PID with both SSR and CONTACTOR

**proto5** · 01-21-2010, 10:12 AM

After reading 25 pages of the Oven Building forum, I haven't come across this idea.....We all love the smooth operation of an SSR..what with no lights dimming, contactor clattering, and perfect temp control. The only negative I've seen is if an SSR fails it can fail in the closed (ON) condition, which may be a safety issue.
Why not add a contactor after the SSR that is activated thru a small relay's NC contacts when the oven is switched on? Then you could use one of the PID's alarms to energize the relay and then open the NC contacts- deactivating the contactor if the SSR fails and an over temp condition is sensed. Seems to me it would be the best of both worlds....and the contactor should last almost forever with it only cycling once per operation. Or am I missing something

For those of you that haven't experienced the joy of an SSR- man are they cool. My oven easily holds temp within ONE degree.

**ed_denu** · 01-21-2010, 10:44 AM

That's an interesting idea, but how about this. Take the pids ssr output that normally goes to the ssr and direct it to the J1 COMMON contact of the pid. Then connect the J1 NC contact to the ssr input. Set the high alarm to whatever, and if the high alarm activates, the power to the ssr is interrupted. For low priced pids this may need another relay as I don't believe the pid alarm contacts are latched. Setting AL1 to a very low number might circumvent this. I think it would work fine on the Watlows, etc.

**proto5** · 01-21-2010, 10:54 AM

Ed-
If the SSR fails closed- interrupting it's control input won't turn it off....the only way to cut power to the elements seems to be a contactor?

**ed_denu** · 01-21-2010, 11:03 AM

Good point, I was only thinking about the over temp. The other option would be a loud alarm.

**SCOTTRODS** · 01-21-2010, 06:11 PM

It's actually a good idea from the get go. I've thought about it before, but I have been "OK" with the contactor on my oven so far,.... I don't get excited about the contactor noises and stuff,... I can go in the house (from the garage) and if I forget about the oven being on, the contactor reminds me it's still there..... But I do wish for better control and a solid "fix" for the possibility of a bad SSR failing closed..... This actually does the job.... I just never put it together like this. Thanks for the post.