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Old 4th September 2010, 01:08 AM   #1
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Default which way to run a LED is more efficient on 12vdc reg or resistor

hey guys thought this might be a good place to ask...
im sure this is an easy one for most people here,
but which way to run a LED is more efficient on a 12vdc battery running them at 3.3vdc or (requierd) voltage by way of a stepdown regulator say a" lm317t" or running them strait of a 12vdc line with the simple resistor attached to the anode??? im only asking because im converting the lighting on my boat to LED's to get the most from my battery life thanks guys here is the specs on the ones im useing to if it helps cheers and thankyou
0.5W 10MM HI POWER
LENS Type:Water clear
Color Temperature: --
Luminous Intensity-MCD: Typ: 200,000 mcd
Reverse Voltage:5.0 V
DC Forward Voltage: Typical:
DC Forward Current:100mA

DC Forward Current

100


mA

Peak Pulse Forward Current (1)

150


mA

Avg. Forward Current (Pulse Operation)

100


mA

Operating Temperature

-40 to +100


0C
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Old 4th September 2010, 01:30 AM   #2
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Armchair thoughts- it makes no difference if you drop the voltage across a linear regulator or a resistor; both ways dissipate the power resistively. What you want to do is eliminate dissipation everywhere but the LEDs by using a PWM scheme (the eye integrates the pulses) or by building a switching supply that controls it's output *current* as needed. I'm pretty sure there are LED driver chips on the market that use these techniques.
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Old 4th September 2010, 01:37 AM   #3
star882 is offline star882  United States
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There are switching LED power supplies out there, some offering very high efficiency. Another solution is to use many LEDs in series, such that the nominal voltage drop is just below the minimum supply voltage and use a simple 2 transistor current regulator.
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Old 4th September 2010, 01:45 AM   #4
Bill_P is offline Bill_P  United States
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High Brightness LED Drivers | National Semiconductor ? High Power LED Drivers for General Illumination
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Old 4th September 2010, 02:40 AM   #5
cbdb is offline cbdb  Canada
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If there 3 volt LEDs and a twelve volt battery, make groups of 4 LEDs in series across the battery. That would use all the power in the LEDs and none in a voltage drop device. If the LED voltages are not a perfect multiple of the battery voltage than use as many LEDs as posssible and a small drop down resistor. (eg 3volt LEDs and a 14 volt battery = 4 LEDs and a resistor that drops the extra 2 volts.)
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Old 4th September 2010, 03:51 AM   #6
wwenze is offline wwenze  Singapore
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12V and 24V LED strips for automotive use are easy to find.
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Old 4th September 2010, 04:52 PM   #7
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It sounds like a sailboat. The LEDs will do fine on the battery, but when re-charging the battery alternators can be hard on LEDs. Lots of the early auto LED tail lights didn't last long.
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Old 4th September 2010, 06:37 PM   #8
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Quote:
Originally Posted by Speedskater View Post
It sounds like a sailboat. The LEDs will do fine on the battery, but when re-charging the battery alternators can be hard on LEDs. Lots of the early auto LED tail lights didn't last long.
Is that because the increased voltage was not properly dealt with because a voltage dropping resistor was the only current limiter?

You can use a LM317 as a constant current source to power series LEDs up to about 2V less than the minimum supply voltage (eg 3x 3V LEDs on a >11V supply).
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Old 4th September 2010, 07:10 PM   #9
T in AZ is offline T in AZ  United States
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Here ya go http://electronicdesign.com/article/...ed--16763.aspx
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Old 5th September 2010, 12:44 AM   #10
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I think that it was large voltage spikes caused by the alternator and voltage regulator combination.
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