HI Folks
Some advice please.
I'm working on the control gear for a homebuilt glass-fusing kiln.
It's using a commercial digital controller, which provides a switched
output @ 12v DC.
Final load is two resistive wire elements, each rated @ 240v / 15A.

I can either feed the controller output through a small relay, to
provide a switched 240v AC, or source a high-current Relay / Contactor
that will accept the 12v. At the moment, I'm leaning towards the 'slave
relay' approach, as I don't know what current the control board can supply.

I'd rather not go the fully electronic route with a solid state relay
for the final device - I rather like things that go 'clunk' grin.

Looking at CPC, high powered relays are available, as are contactors,
but the relays tend to be cheaper. What's best in this application? - is
a contactor 'overkill' - or berely 'prudent' ?

Thanks for any informed comments.
Adrian

Adrian Brentnall wrote:
I'd rather not go the fully electronic route with a solid state relay
for the final device - I rather like things that go 'clunk' grin.

Dad used solid state relays on his kiln controller, instead of going
"clunk", the specialist fuses that protect the relays liked to go "fot"
unfortunately they were more expensive than the relays themselves.

I think he did eventually find a previous owner had incorrectly wired
the elements, and now it's reliable.

On 01/01/2014 16:19, Adrian Brentnall wrote:
HI Folks
Some advice please.
I'm working on the control gear for a homebuilt glass-fusing kiln.
It's using a commercial digital controller, which provides a switched
output @ 12v DC.
Final load is two resistive wire elements, each rated @ 240v / 15A.

I can either feed the controller output through a small relay, to
provide a switched 240v AC, or source a high-current Relay / Contactor
that will accept the 12v. At the moment, I'm leaning towards the 'slave
relay' approach, as I don't know what current the control board can supply.

I'd rather not go the fully electronic route with a solid state relay
for the final device - I rather like things that go 'clunk' grin.

Looking at CPC, high powered relays are available, as are contactors,
but the relays tend to be cheaper. What's best in this application? - is
a contactor 'overkill' - or berely 'prudent' ?

Thanks for any informed comments.
Adrian

Aren't contactors just relays with extra bits to drive high inductive loads?

Being a resistive load a relay should be fine.

Adrian Brentnall wrote:
HI Folks
Some advice please.
I'm working on the control gear for a homebuilt glass-fusing kiln.
It's using a commercial digital controller, which provides a switched
output @ 12v DC.
Final load is two resistive wire elements, each rated @ 240v / 15A.

I can either feed the controller output through a small relay, to
provide a switched 240v AC,

Double pole relays rated at 240V/15A per pole or better will do it. Use
both poles in parallel. I made a gadget for switching the caravan
electrics automatically between two supplies using relays, and it's been
fine for 10 years.

Bill

In article om,
"dennis@home" writes:
On 01/01/2014 16:19, Adrian Brentnall wrote:
HI Folks
Some advice please.
I'm working on the control gear for a homebuilt glass-fusing kiln.
It's using a commercial digital controller, which provides a switched
output @ 12v DC.
Final load is two resistive wire elements, each rated @ 240v / 15A.

I can either feed the controller output through a small relay, to
provide a switched 240v AC, or source a high-current Relay / Contactor
that will accept the 12v. At the moment, I'm leaning towards the 'slave
relay' approach, as I don't know what current the control board can supply.

I'd rather not go the fully electronic route with a solid state relay
for the final device - I rather like things that go 'clunk' grin.

Looking at CPC, high powered relays are available, as are contactors,
but the relays tend to be cheaper. What's best in this application? - is
a contactor 'overkill' - or berely 'prudent' ?

Thanks for any informed comments.
Adrian

Aren't contactors just relays with extra bits to drive high inductive loads?

Being a resistive load a relay should be fine.

Contactors are relays which can pass higher currents on the NO contacts,
because the high magnetic coil force is used to generate a high contact
pressure. Whilst a contactor can also have NC contacts, they can only
pass a lower current because the contact force is just from the return
spring.

Relays generally have NO and NC contacts identically rated, and therefore
they can't go up to the same current ratings and life expectancy.
The coil is significantly lower power, and the whole thing can therefore
be much smaller.

I would go with a contactor for your application (assuming the kiln is
something that fires for some time, like a pottery one).

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On 01/01/2014 16:31, Andy Burns wrote:
Adrian Brentnall wrote:
I'd rather not go the fully electronic route with a solid state relay
for the final device - I rather like things that go 'clunk' grin.

