RVer put dimmer switch in bathroom — Why didn’t it work?

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:

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 of 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.

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. As long as they’re rated for sufficient voltage and amperage, they’re generally interchangeable between AC and DC systems.

See, you learned something cool about electricity today…

Let’s play safe out there…. —Mike Sokol

For our RVing musician readers, here’s Part III of Mike’s semi-silent-stage series of articles, which recently was the featured Top Story at ProSoundWeb.com. (Parts I and II are linked in the article.) It’s an overview of gear and techniques that allow musicians and engineers to work together in the quest for a quieter stage.

Mike Sokol is an electrical and professional sound expert with 40 years in the industry. Visit NoShockZone.org for more electrical safety tips. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.

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2 Thoughts to “RVer put dimmer switch in bathroom — Why didn’t it work?”

1. Wolfe

There ARE 12VDC dimmers, but as Mike said, they are not the same as triac-based AC dimmers. “Similar” to how triacs remove part of the AC wave, the DC dimmer creates an on-off cycling of the DC power at variable duty cycles (called Pulse Width Modulation or PWM) This is the only way to dim LED lights effectively, but it works on incandescent as well (sometimes in a less linear fashion). You can get wired-in PWM dimmers or even ones with remote controls for around \$5 if you search around.

Editor: Here’s the link to the remote control dimmer switch Wolfe is referring to at Amazon.

2. Mel Johansen

Instead of a dimmer switch, use a night light. It will light the room enough to use, but not too bright to be uncomfortable. Get the 12V light at any RV supply store.

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