DIY Kiln Contoller

I keep meaning to design and build a posh one but so far I've not found the time. The principle isn't hard and a kiln is a relatively benign load compared to motors and other nasty inductive things.

Dave

"Dave {Reply Address in.sig}" wrote in news: snipped-for-privacy@robinton.llondel.org:

Hi Dave, go on have a go !

what are the basic components needed? some sort of programmable voltage controller ?

cheers

Rob

In its simplest form it's just a triac per element (three elements on all the kilns here) and a small microprocessor to turn them on as required. Obviously there's a lot more work than just that.

Dave

I'm also interested in this, and have gone so far as rounding up parts but haven't built anything yet.

For electric kilns, there are two basic ways of controlling the heat. The simplest one is like that used by the "infinity" controllers you find on an electric range: When the temperature falls below a setpoint, they turn on the element; when above the setpoint they turn it off.

The more sophisticated kind is proportional, so that the current to the element is proportional to the temperature. This is done by turning the current on only for some portion of each mains cycle. The elements see a lot of partial waveforms that average out to a lower effective current.

The first kind is easy to build, but due to thermal cycling is reputed to be hard on certain kinds of kiln element, namely silicon carbide "glow-bar" elements, which are what I'm using. (It's a home-built kiln.) So I need to use the fancier proportional control.

However, for most kilns, the simple inifinity control may in fact be better since it produces less noise on the power lines. (I have a huge transformer to help block that.) The basic idea is that you have a comparator that looks at a setpoint and a thermocouple and toggles the current via an opto-isolated TRIAC or something similar. (Mercury relays were very popular for this in the past, and may still be the best choice in some applications, since they have less losses than TRIACs.)

Then the setpoint is placed under computer control, to set up differetn firing schedules, and there you have it! In fact, depending on the computer, you may want to have it read the thermocouple, do the comparison, and activate the TRIAC itself. The you can have it plot a graph of the thermocouple temperature vs time, integrate to find an approximation to heat work, etc.

Someday, just as soon as a few other projects get out of the way.... ;-)

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Hi Rob,

I've been giving just such an idea a lot of thought and testing of late.

The route I'm currently taking is perhaps a little odd, but here goes.

#1 A digital Multimeter with a K-Type thermo-couple and a serial connection to a computer.

#2 A Solid State Relay for each of the 6 elements in the kiln. Each SSR is triggered On/Off thru the parallel printer port of the computer.

#3 Some Visual Basic code to read the thermo-copule gauge, compare the temp to a firing schedule, then adjust the PWM rate being sent to each SSR, after being filtered thru some proportional derivative software.

#4 For Safety, the kiln still has its old Kiln Sitter installed set a temp just above the peak in the firing schedule.. (to prevent over-temp if/when the computer decides to lock up!)

To date I've had great success controlling a simple 110V toaster oven :0 I'm currently researching a source for the 220v SSRs needed to control the kiln...

Take Care, James Lerch

(My telescope construction, Testing, and Coating site)Press on: nothing in the world can take the place of perseverance.Talent will not; nothing is more common than unsuccessful men with talent.Genius will not; unrewarded genius is almost a proverb.Education will not; the world is full of educated derelicts.Persistence and determination alone are omnipotent.Calvin Coolidge

Hi all,

Fab - this thread looks like its yeilding some fruit !

Dont know how many of you are from UK - but there is an old computer here called a BBC micro which has a built in 8 bit 4 channell A/D convertor and is simply programmed using an old style basic. Although old - these machines never fall over (in my experience... touch wood) and are quite happy sitting in sheds - also people now give them away .... even better!. Its a simple enough task to take the V output from a thermocuple, read it and then generate a control signal... to ... something.... I must admit I dont know what a triac is ... but I'm just about to google and find out -

Would any of you guys like to share the designs and rationale behind the controllers you are thinking of or have built ?? Can we post jpg's or files to this site ??? if not I could put up some webspace for the sharing of circuit diagrams and programs. if any of you are happy to do this I will supply an email address to this thread.