Dad used solid state relays on his kiln controller, instead of going
"clunk", the specialist fuses that protect the relays liked to go "fot"
unfortunately they were more expensive than the relays themselves.

I think he did eventually find a previous owner had incorrectly wired
the elements, and now it's reliable.


HI Andy
Thanks for your comments. I've always been a bit wary of high-power
semiconductors protected by expensive fuses. Many years ago at
university we used a big homebrew lighting rig, and when the explosive
fuses blew in that (which they did fairly often, with a 'crack' like a
gunshot and a bright purple flash), they often took out the triacs as
well...

I'm sure that ssr's should be more reliable, but the luddite in me likes
an electro-mechanical device - if for not other reason that you can hear
it working...

On 02/01/2014 00:03, Andrew Gabriel wrote:
In article om,
"dennis@home" writes:
On 01/01/2014 16:19, Adrian Brentnall wrote:
HI Folks
Some advice please.
I'm working on the control gear for a homebuilt glass-fusing kiln.
It's using a commercial digital controller, which provides a switched
output @ 12v DC.
Final load is two resistive wire elements, each rated @ 240v / 15A.

I can either feed the controller output through a small relay, to
provide a switched 240v AC, or source a high-current Relay / Contactor
that will accept the 12v. At the moment, I'm leaning towards the 'slave
relay' approach, as I don't know what current the control board can supply.

I'd rather not go the fully electronic route with a solid state relay
for the final device - I rather like things that go 'clunk' grin.

Looking at CPC, high powered relays are available, as are contactors,
but the relays tend to be cheaper. What's best in this application? - is
a contactor 'overkill' - or berely 'prudent' ?

Thanks for any informed comments.
Adrian

Aren't contactors just relays with extra bits to drive high inductive loads?

Being a resistive load a relay should be fine.

Contactors are relays which can pass higher currents on the NO contacts,
because the high magnetic coil force is used to generate a high contact
pressure. Whilst a contactor can also have NC contacts, they can only
pass a lower current because the contact force is just from the return
spring.


Ah - that's interesting - thanks

Relays generally have NO and NC contacts identically rated, and therefore
they can't go up to the same current ratings and life expectancy.
The coil is significantly lower power, and the whole thing can therefore
be much smaller.

I would go with a contactor for your application (assuming the kiln is
something that fires for some time, like a pottery one).

A typical firing cycle will be three or four hours, but, as you're
aiming for a particular profile of ramp-hold sequences over time, the
elements aren't 'on' for the whole of that period.

I'm torn between installing the whole control in 'a box', or using a
stripped-down consumer unit, which would have the advantage that
din-rail-mounted contactors and mcb's can be easily mounted. I may even
use a separate box to house the controller, its mains transformer, the
over-temperature 'watchdog' and the low-power relays - leaving the 'real
power' stuff in a separate CU..

Thanks
Adrian

On 01/01/2014 20:47, Bill Wright wrote:
Adrian Brentnall wrote:
HI Folks
Some advice please.
I'm working on the control gear for a homebuilt glass-fusing kiln.
It's using a commercial digital controller, which provides a switched
output @ 12v DC.
Final load is two resistive wire elements, each rated @ 240v / 15A.

I can either feed the controller output through a small relay, to
provide a switched 240v AC,

Double pole relays rated at 240V/15A per pole or better will do it. Use
both poles in parallel. I made a gadget for switching the caravan
electrics automatically between two supplies using relays, and it's been
fine for 10 years.

Bill
HI Bill
I'm always a bit nervous of the 'commoned-contacts' approach, as
Murphy's Law suggests that one set of contracts will operate slightly in
advance of the other set, and will end up making or breaking the lion's
share of the current.

I'm deliberately over-engineering this project! (Partly because I want
it to be reliable, and partly for safety reasons - as it's a box of
heat, operating unsupervised, in a timber workshop.
For this reason I'm forgoing the usual 'let's see what's in the junk
box' approach, and speccing / buying new components. Call me paranoid!

Thanks
Adrian

On 02/01/2014 07:39, Adrian Brentnall wrote:
HI Bill
I'm always a bit nervous of the 'commoned-contacts' approach, as
Murphy's Law suggests that one set of contracts will operate slightly in
advance of the other set, and will end up making or breaking the lion's
share of the current.

I'm deliberately over-engineering this project! (Partly because I want
it to be reliable, and partly for safety reasons - as it's a box of
heat, operating unsupervised, in a timber workshop.
For this reason I'm forgoing the usual 'let's see what's in the junk
box' approach, and speccing / buying new components. Call me paranoid!

