Joseph Gwinn wrote:
In early June 2008 I bought an old Vigor Burnout Furnace (model CA 1065)
for $95 from a seller on Craigslist. The furnace appears to have been
built in 1979, but does work.

The furnace came with a so-called "infinite" control, which is an
electromechanical contraption with ten intensity settings (from mostly
off to always on). What varies is the average duty cycle of power on
versus power off. There is no feedback control. This is far too crude
and clumsy to heat-treat metals, so I decided to add an electronic
furnace controller.

So I bought a 1/32 DIN PID controller and solid-state relay (SSR) from
Omega Engineering. I'm still using the thermocouple that came with the
furnace, but not for long, as the original thermocouple is old and
oxidized and a bit off. But it works well enough for now, and will hold
to within a degree or so of a temperature, far better than is required
in heat treating of say O1 oil hardening steel or A2 air hardening steel.

The big problem was that the original design didn't have a big enough
heatsink on the SSR, and there wasn't enough thermal isolation between
the control box and the furnace box, so the controller and SSR both got
too hot when the furnace went to max temperature (about 990 degrees C).
The controller would get to 50 degrees C, which is right at its upper
temperature limit, which is asking for reliability problems. The SSR
would get almost to 60 degrees C, which is also in its range, but asking
for trouble.

So, for the SSR I got a large heatsink from Omega, which dropped the
temperature to 36 degrees C.

The controller was a bit more difficult. I ended up making a set of
spacers from 0.5" diameter gummy aluminum rod on the lathe, and cutting
two aluminum plates with clearance holes, all to space the control box
away from the body of the furnace, with two parallel plates between,
thermally isolating control box from furnace body. Now, the PID
controller temperature is 35 degrees C max.

At this point, I have declared victory, as 35 or 36 degrees C is low
enough that reliability won't be much reduced.

Joe Gwinn

I've got an 18" 4kW top loading ceramic kiln that I use for heat
treatment and other things and I similarly fitted a temperature
controller to it. It still has the 2 "energy controllers" fitted, one
for each bank but they are normally set to max or off if one bank is not
needed.
I added a thermocouple to the kiln and connector into the box on the
side. The temperature controller is housed in a separate box with tails
out the back and mains plugs on the end so I can use it for controlling
other things if required, it also allows it to be placed away from the
heat. The thermocouple lead currently plugs into the front of the
controller box. The finned heat sink is about 3"H x 3"W x 1.2"D and gets
warm to the touch when running max duty, like when firing to 1200C, but
very acceptable.


One of these days I'll add a temperature controller I have to my small
Gallenkamp heat treatment furnace as the energy controller on that is a
pain, but the thermocouple read-out is accurate from checks I've done.