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Old 08-02-2011, 23:07   #1
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Led Noise Problem

I just installed a new led dome light and now my fm radio has noise on it when the light is turned on. The manufacturer said to try a .1 mfd capacitor between the positive and negative leads. And to try different sizes if that didn't work.
Anyone have this problem? What size cap? Or?
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Old 08-02-2011, 23:35   #2
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Send that trash back to the manufacturer. Are you their engineering department?
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Old 09-02-2011, 00:14   #3
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What Daddle said. The problem is that there are three ways to provide the constant current source needed by LEDs, none of which are ideal.

The old-fashioned method is a series resistor, but this is only useful when there is a constant voltage source (not a boat with battery-charging fluctuations). Also, it's extremely wasteful.

The second, which it sounds like you have, is a switching power supply (or, sometimes, just pulse-width modulating the LED). Either of those involves lots of sharp on/off transients, which create harmonics into the hundreds of Megahertz. (Technically, a square wave is a monotonically decreasing sum of all odd harmonics... I had one in a homebrew LED bicolor circa 2000 that completely blanked marine VHF in the vicinity.) Without some good engineering, this is basically a big sloppy transmitter, and it's not easy to go back after the fact and provide enough filtering to fix it. The advantages are high efficiency and small/cheap parts.

The third method is also a constant-current source, but it is done linearly. This costs more power, but does not generate noise. An operational amplifier or other active device controls a "pass element" (a field-effect or regular transistor) to adjust the current provided to the LED, keeping it at the same level regardless of applied voltage.

For the manufacturer of yours to suggest a tiny cap like .1 and then suggest you "try other sizes" is really cheesy!

The trade-off that we get down to is either a well-designed switcher (best power performance) or accepting the lower efficiency of a linear regulator. Fun. eh?

Cheers,
Steve
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Old 09-02-2011, 01:59   #4
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Ditch it. There are much better ones available now. Even if the power supplies are less effiecient you won't notice as it's still drawing tenths of an amp for a good led.
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Old 09-02-2011, 05:11   #5
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I just installed a new led dome light and now my fm radio has noise on it when the light is turned on. The manufacturer said to try a .1 mfd capacitor between the positive and negative leads. And to try different sizes if that didn't work.
Anyone have this problem? What size cap? Or?

Please post the manufacturer so we all know to avoid. Thanks.
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Old 09-02-2011, 10:16   #6
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Please post the manufacturer so we all know to avoid. Thanks.
plse do this
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Old 09-02-2011, 13:58   #7
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I知 reluctant to disclose the manufacture until I rule out any electrical issues I may have. I知 in the middle of rewiring the entire boat. They did say that another boat had the same problem and thought it was related to the wiring. I purchased this brand because of good reviews here and in Practical Sailor.
That said I have no noise coming from the incandescent and CCFL痴. I have a temporary ground for the breaker panel ran directly to the negative side of the batteries until I add a permanent -12v buss. The am/fm radio is fed from a different breaker panel. Could a poor negative or positive connection from the secondary breaker panel contribute to this? I知 finding a lot of loose connections as I rewire.

Old boats sure are fun, every project I start creates at least two more

I will get done someday? Right?
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Old 09-02-2011, 14:07   #8
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The Roberts Law of Fractal To-Do List Complexity: The closer you zoom into one task, the more it expands into a cluster of component tasks.
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Old 09-02-2011, 14:40   #9
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Could a poor negative or positive connection from the secondary breaker panel contribute to this?
Only with a poorly designed device. If they are relying on the low impedance of boat wiring to suppress the noise their device generates they are even more foolish than we thought before.
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Old 09-02-2011, 14:47   #10
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Indeed. A long wire leading to something with fast-changing current draw (as opposed to the steady-state that would be exhibited by something with adequate on-board energy storage, whether inductive or capacitive) will be, essentially, an antenna.
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Old 09-02-2011, 14:52   #11
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I知 reluctant to disclose the manufacture until I rule out any electrical issues I may have. I知 in the middle of rewiring the entire boat.
I still feel naming the manufacturer is reasonable considering the advice they gave you in the orginal posting.
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Old 09-02-2011, 15:03   #12
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Hello, I have been reading CF for some time, but thought I might have something to contribute, so I finally registered. Hi to everyone.
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I知 reluctant to disclose the manufacture until I rule out any electrical issues I may have.[...]
Lennie, you may perform a very simple test to point to or rule out this LED light. Switch everything off, except your radio. Tune to some broadcast or just open up squelch and listen to noise. Then switch on _only_ this single LED light. There should be no influence on radio reception at all. If there is any, return this product or try the capacitor across power supply route. If you go this way try a ceramic 100nF type and solder it across power supply pins on leads as short as possible.

LEDs are constant current devices and in #3 Microship summed up supply options a LED light designer has. It appears you have a device with a switching power supply (current regulator). Very easy to poorly design, difficult to get right (btdt).

If you suspect some wiring problem, you may connect this LED light directly to your home battery bank with a pair of leads, but I don't think there will be much of a difference.
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