Relay coil failures

What has been your experience with coils that go bad? Do they typicaly burn open? Can they short to something and draw high current? I'm looking at the need to protect my digital outputs which drive them. Sometimes I use relay outs and othertimes I use triacs.

In my experience, of the three components (relay coils, outputs, and relay contacts) the coil is the least likely to burn out. I generally see contacts fail most often (welded or so pitted they don't make good contact any more). The PLC outputs fail next in frequency, but that is generally rare. The relay coils themselves usually outlast the contacts by a good bit, unless you put the wrong voltage on them or else you are cycling them very very fast.

Here is a problem that I have dealt with in the past. Mechanics used to take the coil off the valve stem with voltage still applied, thus burning up the coil. Besides that, i have never had a bad coil. However, you should still fuse each module output, money permitting.

Back EMF is the serial killer of digital i/O

Protect your outputs using flywheel diodes for DC or VDRs for AC and you should have very few failures.

I have never seen a relay coil take out an output. Solenoid coils are another thing. They are out in the field after all. Our mechanical brothers hit them with iron bars "it was sticking", they pour lubricant and water over them then hit them with the iron bar "it was sticking because it was too dry/hot". They sometimes gang up on you to replace it because its not as strong as it used to be "CHANGE IT!"

Its a coil, it works as long as you don't put too much energy into it.

If you put too much energy into it It usually goes O.C. Sometimes it goes S.C. but thats what overcurrent protection is for.





"include your own elec-based humourous quote here"

godfrey,

The simple answer is YES, they typically burn open and they can short to ground.

Whether it is a coil in a relay or a coil in a sloenoid, the coil can fail and it can fail open or shorted.

The name of the game is to find an intermediate relay that is reasonably reliable to protect the PLC Output, relay or solid-state.

What a shame it is to lose a "brick" just because one output failed.

I typically use dry relays (that is, relays that are subject to minimum environmental conditions, such as water or oil) to provide PLC Output signals. It adds a little expense to the project up-front but it pays-off on the tail-end.

And then, in agreement with Dark-Knight, I fuse the output of the relay to save the relay.

Then it becomes a question of knowing when the fuse has blown.

I use indicating fuse-holders.

As Tom says, cycle-time can be a killer.

Paul...

Shouldn't "flywheel" be "free-wheeling"?

Does O.C. mean "Over Current"?
And does S.C. mean "Short Circuit"?

If so, why didn't you simply say so?

Begin Tangent:

The world seems to be turning into a place where only the guy that did the "Match-Box Cars" and "Federal Express" commercials can get on.

What is wrong with slowing down to the point where communication can be understood by hearing (or reading) instead of E.S.P. (Extra-Sensory Preception).

End Tangent:

Hi guys

I usually use low current DC contactors or relay and low current valves if I have valve or contactor more then 2.5W I use relay or solid stat relay.
I always use switched power supply to protect against short cut or over current.
I never have any problem with that only in one case that I had 16 relays who operated
By Omron CQM1 transistor output card. After 2 years the outputs start to burn .I replace all the relays to different model and it happened again.
Someone have any idea?