Kilns again!

[Adrian Brentnall]

Hi Folks

Still working on that DIY glass kiln - done all the nasty itchy stuff (it's lined with ceramic fibre - think fibreglass roof insulation & then some!)

So the kiln controller is powered by AC, from a 12 - 0 - 12 v centre-tapped transformer - which is half-wave rectified on-board the module to give +12v.

My other bits of circuitry (the thermcouple conditioner and the famous latching relay / logic module) wants 12v dc, plus a stabilised reference voltage in the region of 10v. Will add suitable c's to take any relay switching spikes off the 'instrumentation' dc line.

Both modules share the same 0v line.

Can anybody see any snags in running all the electrics from the same centre-tapped mains transformer (it's got sufficient current available)

- simply by adding another half-wave (2 diodes) circuit, smoothing cap & so on...

- see diagram here

I had bought two c/t transformers, but if I can get away with just one then that frees up some space in the enclosure.

Thanks Adrian

[mick]

From a basic electrical view it will work fine. Just make sure that all coils are suitably suppressed - you don't want back-EMF spikes flying round when they open.

(Note that using a full-wave rectifier system like that the DC current available from the transformer is equal to the AC current rating, so you may have even more spare capacity than you expected. :) )

[Adrian Brentnall]

HI Mick Thanks for the reassurance... Yes - I've addressed the back-emf from the relays & contactors - the smaller relays come 'ready-suppressed' (they're din-mount, complete with diodes & led indicators). The contactors are going to get an MOV each... and the actual controller module seems to have its own mov's built-in...

"Nothing can go wrong....." A

[Cursitor Doom]

I use reasonably beefy snubber diodes (reverse-biased) in parallel with any large, switching inductances to short out the spikes, myself.

What temp are you hoping to achieve inside this kiln and what's it for, btw? I was thinking of making one myself as part of the process for hardening medium carbons steels.

[Adrian Brentnall]

There's a mix of 12vDC "control" relays and 240vAC 'grunt' contactors in this control box (2 boxes, actually)

The 12v relays are DIN-rail mounted, and very civilised as they come with their own diodes / leds etc.

It's the mains-driven contactors I'm more concerned about - but I have some MOVs which I'll be connecting across the coils, to hopefully reduce any spikes when they drop out.

This one's for fusing / slumping glass. Probably no more than 800 centigrade, or thereabouts - with my current (commercially-built) 14" kiln I get a 'full fuse' between two pieces of glass at an indicated

780C, bend glass over a former at about 660C or tack it together at 720C.

The commercial kiln is lined with firebrick, which means it takes a long time to heat up & cool down. The new kiln (2ft x 4ft) is lined with ceramic fibre, and has 2 x 3kw heating elements - so it should be able to warm up (and cool down) faster.

Only trouble is - reorganising the workshop to get the beast in has resulted in quite a lot of tidying (2 van-fulls of rubbish to the tip today!)

- so the project's taking a little longer than expected....

Shouldn't be too difficult to make a kiln for heat-treating metal - one of the gurus in the USA who helped with the design ideas for my kiln said 'A kiln is just a box that gets hot inside'...

Adrian

>

[meow2222]

12v transformer hw rectified would give 12x1.414 -0.6v = 16.4v on full load. No load would be plus transformer regulation of anything from 5% - 33%.

Should be ok, but if you're feeding anything sensitive I'd use a voltage regulator.

PS whats shown there is full wave rectification

NT

[Adrian Brentnall]

Yup. The module specifies 12vAC - so we'll hope that it's OK. I might check it against the other (commercial) kiln - which uses the same module - see what AC input that transformer gives.

Yes - the AD594 doesn't seem to mind too much - but there's a need for a reference voltage of about 8.5 - 9v for the 'over-temperature' facility

- so I might just regulate the whole lot, apart from the relay power.

You're right.... I was thinking 'half a diode bridge' and making that mistake....

Thanks Adrian

[Cursitor Doom]

Would it not be an awful lot cheaper to run this thing off kerosene or bottled gas? Your electric bills are going to be on the HIGH side!

[Adrian Brentnall]

Trick is - unlike a pottery kiln, the thing isn't running 100% of the time during the process... ..typically (with my old kiln - rated 3kw) a 12-hour firing cycle only uses about 4 - 5kWh - as the controller pulses the heating elements on/off in order to achieve a sequence or 'ramp / hold' over time...

The new kiln has twice the electrical power of the old one, but at least

5 x the capacity (in terms of square feet) and a much smaller thermal capacity (so I'll not be wasting electricity in heating up a massive set of firebricks...

..at least - that's the theory! Adrian