Saturday, November 29, 2008

Heat-Treat Furnace Controller

The furnace controller is complete, all the bits fit into an ATX PS case. It has two safety features to avoid electrocution, as the furnace heating elements, made from kanthal wire, are conductive and therefore pose some hazard.

1.) Since each leg of a 220V circuit is "hot", each pole is isolated by its own solid state relay. I also chose solid state relays because of reliability, and better heat regulation - with an SSR the furnace controller can be configured to switch on/off much more often which would otherwise cause excessive wear to a mechanical relay's contacts.

2.) The relays control terminals are interrupted by a microswitch which I plan to fit to the door latch as an interlock. Any time the door is open, the coils will be switched off, yet the controller will still be on.

I had originally built the furnace frame using galvanized angle iron and sheet. Given the higher temperatures this device will operate at, I didn't want to risk any of it being burned off; zinc is a toxic metal and the fumes will kill you. It probably wouldn't burn off, but I decided to go on the safe side and rebuild the frame after dissembling it and dipping the the various parts into concentrated hydrochloric acid. This dissolves the zinc coating and leaves the metal bare. Since bare steel will rust very quickly (especially if heated!) I coated it with Rustoleum high heat BBQ paint. This can supposedly stand up to 1200 F, so it should work.

The last order of business is the door construction. The door will contain several 2,300F firebricks of the same type as used in the oven, and for a gasket material I will be using ceramic fiber blanket cut to shape.

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