I have a mirror defogger heating element that is intended to be hardwired to a 120V circuit and attached to the back of a bathroom mirror.

The heating element says the following -Power supply: 120V/60HZ -Input: 60W -Temperature rise: 50-77F -(Resistance of heating element: 342 ohms)

I would like to use this same heating element in a mirror in the shower but I need to drop it down to a lower voltage.

I will connect this heating element to a 24VAC power adapter but I'm not sure of the correct amps i need in the 24VAC power adapter.

The current of the heating pad when connected to the 120V power supply is 0.5A.

Does this mean that I would need a 24VAC power supply that supplies a current of 0.5A?

thank you in advance


closed as off-topic by Olin Lathrop, Andy aka, Michael Karas, MCG, PlasmaHH Jun 13 '18 at 11:07

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – Olin Lathrop, Andy aka, Michael Karas, MCG, PlasmaHH
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  • \$\begingroup\$ How much heating will be produced when you lower the supply voltage by 5? \$\endgroup\$ – Andy aka Jun 13 '18 at 10:59
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    \$\begingroup\$ This question seems to be more about "how do plug dis inta dat?" than about real electrical engineering. The fact that you're asking what "correct amps i need" makes it clear there is no understanding of electronics here, nor any interest in learning. If you don't understand why a 120 V heater won't work on 24 V, then this isn't the site for you. Maybe try home improvement. \$\endgroup\$ – Olin Lathrop Jun 13 '18 at 10:59
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    \$\begingroup\$ You won't get 0.5A through 342 ohms using 24VAC. Can't happen. You'll get like 50mA, and the consumed power will drop to like 1W. Won't do much to defog your mirror. \$\endgroup\$ – JRE Jun 13 '18 at 11:06
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    \$\begingroup\$ Read about ohms law for a starter \$\endgroup\$ – PlasmaHH Jun 13 '18 at 11:07

If the heater is rated for 120V and the resistance is 342 ohms then the current is 0.35A. When you try to operate the mirror heater at 24V you are operating at only 20% of the designed to voltage. This means that the current drawn at 24V will be only 20% as well or something on the order of 0.07A.

At this situation the mirror will heat up way way less at the lower voltage and will not achieve the defogging that you would expect.

You would do better to look for a shower mirror that uses a guided stream of water from the shower spout to keep the mirror clear.


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