Anyway thanks for the info and inspiration - I'm off to find out what the hell a triac does. hmmm I see a possible world full of cheap, efficient, programmable home made kiln controllers !

:-)

I like the idea of using old PCs for this. The BBC micro sounds ideal. (I'll be using an old PC/XT with an add-in A/D board.)

One caveat: If you are trying to read the thermocouple directly, you will need some "signal conditioning" before it goes to the A/D. The A/D no doubt has a range of 0-1V or more, whereas the thermocouple range is more like 0-50 millivolts. So you definitely need some gain here. But this must be done *very* carefully, since simple DC amplifiers tend to have drift in their outputs that would be confused with temperature changes. So you will probably want a slightly more exotic "chopper stabilized" op-amp.

My approach will be a variant of this. I will use standard CMOS switches to alternately send either a normal or inverted version of the amplified thermocouple voltage to the A/D. Besides the thermocouple signal, I will also send a temperature-stable reference voltage, also in normal and reverse polarity. By judicious use of these values, all the drift can be compensated.

A TRIAC is the same gizmo inside a standard lamp dimmer. If you know what an SCR is (Silicon Controled Rectifier), it's just two of those wired back-to-back internally. The idea is that when the incoming AC line voltage rises above some threshold, the SCR for that polarity is fired (by a little trigger circuit), after which the SCR will stay on for the rest of that half-cycle. When the polarity reverses, the same thing happens on the opposite-polarity SCR.

To get a proportional control like a lamp dimmer, you just change where the trigger point is in the AC half-cycles: Earlier and you get more of the power, later and you get less. To have the whole thing act like a simple switch (Solid State Relay), just fire the trigger right at the zero-crossing.

I'd be glad to share whatever I come up with. Can't post JPGs here. I thnk such things are usually posted to some .alt.binaries NGs that exist just for this purpose. though a Website has the advantage that it keeps related stuff together, and can include descriptive text pages, etc.

I suspect this world will be on the small side, consisting only of electronically-inclined potters. But we'll have fun!

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Oops! Just re-read your post regarding the A/D converters. I'm afraid 8 bits won't do the job, since it only encodes 256 levels. If you scaled the thermocouple range to fit this, you'd end up with several degrees per A/D step. True, that may be closer than you'd control it manually, but it would be better to use something like a 12-bit A/D (4096 levels) or better yet 16 bits (65536 levels).

One of the benefits of my complicated chopper scheme is that the signal output from there to the computer is AC, so it can be monitored with a cheapo 16-bit sound card. (Sound cards don't pass DC or low frequency signals like thermocouples put out.) Plus, as a side benefit, it is easily expanded to any number of input channels just by using CMOS switches with more channels.

When I get this ready, I'll post full info with schematics and circuit board layout on my site and announce it here. Probably not any time soon, but it's definitely in the works!

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

To me, a controller running the schedule, with a kiln-sitter switching it off is the best of both worlds: the convenience of a controller with the heat-work reaction of a cone. This is exactly how I fire my electric kiln and it can't be beat!

Steve Bath UK

Look at

I built a single zone controller for a test kiln using one of their controller boards. Was not too hard to do. Hardest part was making the enclosure.Bartlett offers plans at the website.

Hi all ,

Been away for a while.

I hope some of you electonic potters out there are scribbling and soldering away! If any of you want to share your circuits and ideas my offer of putting up a wwwpage is still good - you can email me at soyaheartreplica@ aol.com (sans spaces) with plans and txt and I'll put up a simple page per design.

I'm not to sure about having my kiln controlled by a chopper tho'... that means something else here in UK ... :-)

Come on Ian, Don, Bob and James let me know your thoughts please - I want to build something!

cheers all

R

"ian" wrote in news:xek%b.31907$% snipped-for-privacy@twister.southeast.rr.com:

What was your design Ian ?