solid-state relays - how to spec ?

[Martin Brown]

I don't know if it is still a vulnerability of high power SSRs these days but read the signal drive specification very carefully. We had some early ones self immolate due to internal dissipation caused by incorrect current voltage characteristics of the control circuit drive signal. I forget the details but remember the fire. When driven correctly they were fine and long lasting with no moving parts.

You might want to add some primitive failsafe interlock to prevent it going crazy in the event of controller failure and continuous heating.

[Adrian Brentnall]

Thanks for the warning ! The controller I'm going to use for starters just puts out 12v (in the original design it operates a standard electro-mechanical relay) - in a very brief scan of some SSRs they seem to be fairly un-fussy about their drive requirements - somewhere between 4v and 30v - but I'll read the small print.

Funny how these things stick in your memory, isn't it ?

Sounds good

Yes - an independent thermocouple & 'hi-temperature' limit seems favourite at the moment - with the ability to drop out a big contactor and take the power off the heating elements. The commercial controller that I'm using isn't clever enough to say 'I've had the heat on for 30 mins now, and the temperature isn't increasing, perhaps I should stop' - as I know from personal experience when the thermocouple failed!

The whole thing's a project for 'after September' - when things have calmed down in the shop and I'm not up to my ears on 'production'.

Thanks

[dennis]

Or kiln sitter as the OP said?

I looked at some proportional power controllers that would handle 40A recently. I think they were on CPC or RS.

One of those and a RPi to monitor the temp and set the power would probably do the job far better than switching the heaters on and off.

Something like

or

I would fit a thermal fuse to trip the whole thing if the kiln gets too hot. I know you can get them for furnaces but I have no idea where.

[Harry Bloomfield]

dennis@home was thinking very hard :

I agree, proportional control would be much better than straight on off. Switching elements on and off causes much more wear and tear on the elements, than modulating them.

[Adrian Brentnall]

Perhaps on the Mk II version! Don't want to run before we can walk

[docholliday93]

The ovens I use which don't have this mormally have a thermal fuse instead, which can be a pain to replace if it goes. Not as much of a pain as the chamber which for some reason missed out and suffered thermal runaway, though... Several of them also have interlocks to prevent you opening the door if the temperature is too far from ambient.

[dennis]

Well if you fit the controller you can loop the control input via the stat to make them into on off switches.

Then add the proportional control later.