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 http://www.ambquality.co.uk/power-supply.html

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

On Wed, 19 Feb 2014 17:27:03 +0000, Adrian Brentnall wrote:

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 http://www.ambquality.co.uk/power-supply.html

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

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. )

On 19/02/2014 18:47, mick wrote:
On Wed, 19 Feb 2014 17:27:03 +0000, Adrian Brentnall wrote:

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 http://www.ambquality.co.uk/power-supply.html

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

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. )


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....." g
A

On Wed, 19 Feb 2014 17:27:03 +0000, Adrian Brentnall wrote:

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.

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.

On 20/02/2014 21:37, Cursitor Doom wrote:
On Wed, 19 Feb 2014 17:27:03 +0000, Adrian Brentnall wrote:

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.

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

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.

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.

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

On Wednesday, February 19, 2014 5:27:03 PM UTC, Adrian Brentnall wrote:

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.

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%.

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 http://www.ambquality.co.uk/power-supply.html
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

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

On 20/02/2014 23:09, wrote:
On Wednesday, February 19, 2014 5:27:03 PM UTC, Adrian Brentnall wrote:

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.

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%.

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.


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 http://www.ambquality.co.uk/power-supply.html
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

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

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.


PS whats shown there is full wave rectification

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

Thanks
Adrian

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!

On 21/02/2014 09:02, Cursitor Doom wrote:
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!

Not really....
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