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Old 05-28-2013, 02:52 PM   #1
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Default STC/relay mash temperature controller question

I have an STC-1000 temperature controller which I use to drive my fermentation heating/cooling system, it's wired as follows (except on my STC the hot and cold are the other way round:

It looks like this (shown here during testing):

H is obviously hot, and cold is the one with a plug in it. Those are UK standard mains plug sockets with Live, Neutral and Earth pins. Mains electricity in the UK is 240v. The controller works fine; I've probably run it for 12+ weeks now in total with no problems.

That's the background, here's my problem/question:

I use the temperature probe to tell me my mash temperatures already (easier than using a glass thermometer). I'm thinking of making an electric kettle for BIAB. I would like to use the STC-1000 to switch the kettle element, however the relay in the STC is only rated to 5 or 10A (I forget which - adequate for my fermentation control system but not for an element). Therefore, I would need to use some sort of relay.

I have two options currently, unless there's some I haven't thought of -

1) build a standalone relay which plugs into the hot side when I want to use it and has it's own mains supply. The STC would switch the relay which would handle the higher demand of the element.

2) wire a relay into the current setup between the STC and the plug, same idea but neater execution. I would have to change out the power cable too as I only used 5A cable (better safe than sorry/it was cheaper).

What sort of relay should I be looking at? Does this sound like a reasonable solution?

I probably haven't explained myself very well, so if you need some more information just ask...

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Old 05-28-2013, 09:40 PM   #2
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You will need a power relay AKA contactor rated for 240v @ 30amps. It looks like you will want one with a 240v coil (output from the SCT1000)

Most electric system are using an SSR to enable/disable one phase of the 240v power. This requires approximately 12V DC to enable the SSR.
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Old 05-29-2013, 08:56 PM   #3
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Yes just cascade the signal to a contactor with a 240V coil and a higher power rating. But a 30A contactor is probably not necessary -- I am guessing you are probably running a 3kW element at around 13A.

Also you will just need a single pole contactor to switch the live wire. In the USA we have to use double pole contractors to switch 240V because both wires are "hot".

At the bare minimum you want 1.5mm2 cable to run 13A to the contactor, and thicker (preferably 2.5mm2 butyl) for the flex that actually connects to the element.
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