Help with relay for thermostat kit

Discussion in 'The Projects Forum' started by booshank, Sep 20, 2011.

  1. booshank

    Thread Starter New Member

    Sep 20, 2011
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    Hi all, I am trying to set up a thermostat to control a small 240V heater (no more than 60W) that will heat a plant growing cabinet, to maintain a temperature of at least about 18C (exact temperature isn't critical).

    I have a Velleman MK138 thermostat kit, but the relay power rating is only 24VDC, so I'm thinking I will use a mains (240V) relay, say this one.

    My question is will I have to use the mains relay in addition to the Velleman kit, or can I replace the relay in the Velleman kit with the mains relay? If I could just replace the relay, it would be neater as it may all fit on the same board.
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The relay in the three MK138 kits that I have put together are rated to handle up to 10 amps at 240 vac. Of course, I haven't bought a new one in over a year, so perhaps they have changed.

    ETA: My memory was wrong. Here is a photo of the relay that came with one of the MK138 kits. As you can see, it marked as being rated for 15A 125VAC, 10A 28VDC, 10A 240VDC, and 10A 28VDC. I find these ratings strange; I will look for a datasheet for the relay.
     
    Last edited: Sep 20, 2011
  3. booshank

    Thread Starter New Member

    Sep 20, 2011
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    Thanks for your reply. That relay in the photo you posted is exactly the one I have as part of my kit.
     
  4. tracecom

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    Apr 16, 2010
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    I wonder if the rating that says 10A 240VDC isn't really a misprint and should say 10A 240VAC, but don't try it on my authority. Maybe you should post a question about it on the Velleman forum?
     
  5. mcgyvr

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    Oct 15, 2009
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    Technically the rating printed on the relay doesn't matter as Velleman has chosen to "self-derate" the contact side of the relay to 24V @3A max as printed right on the circuit board in front of the associated terminal blocks.

    But a simple check of the trace to trace spacing (adequate for 240V) and ensuring that the contacts go directly to the terminal blocks and nothing else is placed on that relay contacts should allow you to use it up to the rating stated on the relay. (based on your judgement not mine)

    Safest bet is to use that Velleman relay to power the coil on another suitable mains rated relay (I'd find one that fits into a relay socket or something you could just mount next to the Velleman unit.) I would not replace the one on the board already.
     
  6. booshank

    Thread Starter New Member

    Sep 20, 2011
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    Just thought I'd let you know that on the Velleman support forum they have advised that the original relay can indeed take 240V AC. I'm a bit puzzled why it says 240V DC max on the board though.
     
  7. mcgyvr

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    Oct 15, 2009
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    The original design was probably only intended for 24V thermostat systems so that's what the board layout guy put on the silkscreen.
     
  8. tracecom

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    Apr 16, 2010
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    Thanks for posting a follow up, which may help someone else who has the same question.
     
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