Why not look for a solid state power controller..

You can get a 50A rated one for about £50.

They come in mark space control and in phase control.
I think you want mark space for your application.

The power follows a DC input voltage of about 0-5V so you can drive it
with a Pi or similar.

Don't forget to fit a heat rise fire detector to trip the supply.

In message , Adrian Brentnall
writes
HI Folks
Some advice please.
I'm working on the control gear for a homebuilt glass-fusing kiln.


I'm interested in the homebuilt kiln, do you have any links?

Thanks

Chris French

On 02/01/2014 12:27, chris French wrote:
In message , Adrian Brentnall
writes
HI Folks
Some advice please.
I'm working on the control gear for a homebuilt glass-fusing kiln.


I'm interested in the homebuilt kiln, do you have any links?

Hi Chris
Drop me a direct email (the email address is valid).
I've got lots of links from the research I did last year -
but it all depends on what you're wanting to do with it

Adrian

On 02/01/2014 07:27, Adrian Brentnall wrote:
On 01/01/2014 16:31, Andy Burns wrote:
Adrian Brentnall wrote:
I'd rather not go the fully electronic route with a solid state relay
for the final device - I rather like things that go 'clunk' grin.

Dad used solid state relays on his kiln controller, instead of going
"clunk", the specialist fuses that protect the relays liked to go "fot"
unfortunately they were more expensive than the relays themselves.

I think he did eventually find a previous owner had incorrectly wired
the elements, and now it's reliable.


HI Andy
Thanks for your comments. I've always been a bit wary of high-power
semiconductors protected by expensive fuses. Many years ago at
university we used a big homebrew lighting rig, and when the explosive
fuses blew in that (which they did fairly often, with a 'crack' like a
gunshot and a bright purple flash), they often took out the triacs as
well...

I'm sure that ssr's should be more reliable, but the luddite in me likes
an electro-mechanical device - if for not other reason that you can hear
it working...

I built a controller for a small brewery "hot liquor tank", basically
four 3 kW immersion heaters. I used octal base relays (notionally
adequately specced) in the first version but they didn't last all that
long. Solid state ones have been fine.

Adrian Brentnall wrote:

HI Bill
I'm always a bit nervous of the 'commoned-contacts' approach, as
Murphy's Law suggests that one set of contracts will operate slightly in
advance of the other set, and will end up making or breaking the lion's
share of the current.

I did wonder about that, but it doesn't seem to have happened.
Since I wrote before I've remembered that I made a unit that switches in
two sets of batteries on different time delays, the idea being that the
alternator wouldn't find itself supplying 24V to the vehicle battery
plus two 110Ah batteries both of which might theoretically be flat. That
device used 20A commoned contacts relays and it's had some right hammer
over the years, and it's been fine.


I'm deliberately over-engineering this project! (Partly because I want
it to be reliable, and partly for safety reasons - as it's a box of
heat, operating unsupervised, in a timber workshop.

How's your insurance?

For this reason I'm forgoing the usual 'let's see what's in the junk
box' approach, and speccing / buying new components. Call me paranoid!

You need to be paranoid by the sounds of it.

Bill

On 02/01/14 17:45, newshound wrote:
On 02/01/2014 07:27, Adrian Brentnall wrote:
On 01/01/2014 16:31, Andy Burns wrote:
Adrian Brentnall wrote:
I'd rather not go the fully electronic route with a solid state relay
for the final device - I rather like things that go 'clunk' grin.

Dad used solid state relays on his kiln controller, instead of going
"clunk", the specialist fuses that protect the relays liked to go "fot"
unfortunately they were more expensive than the relays themselves.

I think he did eventually find a previous owner had incorrectly wired
the elements, and now it's reliable.


HI Andy
Thanks for your comments. I've always been a bit wary of high-power
semiconductors protected by expensive fuses. Many years ago at
university we used a big homebrew lighting rig, and when the explosive
fuses blew in that (which they did fairly often, with a 'crack' like a
gunshot and a bright purple flash), they often took out the triacs as
well...

I'm sure that ssr's should be more reliable, but the luddite in me likes
an electro-mechanical device - if for not other reason that you can hear
it working...

I built a controller for a small brewery "hot liquor tank", basically
four 3 kW immersion heaters. I used octal base relays (notionally
adequately specced) in the first version but they didn't last all that
long. Solid state ones have been fine.

Non inductive loads like heaters is the one case where I WOULD use SSRs
without hesitation.

