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