Solid state or relay outputs

I would appreciate opinions on the debate over using relay outputs to energize contactors, relays, and solenoids (Inductive loads). I have had people tell me that they eat the contacts up, and that I should use the triac outputs. Any opinions, or experiences?

Traics are good for relays,Solenoids,Contactors,etc. When they are used with lights and horns sometimes the voltage bleed can false trigger those devices and additional loading has to added to the end devices. The caveat to most plc triac cards is they do not handle high current. Some brands offer high current traics that go up to 2.0 amps per point but most traics are .5 amp per point.

Relays can pass more current than most traic cards but they do not last as long and you need suppression at the device eg diode on a coil and you need suppression on the plc relay contacts also. On many higher end cards this is built in like Allen bradley models but on some brands and models it is not so that is something to check before purchasing.

Anying thing that has a normal high cycle on/off rate should be solid state IMO.

Well any time you use a relay you will have arc so it will eventually fail. To exend life a MOV or snubber can be used. Soild State devices will almost always last longer due to the fact they are solid state.

I generally only use relay outputs on things that do not have a high cycle rate or I need a voltage other than the output voltage.

Rojay,

relay contacts do in fact wear out, break, weld together.
personal experiences.
replaced relay output card every week on an Omron unit powering solenoid valves - over 1 million operations per week.
went to solid state outputs - problem solved. burning out output is now rare. the life of the relay contacts was 1 million cycles.

we use relay contacts for lights, and light load applications, but you still must look at the application.

contactors, valves, relays, and other inductive loads (ac or dc), causes the contacts to arc when the load is removed. that's why we use reversing diodes for dc and mov's or varistors for ac loads.

you still need to look at the application in regards to frequency of use and the load. solid state outputs leak and cause light loads to remain energized even when the output is off. in that situation, you need load resistors to load the output circuit keeping the output off.

you also will need to train/remind the mechanics that solid state outputs will remain on when the load is removed, protective fuse blown. care must be taken not to get into the circuit, it will bite (first use of solid state outputs).

regards,
james

Thanks, I kind of thought so, but wanted another opinion. I will break the inductive loads out from now on. Is supression needed on triac outputs?

The Plc Kid said:

Anying thing that has a normal high cycle on/off rate should be solid state IMO.

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That's my only feeling on the matter.

rojay said:

Is supression needed on triac outputs?

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Yes, to protect from noise and voltage transient spikes when used to switch highly inductive or capacitive loads.

I read Jame's reply with interest. I'm working on a project whereas I'm using relay outputs to operate ice-cube relays, 24vdc (very light loads). Should I use reversing diodes for this application and if so how do I size them and what do I order. I usually purchase my electrical supplies from Allied.

Thanks, Dave

For a high current load such as a solenoid use an external relay. It can be solid state or mechanical. Avoid using the plc directly. Its cheaper to take out an external relay then a card.

Don't use the same power source for the processor and solenoids. You'll take out the processor.

Dave,
Are you planning on using the diodes across the coil of the 24VDC relay? If you are, then you can use a basic diode to do this.

Triacs are good for relays, Solenoids, Contactors, etc. When they are used ...
....
Anything thing that has a normal high cycle on/off rate should be solid state...

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Kid, that was a good write-up. I don't think "Kid" describes you any longer. You are just going to have to change your name from PLC Kid to PLC Guru.

Plcwill said:

Dave,
Are you planning on using the diodes across the coil of the 24VDC relay? If you are, then you can use a basic diode to do this.

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Thanks William, That is what I plan on doing. The relay coil will draw .7 watts. Should I use a diode or not. I don't need any problems with this plc. What criteria do you use in selecting a diode for this purpose...i.e. voltage, and current ratings. In other words, I know the voltage rating should probably be 24vdc but how should I chose the current rating???

Thanks, Dave