# powering white LED array from a car battery

Discussion in 'Electronic Design' started by [email protected], Apr 22, 2007.

1. ### Guest

hi,
i need to build a LED light source using an array of LED's. (about
100). the supply voltaeg is 12DC(from a car battery). i tried powering
multiples of 3 series LEDs (without any resisters) , but the intensity
of LED's were not uniform. many LED's were destroyed when powered
continuously.

i need to know the reason for this and the proper way way to do this.

thank you

2. ### D from BCGuest

ewwwww

1) LED Binning
2) LED temp coefficient
3) current limiting methods active and passive
4) car system voltage
5) heat sinking?
6) LED spec variation
7) LED series parallel combinations
8) Maximum drive DC and pulse
D from BC

3. ### Homer J SimpsonGuest

http://led.linear1.org/led.wiz

4. ### defaultGuest

You should be posting basic questions to the basic newsgroup - this

Lurk and learn

White leds typically require 20 milliamps at three point something
volts. You need to limit the current or the leds burn up. You limit
current by adding a resistor in series with three or four leds for 12
volts.

For instance my 12 volt application was using 56 white leds in series
strings of four each - the 12 volt battery was putting out ~14.8 volts
when charging (when the engine was running)

The leds I used required 3.2 volts each X 4 = 12.8 volts for each
string minus 14.8 volts for the battery left 2 volts at 20 milliamps
to dissipate in the resistor. Ohms law says that I need to divide .02
amps into 2 volts for the proper resistor or ~100 ohms. Fourteen 100
ohm 1/4 watt resistors in series with 4 leds each for 56 leds total.

Your choice of led and the voltage and current it requires and your
actual battery voltage will determine the correct resistor - a 12 volt
battery is usually ~12.6 at rest and 13.8-15 when the motor is
running.