I put a dimmer switch in my RV bathroom — Why didn’t it work?

AC and DC devices are totally different things and generally not interchangeable

Hey Mike,

I tried to change out the On-Off wall switch in my camper bathroom to a dimmer switch and it didn’t work. Now I discover that my lighting is 12-volts DC and the dimmer is 120-volts AC. I put a regular On-Off switch back in and now it works. So is it the voltage difference or the AC-DC difference that made the dimmer fail to function? —Todd D.

Hey Todd,

That’s a great question. In a nutshell, your RV has two totally different types of electrical systems, 12-volts DC and 120-volts AC. The 12-volts DC comes from your house battery or converter, and provides 12 volts of DIRECT Current. That is, it’s always at 12 volts (or thereabouts) and doesn’t cycle on and off.

You also have a 120-volt system of ALTERNATING Current that comes from shore power or the Inverter. In the USA this actually alternates between positive and negative polarity 60 times a second, hence the name 60 Hertz (Hz). So the 12-volt DC system is on all the time, while the 120-volt AC system actually pulses on and off 120 times per second. See the diagram below:

How does an AC dimmer work?

Standard AC dimmers work by chopping up this Alternating Current into smaller chunks of power using something called a Triac. You can see a full power waveform on the left (green) side of the diagram (below) which would be a dimmer at 100% brightness. The right (red) side of the diagram shows a dimmer at 50% brightness. You’ll note that the Triac circuit only turns on the power one-half of the time. And so on for 20% or 80% or whatever brightness level you set it to.

This is happening 120 times a second which is too fast to be detected by the human eye (most of the time).

But because the DC power from your battery and converter never does this on-off Alternating thing, there’s nothing for the Triac to chop up, so it can’t dim the lights. There’s a totally different type of circuit needed to dim a DC lighting system.

What’s Interchangeable?

So the general rule is this: You typically can’t interchange electronic gear between AC and DC systems. However, simple contact devices (such as an On-Off switch) don’t have any electronic components so they don’t care if they’re switching AC or DC. So as long as they’re rated for sufficient voltage and amperage, they’re generally interchangeable between AC and DC systems.

Tungsten light bulbs will work on either AC or DC power as long as the voltage is correct. But LED bulbs only work on DC so they require some kind of converter (internal or external) to operate on AC power.

Let’s play safe out there… Mike