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