Kiln control.

I am building (designing at the moment) a small kiln (approx 2kw). I already have the resistance wire and my calcs suggest 110vac is the most appropriate voltage.

To get the 110v I will use a step down transformer. To vary the power i have 3 control options - simple on / off, burst fire or phase angle. The latter two would regulate the 240 v feeding into the step down transformer.

I would appreciate opinions regarding the effect, if any, of feeding the altered waveform into the stepdown transformer - my concern is could it cause the transformer to overheat through inefficiency.

Thanks for reading,

Mark Galimore

A friend of mine built his own kiln using 240v and a simple temperature controller that used a solid state relay (ssr).

So far no problems melting aluminium or brass.

[Edit] Something like this...

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F311998479037

Edited By Brian Sweeting on 14/11/2017 15:41:36

I can't see the point in running on 110Vac. Assuming you're in the UK wind the heating coil for 240Vac. That way you eliminate a large, and expensive, transformer. I agree with Brian, use a simple P(ID) controller and SSR. I bought a single channel temperature controller, thermocouple and SSR on Ebay a while back for about £18. At that price it's a no brainer.

Andrew

Why do you need the transformer? I am also in the middle of making a kiln and am using 240V mains in conjunction with a solid state relay (SSR). The SSR is fed with a pulse width modulated signal derived from a 555 timer chip. This setup works perfectly to alter the power input to the kiln element.

​ I will also use a PID temperature controller with its output used to control the 555 timer through pin 4 of the 555 chip. This gives variable control of the kiln temperature without the need for a transformer.

Doug

Too slow in typing! Andrew has beaten me to it.

Edited By Douglas Johnston on 14/11/2017 15:59:49

In addition to Andrew and Brian's points a transformer isn't a resistive load. They're not really suited to either having either burst fire or triac regulators on the input. However, if you're stuck with a 110v element and already have a transformer, a phase-angle/burst-fire or SSR on the transformer's 110VAC output would be OK.

If I were building a kiln Brian and Andrew's method would be my first choice.

Dave

The simple controller I have drives the SSR directly. It's either on or off. The response time of a kiln is pretty slow so on/off control, with varying mark:space, over minutes is easily as good as "proper" PWM.

Andrew

I hope at least one of you kiln builders is going to write it up for MEW! It's a very long time since anything was described

Please don't ask me to do that, I promised Neil that I would write an article on another matter and that was a year ago and I have not done that yet. I do hope Neil is not reading this since I don't want to be excommunicated from MEW

Doug

GET THEE HENCE TO A PLACE OF CONTEMPLATION!

As long as the time constant for the controlled thing (furnace temperature) is significantly longer than the time constant of the controller all will be fine.

Furnaces take along time to heat up and cool down so almost any control method will work.

I would not advise using a triac controller ahead of a transformer. It can be done...Hewlett Packard did it years back as a preregulator on a fancy power supply....but you must use a controller that ensures that the phase angle on both halves of the waveform is the same. If you don't, you get DC in the transformer primary. The ordinary sort of triac controller does not maintain symmetry very well. I did try it once to see how it would work out for controlling a battery charger, but the transformer was not very happy about it. so I desisted.

John

eBay would seem to have ready made packages for kiln control at very attractive prices. I would definitely have a rethink about using a transformer and look at a resistance wire configuration for 240v. Unless you have one, a 2kVA transformer is a lot of money and I am sure resistance wire would be much cheaper.

Mike

Doug,

Have a careful look at the specs for the ebay PID controllers. Most of them will accept a type K thermocouple input which is good to about 1300C. However, they are often supplied with a TC whose insulation will only take a much lower temperature so they are advertised to reflect this.

HTY,

Rod

I must I must admit that I only had a cursory look at eBay but is does seem that the choice of thermocouple is the limiting factor to the range. Heating elements seem very cheap on the face of it, longevity is unknown though. It certainly seems the basic components of a small muffle furnace would not cost too much. Is it time for an MEW muffle furnace to be built?

Mike

Edited By Mike Poole on 16/11/2017 00:02:32

I did wonder about that Rod and I looked up a data sheet on these PID's. That indicated they were available with different input temperature ranges and each range had its own part number. I can't understand why they would do that if they were suitable for different ranges. Can anybody who has one marked for 400 degree C tell us if they are happy to work perfectly well above this value?

Doug

Can anyone tell me why I cannot see the posts by Douglas Johnston, please? They are totally invisible, only known to exist from quotes and the 'lastest forum posts' list!

.

At a guess ... You have inadvertently activated 'Ignore member'

MichaelG.

Surely if one clicks accidentally on the ignore button there is a confirmation request before that poster is ignored?

I see no way of unblocking, if that were the case. I've never blocked anyone, to my knowledge. And usually if a post were to be blocked, the missing posts would be indicated (for continuity), even if the post were missing? Forum has some peculiar quirks, it seems.