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DIY Heated Mirror

Discussion in 'General Electronics Discussion' started by Ricardo da Silva, Aug 17, 2014.

  1. Ricardo da Silva

    Ricardo da Silva

    Aug 2, 2014
    Hi All. So after an unsuccessful attempting to go it from scratch with chromatic wire etc. I found these,

    , self adhesive heating pads, rated are 12V 15W, I am using two per mirror and with a 15V 4A transformer they seem to work a treat although they do take a while to heat up. I only need them to warm up enough to prevent fogging up.

    I was hoping someone could please help me so I do not burn the house down please:

    1. Is there any way to make them heat up quicker, like higher voltage, or would that possible damage them.
    2. Do I need to wire in a temperature switch to ensure they do not overheat or is that not really an issue at these low voltages.

    Many thanks in advance
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    What sort of mirror are you heating, and how large? It sounds like it's not on a car.

    And why is it fogging up?

    A higher voltage will make them heat up faster (because the power used is proportional to the SQUARE of the voltage. You shouldn't need a temperature switch as long as you don't exceed the rated voltage -- I'm pretty sure they're designed to be run continuously.

    A higher voltage could damage the resistors as they might be asked to dissipate too much heat.
  3. Ricardo da Silva

    Ricardo da Silva

    Aug 2, 2014
    Thanks so much for your reply. The mirrors are bathroom mirrors, which are about 3mm thick and 90x50cm. The heating is to stop them misting up during showering.

    The reason I asked about the switch was that the site states
    "It is not thermostatically controlled and should not be operated continuously without a suitable thermal regulation system in order to prevent overheating and possible damage", but I am not certain that it not just standard blurb, because I imagine that given 15V 4A those resisters could never get that hot regardless how long it runs for or am I wrong.

    If I do need a switch would something like this a Thermostatic Steel Temperature Switch do the trick.

    Although they seem to be rated at 250V 5A; could I wire it in on the "in" live AC wire of the transformer ?
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