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Replaced compass light with LED, one problem

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tmhansen
Captain

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USA
397 Posts

Initially Posted - 02/21/2005 : 00:40:33

The light on the pedistal mounted compass on my boat appeared to never have been connected. The red dye on the bulb was faded away. I decided rather than fix it I would install a LED instead.

Drove down to the electronics store and got a red LED that puts out 10 Candella (the best I could find at Radio Shack was 80mCd) The package conveniently gives the size of 1/2 watt resister to use for 12 v, (680 ohms).

The mount for the LED was easy. I used a small piece of aluminum sheet with a hole sized for the LED that I was able to install in the same screw holes as the original. I soldered the resister right to the anode of the LED and then attached about 16" of twisted pair wire. To this I attached a pair of plugs so I could remove the compass if need be. The wires then run in a sheilded pair of multistrand to the switch panel.

I tested it out on the bench with a transformer and it worked great. Once installed on the boat the frustration begins. I put it on the circuit with the bow and stern lights (two bulbs, each 10w or .84A) The nav lights work fine but the LED remains dark. If I isolate (remove) the nav lights the LED works fine. I'm wondering if the lights draw down the power on the circuit too much? Do I need a resister with less resistance? I'm stumped. I suppose I could put it on its own switched circuit but that seems like a poor solution to the problem.

By the way the LED is by Linrose and is model # B4304H91. I can't understand how much current it draws and I forgot to measure it, but it looks like it is 2 or 2.5 MA. The bulb it replaced was 125MA if I read the meter right.

Todd Hansen
Fiorghra'
San Luis Obispo, CA

Edited by - tmhansen on 02/21/2005 00:46:11

eric.werkowitz
Captain

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USA
283 Posts

Response Posted - 02/21/2005 : 08:45:15

Todd,

The LED circuit is in parallel with the nav lights, right? If so, my guess is the nav lights drop the voltage enough that the LED is just below where it will work. Try a smaller resistor, or put a potentiometer (variable resistor) in so you can start at maximum resistance and turn it down until the LED looks bright. Then measure that resistance and install a resistor that is close. Be careful, though, if you apply too little resistance while you're experimenting you might burn out the LED.

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Leon Sisson
Master Marine Consultant

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USA
1893 Posts

Response Posted - 02/21/2005 : 11:35:54

Todd H,

I think the compass LED is a fine idea. It may just need some fine tuning.

Unlike incandecent bulbs, LEDs are polarity sensative. When you tested using a transformer, you were feeding it AC voltage, so it was getting the polarity it wanted half the time, no matter how you wired it. So recheck your polarity using a 12VDC power source.

Yes, LEDs typically draw around 2.mA for max brightness without frying. For 12V use, try a 470.ohm 1/2W resistor. Since the resitor is in series, it can go anywere along either lead to the LED -- it doesn't have to be in the compass if there's a more convenient location.

I believe there are LEDs available with built-in voltage or current regulators which don't require an external resistor, and which may be able to compensate for the 2.V or more swings of a sailboat battery going from on the charger at nearly 14.V, to needs to be back on the charger because it's down to under 12.V.

This sounds like a clever and useful project. Please let us know how it works out for you.

-- Leon Sisson

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ilnadi
Captain

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452 Posts

Response Posted - 02/21/2005 : 11:43:07

Most LEDs have a forward "ON" voltage 1.5 to 3.something volts, depending on technology (it should be printed on the packaging). Hard to imagine the nav lights could pull the battery below that. More likely is a wiring fluke (like the nav lights and the LED are grounded very differently and one of the grounds floats above the other.

OR, as Eric said the voltage is dropping low enough that the light is too dim to see, in which case the potentiometer will tell you (just do not crank the pot until the light is bright then disconnect the nav lights; LEDs DO explode and usually the plastic cap goes off like a mini bullet).

Simpler to find a voltmeter and measure the drop across the LED and LED+Resistor. Also you can check if there is a voltage from the ground side of the LED+Resistor to the battery ground when the NAV lights are on. In one of the Good Old Boat's this fall there was an article about floating and phantom grounds.

Good luck,
<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by eric.werkowitz
 Todd,

The LED circuit is in parallel with the nav lights, right? If so, my guess is the nav lights drop the voltage enough that the LED is just below where it will work. Try a smaller resistor, or put a potentiometer (variable resistor) in so you can start at maximum resistance and turn it down until the LED looks bright. Then measure that resistance and install a resistor that is close. Be careful, though, if you apply too little resistance while you're experimenting you might burn out the LED.
<hr height="1" noshade id="quote"></blockquote id="quote">

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roberoo
Navigator

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USA
182 Posts

Response Posted - 02/21/2005 : 21:02:31

Hi I don't have any technical info, but this may be of even better help.

http://www.superbrightleds.com/cgi-bin/store/commerce.cgi?product=OTHER

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Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 02/21/2005 : 21:38:10

Leon is right in that a 470 ohm resistor works with most leds at 12v but it can be assumed that the led is not hooked up backwards or it wouldn't function when the nav lights are disconnected.

I suspect that the battery may not be in good condition.

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