What Is a Mirror Defogger and How Does It Work?

What is a mirror defogger?

A mirror defogger is an electric heating film fixed to the back of the glass. Our TSP-3030 mirror defogger pad, for example, is a 300 x 300 mm PET film with a 20 cm lead that sticks to the back of the mirror and keeps that area clear of fog.

In an LED mirror, the bathroom mirror defogger usually works alongside the lighting: one touch switch controls the light and the defogger, either from one button or from separate keys. The pad itself has no controls; it heats whenever power reaches it.

Why does a bathroom mirror fog up?

A mirror fogs because warm, moist air touches glass that is colder than the air's dew point. The water vapour condenses into small droplets on the surface, and those droplets scatter light, so the reflection turns grey.

During a hot shower the air in the bathroom carries a lot of moisture while the mirror is still at room temperature. Wiping a fogged mirror only clears it until more vapour condenses. The fix is to keep the glass warmer than the dew point of the room air.

How does a mirror defogger pad work?

The pad turns electrical power into heat through a resistive track inside the film, and that heat passes into the glass. The TSP-3030 is listed for a 10-20 °C temperature rise on the glass, which is meant to keep the covered glass warmer than the steamy air around it so the vapour does not condense there.

Defogger pads are specified by power density, in watts per square decimetre. The TSP-3030 is rated 1.5-2.5 W/dm². Its area is 3 dm x 3 dm = 9 dm², so its power is 9 x 1.5 = 13.5 W to 9 x 2.5 = 22.5 W.

Because heat spreads only a short way through the glass, the clear zone follows the shape of the pad. That is why the pad goes behind the area people look into, not in a corner.

How is a mirror defogger powered and switched?

A defogger pad runs at the same voltage as the circuit that feeds it, and it is switched by a mirror touch switch with a defogger function or by a separate switch. The TSP-3030 is available for 230V AC, 120V AC, 12V DC and 24V DC.

For 12V low-voltage mirrors, our TD103 / TD103D, MTD-B02 and TQ-303SW switches all combine light and anti-fog control. TD103 and MTD-B02 are rated 12V DC and 60W. TQ-303SW has separate light and anti-fog keys and is listed in 50W and 100W versions. For 24V mirrors, the FWS-6D dual-button switch has one button for the light and one for defogging.

For mains-input mirrors, the TSP-002 switch is built for 110V to 240V markets with a 12W light load, and the TSP-003 switch adds a time and temperature display to light and anti-fog control, listed for AC 110V or 220V. Check the pad voltage against the defogger output of the switch you choose.

Should I choose a 12V, 24V or mains defogger?

Choose the voltage that matches the rest of the mirror, so the pad can share the existing supply or switch output. Lower voltage means higher current for the same power, which affects the driver and switch rating.

For one TSP-3030 pad at full power: 22.5 W / 12 V = 1.875 A; 22.5 W / 24 V = 0.94 A; 22.5 W / 230 V = 0.098 A. A 12V or 24V pad keeps all mains wiring inside the driver, but its current adds to the driver load. A mains pad leaves the driver load unchanged but puts mains voltage on the pad circuit, which must then be wired by a qualified electrician.

When the pad shares a 12V driver with the LED strip, add the two loads. A 30 W strip plus a 22.5 W pad is 52.5 W, and 52.5 W / 12 V = 4.375 A. Size the supply for that total, and check it against the rating of the switch.

How big should a mirror defogger pad be?

Size the pad to the area that must stay clear, usually the face-height zone in the centre of the mirror. One TSP-3030 covers 300 x 300 mm. A wider mirror can use more than one pad, or a custom pad size, provided the total power stays within the defogger output.

To check the total, add the pad powers: two TSP-3030 pads at full rating are 2 x 22.5 W = 45 W. At 12V that is 45 W / 12 V = 3.75 A for the pads alone, before the LED strip is counted. Send us the pad size, quantity and supply voltage and we will confirm which switch output suits it.

Is a mirror defogger safe in a bathroom?

A defogger is designed for bathroom mirrors, but the installation must suit a wet room. The switches listed here carry IP20 or IP22 ratings, which are for positions protected from water, such as behind the mirror glass and inside the mirror housing.

Switch off the power before working on the mirror. Any connection at 110-240V, including mains pads and switches with a built-in power supply such as the TSP-002 and TSP-003, must be made by a qualified electrician following local bathroom wiring rules. Keep leads away from the heated pad surface, and never cut or fold the pad.

Figures are from our product listings or worked out from them. Check the specific model before installation.