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Son of a... PWM Dimmer Modules

2367 Views 3 Replies 3 Participants Last post by  k7cej
Somebody here hopefully is smarter than me. Added 2 more rows of ceiling lighting to my Cargo using the factory wiring harnesses and lights. They clip right in like they came that way from the factory. Then I clipped the ends and built my own harness to gang all the lights in parallel and plug them into my house power setup. Added a relay and 3-way switch to the mix so I can have them all turn on when the door opens or have them full on. Works like a dream. Then I had the great idea to replace all the bulbs with 16LED festoon replacements. Great idea and super cheap off Amazon. Works great. THEN I decided to add a PWM dimmer for the LEDs so I could dim down the grow op. Works awesome... except.

I've tried two dimmer modules now—one cheap one off Amazon and one $28 one from Home Depot. Both work great, but both of them make a high pitched squeal when dimming < 95% or > 0%. On or off, no noise. In the middle... worst mosquito ever. I've tried switching back to incandescents, unplugged each row of lights, changed ground locations, etc. No dice.

The kicker? The high pitched whine isn't coming from any specific source. Not the dimmer, not a speaker, nothing electrical. It's COMING FROM THE SHEET METAL JUST IN FRONT OF THE REAR LEFT WHEEL WELL. No ****. I even hooked up my portable recorder and amplified the sound 20x and the loudest spot is inside the round cutout in the body between the rear left wheel well and the center vertical dividing rib.

I've tried blocking RF and EMI with a few different big hunks of metal between wiring and the body or the dimmer and the body or wiring. I've clipped all the nice zip ties on my wiring and separated it all out to make sure it wasn't acting as an antenna. Nothing. I've tried touching every surface and nothing changes the sound. I've tried short grounding runs to the body, I've tried using the rear factory grounding points, I've tried my house power grounding point.


Anyone ever heard of such a ridiculous problem?
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Not sure where it's located but the fuel pump makes some wonderful sounds, there are some manuals available for download that I have seen links to from this site. Some lights are not dimmable, this includes the led lamps them selfs. So the lamp needs to be dimmable and also the dimmer needs to be rated for the proper voltage and lamp type. Sounds like you understand that already just be aware that dimmers can cause many types of issues including RF and harmonics.
Good luck!
Yeah, I was thinking fuel pump or something similar as well, but the vehicle is off and the sound isn't audible from outside or underneath. I immediately though it had to do with the LED lamps so I put the incandescents back in and nothing changed. Both dimmers I've tried are 12v 96 watt (8 amp) units. They supposedly work for both incandescent and LEDs.

So after a couple days of kicking it around I came up with a functional solution. After much experimentation I found that the incandescent bulbs were a large source of the PWM high pitched noise. The filaments themselves emit the PWM pulse sound. By switching to all LEDs, removing 2 of the 8 lights and putting them on their own separate circuit—thereby lowering the amp draw for the dimmer circuit—and shortening the ground distance a bit reduced the noise to a barely noticeable level.

Short story is that PWMs in tandem with a moderate amp draw (3-5amps) are indeed noisy and they emit noise—through regular incandescent bulb filaments as well as the unit itself.

I'm happy to say that now that it's buttoned up, the finished install is pretty slick. Six LED lamps on the dimmer circuit, plus switches added to the rear two overhead lamps (with dedicated power) for nighttime reading lamps.
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could noise be from poor ground connections?

Re the noisy dimmer system for your lights. If the sound is really coming from the sheet metal and not the dimmer, I wonder if it is caused by the current passing through the sheet metal as a ground. Are there separate ground wires, or do all of the lights use the chassis as the -12V connection?

PWM circuits can have very high peak currents when the switching semiconductors change state.One solution might be to put a large elctrolytic capacitor across the input to the PWM module. If the noise is coming from the lights, try putting a cap across the output. For a starting point, try 470uF/25V low ESR.
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