (l imagine long gone for other F#$% up's).

BD[/QUOTE]

Andybr said:

Duckman. - Resistance increases with temperature. When an incandescant lamp is energised the filament temperature increases from room temp to over 2000C in a few milliseconds. The inrush takes place because the filament is cold when the power is applied.

Click to expand...

Looks like l am having to get my tech spec's out, as l would be surprised if the current inrush on a indicator lamp even registers, as well as most lamps are now LED, all thou this question was incandescent. l was aware of incandescent lamp are in a vacuum, but are you a believer of a interposing relay just to run a indicator?