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Connecting two heaters of 240V to a relay rated at 240V

  1. Jan 31, 2012 #1
    Ok this might be a really stupid question but I would appreciate some help.

    I have a controller with a relay output rated at 240 V 3A. I was wondering what happens if I connect 2 heater elements of 240 V each in series to this relay. Will it ruin the relay over time? Will the output of the heaters be affected?

    Help is appreciated. :smile:
  2. jcsd
  3. Jan 31, 2012 #2


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    Science Advisor

    Welcome to PhysicsForums!

    Making the naive assumption that your heating coils have constant resistance at all temperatures (they almost certainly won't, but also shouldn't change drastically for large values of drastic), what'll happen is that the in-series coils will only output a quarter of their power, and the sum of the two will result in only half the power output of a single coil.

    You can see this by rearranging Ohm's Law, V=I*R, and the power formula, P=I*V:

    To double your power output, you should really connect the coils in parallel (so that 240V is present across each coil).

    HOWEVER, you don't mention how much power your heating coils are rated for (or equivalently, how much current they consume). If they individually or together draw more than 3A (720W), you have a problem.

    If this is the case, your relay may burn out because the coils are drawing too much current. Your controller might need to drive a relay driving the coils (either one supplying both in parallel, or two supplying one each)
  4. Jan 31, 2012 #3


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    Staff: Mentor

    If you connect in series two identical heating elements, each rated for 240 VAC, then when you power that series circuit with 240 VAC you will be disappointed at the low amount of heat produced. For full power, they'd need 480 VAC.

    Though at 240 VAC, so long as the series combination doesn't draw more than 3A, the relay shouldn't be upset.
  5. Jan 31, 2012 #4
    3 A at 240 V gives about 720 watts, check the heater ratings if they add up to more then 720 you need a bigger relay.
  6. Feb 1, 2012 #5
    Thanx so much guys for your great explanations and quick response!!!! :)
    The three of your answers put together really explained the situation to me!!! Thanx a ton!
    Am already feeling good about joining PhysicsForum!
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