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Old Jan 03, 2013, 05:14 PM
Registered User
farmington, mn
Joined Sep 2003
884 Posts
led voltage and cooling requirments

i am going to be hooking up four 5mm leds just curious how sensitive leds are to voltage changes. the boat battery is 12v but the alternator kicking in and it goes to 14v, should i use a voltage regulator or should i just calculate the resistors for 13v. also the array and resistors will be placed in a epoxy puck so they can be put under water. Will the leds over heat or the resistor burn itself up if there cased in epoxy? it will always be in water so there will be some cooling but just thought i would ask first. here are the current usage from the array

10 ohm resistor dissipates 4 mW
together, the diodes dissipate 256 mW
total power dissipated by the array is 260 mW
the array draws current of 20 mA from the source
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Last edited by jeff262; Jan 03, 2013 at 10:46 PM.
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Old Jan 03, 2013, 09:52 PM
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boaterguy's Avatar
Canada, ON, Ottawa
Joined Apr 2009
2,444 Posts
What are the specs (forward current/ voltage) on the specific LED's? What kind of circuitry are you running? Are they all in parallel with their own individual resistor? In parallel with a single resistor? In series?
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Old Jan 03, 2013, 10:44 PM
Registered User
farmington, mn
Joined Sep 2003
884 Posts
they would all be in series with 4 leds running a forward voltage of 3.2-3.4 volts. each led has a forwards current of 20mA. i would need one 1/4W resistor to drop the current
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Old Jan 04, 2013, 06:31 AM
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Julez's Avatar
Joined Dec 2003
5,413 Posts
LEDs with over 3V should only be connected as 3 pieces in series for 12 V operation.

3.3V * 3 = 9.9V



Take 100-150ohm then for each set of 3 LEDs.
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Old Jan 04, 2013, 08:08 AM
Dave the Rave
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Joined Jun 2007
894 Posts
When you wire LEDs in series, you add the Forward Voltages together and calculate the current limiting resistor's value accordingly. But the supply voltage needs to be a couple of volts greater than the combined FVs of the LEDs, so you will not be able to wire 4 in series on a 12 volt supply, since 4 * 3.2v = 12.8 volts. A good option is to wire 2 sets of 2 LEDs in series, then wire the 2 sets together in parallel. If you do it that way, a 390 ohm 1/4 watt resistor in series with each pair of LEDs will limit the current through each LED to about 19mA, and the resistor should stay plenty cool, especially if it's under water. The 390 ohm resistor is based on a supply voltage of 14 volts, it's better to size the resistor to the highest voltage the circuit will see, to protect the LEDs from too much current (not voltage), and the LEDs should continue to conduct until your battery voltage drops way down, maybe as low as 9 volts or so.

LEDs are very different from a regular filament-type bulb, they don't really care about voltage, and will work on any commonly found voltage, even AC, as long as the current is limited via a resistor. And they don't produce much heat (except when you get into the bigger Luxeon-type higher wattage varieties) because they don't have a filament.
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