In article ,
David Billington wrote:

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.

[snip]

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.

[snip]

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.

The Vigor furnace is rated at 1.6 KW, being 14 amps at 115 volts.
However, the heater coils are old, and only draw 11 amps at 113 volts,
or 1.24 KW. At full power (with no voltage drop in the controller),
after closing up a bunch of heat leaks, it gets to 1000 degrees C.

If I understand, you electronically control only one of the two banks,
the other being on or off as needed, so the electronic controller
handles 2 KW at 220 volts.


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.

I thought of having a separate box, but all-in-one is more convenient,
and the commercial furnaces have the same scheme that I used, thermal
isolation using a set of parallel plates and spacers, so I knew it would
work.


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.

Operation at 220 volts cuts the current in half. A solid-state relay
(SSR) has a constant voltage drop, being 1.2 volts for the SSR I'm
using, so half the current is half the power to be dissipated in the
heatsink. This (and the distance from the furnace and its heat)
explains the difference in needed heatsink size.


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.

For the record, the old thermocouple that came with the Vigor is made of
14 guage chromel-alumel (Type K), appears to be original from 1979, and
reads about 5 degrees C low. It's still perfectly useable.

Joe Gwinn