Relay or controller?

[hashmasta-kut]

OK i went and read a bunch of threads that i didnt glean the info i need from.

i replaced my relay last month, june 20th or so, as it wasnt holding a consistent temperature. at first it was fluctuating from between around 850-1000 when set at 1000, then it started sitting around 260 or 300. anyhow, after replacing the relay it worked fine, so i thought ok thats it.

But now, just one month later, the same thing is starting to happen. i am trying to have it sit at 1050, but it drops to like 920 or so, then rises back up to around 1030 usually, then drops again.

So is this the digital controller maybe? is it killing relays, or is it just another relay that died prematurely? when i had a relay go before, it just got stuck on.... only thing i notice odd otherwise is it doesnt seem to make the buzz sound it usually does when going into a new cycle. or whatever the buzzing indicates, it seems the kiln operates silently, with only clicks, and no buzzes ever like it always has done...

[Skipjack]

I would add the thermocouple to the list of possible suspects.

[hashmasta-kut]

arggh, i was hoping there wasnt more things it could be. I'll go research it, thanks.

[Mike_Aurelius]

Either the thermocouple or the relay, my bet is the relay.

Kiln makers have a tendency to use a smaller relay then they should. Typically, a relay should be rated at about 120% of the amperage the kiln draws, for example, a kiln that draws 14.5 amps should use a 17.4 amp relay (upsized to the next available, usually a 20 amp). What I've seen in practice is they use the closest amperage to the actual draw, in the case of the example, a 15 amp relay.

While it's numerically correct, it overlooks the fact that each time the relay clicks, full amperage is drawn through the contacts, and no allowance is made for repetitive on-off cycles, which is what the additional 20% is for. The difference between a 15 amp and 20 amp relay is the area of the contact point. The larger the contact point, the less resistance to current flow, and therefore, less sparking and carbon build up.

When I was building kilns, I usually used relays that were rated at 150% of the kiln amperage. They cost a bit more, but you get long life and low maintenance out of them.

You also have the option of going to a mercury relay, however, they are very expensive and can be hard to find. They have a life that is rated many times that of a conventional relay. They also have a positioning requirement, meaning that the tubes have to be mounted vertically, otherwise they do not work correctly.

The other option is SSR, but you have to have a controller that has the SSR option available on it, some do, some don't. SSR's will need a large heat sink and the use of heat transfer goo, but will last a very long time.

[hashmasta-kut]

Thanks Mike.


You know i found some brittle wiring on one connection on the relay, where one wires comes from the back, and the other goes to the power switch. That bad connection cooked the relays.

And for Canadian guys, i found overnight relays for a good deal, digi-key.ca

[Mike_Aurelius]

Replace the entire wire if you can, with high temp stainless braided wire, use proper stainless wire connectors, otherwise it will keep on happening.

[brads]

Everything Mike said is right on the money. The two things I would add to what he posted are:

1) Check your power connections within a few weeks after first installing the relay, again a few more weeks after that, and then periodically every 6 months to a year. They can loosen up over time, especially when new. A loose connection can get hot and ruin a relay, or toast the wire used for the connection.

2) Make sure the cycle time of your controller isn't set too short for the relay you're using. If it is, it will cause premature relay failure. Electro-mechanical relays in particular are susceptible to this type of failure.