The fuses will not protect the SSRS but they will stop the wiring
catching fire.

SSRS fail short circuit though,so its important to have a full
mechanical DP isolator behind them for fault repair purposes.


--
Ineptocracy

(in-ep-toc-ra-cy) €“ a system of government where the least capable to
lead are elected by the least capable of producing, and where the
members of society least likely to sustain themselves or succeed, are
rewarded with goods and services paid for by the confiscated wealth of a
diminishing number of producers.

In article ,
Adrian Brentnall writes:
I'm always a bit nervous of the 'commoned-contacts' approach, as
Murphy's Law suggests that one set of contracts will operate slightly in
advance of the other set, and will end up making or breaking the lion's
share of the current.

It's not just the make or break order, but inevitable difference in
contact resistances means that the current will be badly balanced
between them (albeit, picking the better one at any one time;-)
You can mitigate this to some extent by making sure there's a length
of parallel wires connected to the contacts rather than paralleling up
as close to the contacts as possible - the resistance of that wire
will help reduce the ratio of differential contact resistances.

Relays are not usually rated for paralleling up contacts.

Another common mistake in this area is using a 3-phase contactor on
single phase. A 3-phase 4-wire contactor rated at, say, 100A, will
have 4 x 100A contacts. People will sometimes use this on a single
phase (where the load doesn't require phase rotation), and overlook
that the 100A neutral contact is now carrying 300A. Even when the
contactor is only switching the 3 phases and the neutral is
permanently connected, the connections and neutral conductors through
the device are still only rated 100A.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On Wed, 01 Jan 2014 16:19:06 +0000, Adrian Brentnall wrote:

HI Folks Some advice please.
I'm working on the control gear for a homebuilt glass-fusing kiln.
It's using a commercial digital controller, which provides a switched
output @ 12v DC.
Final load is two resistive wire elements, each rated @ 240v / 15A.

I can either feed the controller output through a small relay, to
provide a switched 240v AC, or source a high-current Relay / Contactor
that will accept the 12v. At the moment, I'm leaning towards the 'slave
relay' approach, as I don't know what current the control board can
supply.

I'd rather not go the fully electronic route with a solid state relay
for the final device - I rather like things that go 'clunk' grin.

Looking at CPC, high powered relays are available, as are contactors,
but the relays tend to be cheaper. What's best in this application? - is
a contactor 'overkill' - or berely 'prudent' ?



As the load is resistive, look at the AC1 rating of the contactor, not
the usual AC3 "motor" rating. You'll use a much smaller contactor that
way. If you are going for "over engineering" then don't use a
conventional relay. The contacts run hotter at the same current rating. A
contactor-relay is ok as it is really a small contactor with the contact
numbers changed! If you can get a contactor with a 12vDC coil then that's
what I would recommend. It'll be a lot cheaper SSRs by the time you've
got heatsinks sorted out.

On 07/01/2014 23:18, mick wrote:
On Wed, 01 Jan 2014 16:19:06 +0000, Adrian Brentnall wrote:

HI Folks Some advice please.
I'm working on the control gear for a homebuilt glass-fusing kiln.
It's using a commercial digital controller, which provides a switched
output @ 12v DC.
Final load is two resistive wire elements, each rated @ 240v / 15A.

I can either feed the controller output through a small relay, to
provide a switched 240v AC, or source a high-current Relay / Contactor
that will accept the 12v. At the moment, I'm leaning towards the 'slave
relay' approach, as I don't know what current the control board can
supply.

I'd rather not go the fully electronic route with a solid state relay
for the final device - I rather like things that go 'clunk' grin.

Looking at CPC, high powered relays are available, as are contactors,
but the relays tend to be cheaper. What's best in this application? - is
a contactor 'overkill' - or berely 'prudent' ?



As the load is resistive, look at the AC1 rating of the contactor, not
the usual AC3 "motor" rating. You'll use a much smaller contactor that
way. If you are going for "over engineering" then don't use a
conventional relay. The contacts run hotter at the same current rating. A
contactor-relay is ok as it is really a small contactor with the contact
numbers changed! If you can get a contactor with a 12vDC coil then that's
what I would recommend. It'll be a lot cheaper SSRs by the time you've
got heatsinks sorted out.


Thanks for the advice!
I need to design some 'relay logic' so that the 'over-temperature'
circuit can latch out if it trips - and so it's a question of just where
things go from 12v coils to 240v coils...

It'd be nice to run 12v control signals everywhere as I have 12v available.

Thanks
Adrian