AC snub?

[roboteacher]

I have a customer that is having an issue with a relay staying closed. They are using this relay to allow their customer to get a "go" signal to another piece of equipment. Sometimes the secondary signal is DC and sometimes it's AC. It doesn't always happen. They've tried solid state relays as well as conventional relays. If the relay gets "fused" closed, they have to remove it and then re-install it. There is 0 volts appearent at the coil on the relay but the signal is holding in. If they were simply allowing the secondary machine to switch a DC signal, I'd tell them to put a diode accross the load side of the relay to essentially bleed of the excess current but I don't think that will work with AC.

Is there a solution or idea for them to try. They have in some instances had to relay stay "fused" and had to completely replace it.

Thanks

 

[timothy48342]

What is being switched? Is it possible that the equipment being switched is drawing so much current that the internal contacts of the relay are getting welded together?

What is the current draw on the equipment compared to the maximum rating for the relay?

-tim

 

[roboteacher]

it's typically just signal to "start cycle" of the accesory. I actually thought the same thing but it even happens with solid state relays. I'll find out the max current draw and let you know.

 

[duke37]

DC relays can stick due to retained magnetism and a little brass screw is sometimes fitted to stop the armature closing fully. This could be adjusted.

 

[roboteacher]

that could be what going on with regards to the mechanical realy but what about the solid state relay failing? They're going to get me all the pertinate data so I can look deeper in to it.

 

[timothy48342]

I can't think of one thing that would cause both the solid state relays to fail and the relay-style relays to fail as well, and for both to fail in the ON position. But failure is unpredictable. If you are in fact driving them beyond their specifications, then fix that.

Sometimes you can drive a device beyond it's specs with some success, but your out on a high wire with that. How a device might fail is a lot less predictable then whether it fails or not.

How much current these relays are passing though, I think is one big question, then if that is not an issue, tell us everything. We (I) don't mind reading. Spill it all. Maybe something you say will trigger something in someone to get at the heart of the problem.

--tim