outputs directly controlling indicators

[Secpcb]

= but are you a believer of a interposing relay just to run a indicator?

I would say unless there is a real need, an added relay would just be added costs for the relay, socket, wiring, wire labels, labor & panel space used.

 

[duckman]

Without going thru manuals tonight, l would suggest people understand switch ratings of contacts, incandescent lamps are NOT inductive loads, coils are. Confirm (in Aust, AC 3/5 rating of switching contacts) what your load is, ratings take in to account inrush.

 

[Andybr]

I am not sure where anybody suggested that Incandescent Lamps are an inductive load. A quick Google search suggests that the inrush into a tungsten filament lamp will normally be around 10 times the normal running current. Whether this is a potential problem or not will depend entirely on the rating of the switching device.

 

[duckman]

A, you are most probable correct about my incorrect assumption (believing inrush was a inductive load issue). What you are saying is that incandescent lamps have more inrush than an inductive load (figures l have seen on Wikky, but when you are talking small amounts which are allowed for in switching category). As a ex sales rep, all l say is read the manual, it is pedantic's about this question. Read the manual/spec and that is what the contacts are rated for. Inductive/ reasistive/ what ever the load is. Manufactures give you the rating for all loads. Read the load spec's, if it can't handle it, put an interposing relay. Normally people don't understand switching categories and that is when failures happen. If people follow manuals most problems are avoided.

 

[Bit_Bucket_07]

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.

Seriously? It's a waste of time and money. I'm all for protecting PLC outputs from damage, but if an output can't handle the load of a tiny panel indicator lamp, then it isn't likely to fare very well in operating a relay coil.

I've been building PLC based control systems since before graphical HMIs were even available. If we had had to use interposing relays for panel indicator lamps on those old pushbutton systems, then we would have had to build a separate cabinet just to hold all of the relays.

 

[CapinWinky]

Your IO should have a datasheet that clearly states the maximum current for each point and for the module as a whole. If you're lucky, it will also mention inrush current tolerance. Unless you are trying to directly light a halogen spot lamp, you're going to be fine. A typical DO can handle half an amp at 24V and many can go up to 2A continuous load (these often have module limits preventing all outputs on the card from using all 2A). Beyond 2A you are looking at IO slices with built in relays anyway.

I'm also finding it hard to believe there is an inexpensive relay that takes less current than a light.

 

[Liam Moran]

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.

An incandescent lamp filament has a low resistance when cold,
and when powered up this filament glows white and its resistance increases, just like any metal.
This inrush current which occurs between the cold and hot state of the filament is the reason an incandescent
lamp will almost always fail at the end of it life as it is turned on.

 

[Lancie1]

Guys, put the load issue in the back of your mind. Somehow we got off into la-la land. The most likely situation is what BikerDude said:

Your machine may have been wired this way for the sole purpose of lamp testing.

I have seen many lamp-test circuits on control panels, and usually 2 functions are required of each lamp: Normal, where some process switch or signal operates the lamp, and Test, where all lamps are switched to a power source when a Lamp Test button is pressed. All lamps cannot be wired together for testing, without some type of switching arrangement, otherwise they would all go on/off together all the time. Now what is the simplest way to create those 2 separate functions?

Answer: By using relays, with normally-closed contacts wired to the normal operating functions (whether that function is from a PLC output or something else), and the normally-open contacts wired to a common lamp test function.

Another option is to buy the type of indicator lights that have a built-in Push-to-Test circuit (more expensive, but simpler field wiring).

 

[The Plc Kid]

This thread just hurts.

No one in their right mind would use interposing relays on pilot lights unless it was for a lamp test circuit or they had a really poor plc output card. As Bit Bucket said if it won't handle a light it would not stand a chance with a coil.

If you want to do a lamp test wire you pilot lamps directly to outputs and write your logic so that the machine has to be in a stopped / safe state and when the lamp test button is pressed turn on all those pilot light outputs. We normally make this in the maintenance routine.

We use the lamps in the link below because they are universal voltage and have a shunt to prevent solid state output leakage current from making the lamps glow dim when they should be off.

http://literature.rockwellautomation.com/idc/groups/literature/documents/pp/800t-pp007_-en-p.pdf

I am all about using interposing relays where they are needed but many people in the industry get carried away with them trying to protect an output module where it's not needed and they introduce many more points of failure into the system and higher chances for increased downtime and add difficulty to the troubleshooting process.

 

[The Plc Kid]

Also if trying to build a lamp test circuit without using the plc I would use pilot lights with that feature built in as Lancie1 suggested because the field wiring would be simple and they have a lower chance of failure than the relay option.

This would be the cheaper route also in most cases if you apply a cost to the panel space needed for relays.

 

[CeeDee]

I love it when I start a juicy thread that a lot of people have ideas and thoughts on. All productive and helpful, in my eyes. Thanks everyone!


Mr. Kid, the suggested A/B 800T/H Universal LED System you posted looks like a good set up. I've been slowly starting to replace all the indicator bulbs on the control panels I work with as the bulbs age and burn out. Most are the typical BA9s bayonet, incandescent style and have been replacing them with commonly found LED versions (ledtronics, digikey, alliedelec, etc.). There are just so many options out there. But knowing that A/B makes the good stuff (and always charges accordingly) I might have to give those a try.