Foundry Electronics

Homemade Foundry - Element and Power Controller

DISCLAIMER: The electronics of this foundry are potentially deadly. It likely fails to meet multiple electrical codes. I am not a certified electrician nor am I a professionally licenced engineer. I am NOT providing this information as advise, simply sharing my methods and experience. If you are not comfortable working with high voltages and currents, this project is simply not for you. Proceed at your own risk!

Element: I had the heating element made for me at a local pottery supply store that makes them for electric kilns. I told them about the application; that I needed the coil OD to be ¼", the overall resistance to be 14Ω, and that the length needed to be about 6' (I ended up having to stretch it out to be about 15' later, which wasn't an issue). It was ready the next day and cost a mere $25 Canadian. In the end, the furnace draws about 17.5 amps at 220VAC, 60Hz. I've heard about people making their own elements, with or without success, but I can't see how it's worth the trouble considering how cheap and good mine has been. I'm still on my original element, and I would guess it has operated for a total of 25 hours over approximately 10 casting sessions.

The hot element

Power Controller: The foundry has a solid state controller to control its temperature. It is based around a modified light dimmer, and I built it as per instructions in Dan Hartman's book. I used two TO-218X 55A SCRs and triac from Littelfuse as free samples and bought the box, light dimmer, and wiring from Home Depot. Everything else came from stuff lying around. The SCRs are mounted on heat sinks but do not have fan cooling. So far I've noticed the box get slightly warm but haven't had any problems with heat.

Finished controller

I've found that the controller does not actually allow for much adjustment; it jumps from about 50% power to 100% over a really tiny spot on the control dial. It still does allow you to let the element and bricks warm slowly though to avoid sudden thermal shock, which is the main reason I built it. I expect that further modifications to the light dimmer would allow for more adjustments, but I haven't bothered.

I made a simple mistake when I first assembled the circuit, causing one of the SCRs to exploded the first time I powered it up. This freaked me out a bit, especially when I couldn't find a problem with my wiring. I exchanged a few emails with Dan about it, and he pointed out that the mounting tabs on the SCR's are not isolated, meaning they have to be on separate isolated heat sinks. This would have stopped the controller from working, but shouldn't have destroyed anything.

The SCR that blew

After a lot of head scratching, I turned my focus to the modified light dimmer. The only modification is exchanging the 110 VAC triac with one rated for 220VAC. It finally dawned on me that unlike the original triac, the new triac was also not isolated. Being mounted on the grounded light dimmer body, it was shorting out directly to ground. After putting it on its own heat sink, the controller started working as it should. Be sure to know if the mounting tabs on your components are isolated or not, and put them on their own heat sinks if necessary.

Dan kindly gave me permission to reproduce his schematic and post it here, but honestly I can't figure out how the circuit actually works so I would prefer not to put myself behind it. If you wish to build a solid-state controller per this design, my suggestion is that you visit Dan's website and purchase his guide on building the foundry.

The Terminals: There are two pieces of stainless steel threaded rod that pass through the foundry wall to power the element. Stainless steel hardware is used for its improved heat and corrosion resistance, as it gets orange-hot when the foundry is hot. I simply trimmed the foundry shell away from the terminals after I had cast the refractory and stuck them straight through the cast refractory. Nuts and washers on either end of the rod keep it in place; there is no other electrical insulation around the terminals. It probably doesn't meet electrical code, but it works for me.

As the terminals are at 220VAC, coming in contact with one of them could easily kill the average person. To decrease the danger, I completely enclosed the terminals by cutting holes in the backs of two electrical outlet boxes and using them to cover the terminals. They are held in place with "tapcon" screws. The main power enters the bottom box and connects to one terminal, and then a single wire runs to the other terminal. I pulled the wires out of a short piece of BX armoured cable and used the empty sheath to enclose the connecting wire between the boxes.

Terminal enclosures (Click for larger view)

The heat inside these boxes is incredible, as the outside of the foundry reaches well over 100°C. I've already had a problem where the insulation burned off the wires and one wire came loose from the terminals. I redid all the wires within a foot of the foundry, and this time used wire rated for a higher temperature. Finally, I put two layers of CSA approved heat-shrink tubing rated to 125°C over all the wiring. I'm not sure if it'll be a long term solution, but it's lasted for about 8 hours of use so far and hasn't shown any signs of damage. If it burns again I'll have to look into high-temperature rated wiring or change how the terminals are enclosed.

The other side of each terminal is connected directly to the element. The element wraps around the terminal and is held with a nut. I've had problems with the element overheating at the connection and snapping. I've been able to shorten it by about half an inch and reconnect it, so it hasn't been a big deal.

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