need prescription on led

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georgetwo

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hi
Im about connecting a 12v 6.5a battery to 45 leds which are arranged in groups of 5 in seperate places about 12Meters apart.
the last time i did this with a resistor to 4 groups, the resistors got burned up due to heat generated
i even tried increasing the wattage of the resistor but the heat transrared to the leds.

How do i connect the leds and the batteries to prevent power loss due to heat?
The leds are white
 

hi,
What colour and current rating are the LED's as the will determine the resistor values etc.
 
Mmmmm. A typical LED.Might not be yours. 20 mA, Vf=2.1 Volts. 5 LEDS * 2.1 = 10.5 V.
(12-10.5)/.020 = resistor value. Power rating > I^2 * R or (0.020^2) * Resistor value

So 5 strings of 5 with a resistor per string.

There's a problem with this approach. As the battery depletes, the LED's get dimmer and there is little or no battery voltage reserve.

What are the Vf and suggested If for your LED's or better yet, p/n and/or datasheet.
 
Last edited:
georgetwo said:
the leds are white which should be rated at 20 mA
Click to expand...

hi,
As your LED's are White, they have much higher voltage drop than 2.1V, look at this image.
White are shown as 3.5V, so you can only connect 3 in series across 12V, using a suitable series resistor.
So 3 * 3.5V = 10.5V this leaves 12V - 10.5V = 1.5V to drop across the resistor.
For a current of 0.02A this makes the resistor 75R.

EDIT:
So for 45 LED's you will require 45/3 = 15 sets of 3 LED's each set with their own 75R
 

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i like that idea. im still afraid of short circuit on the line.
im using a pare of twisted copper wires for transformers. the worse is that the wires are quite small, I used it to make my connection so hidden.
 
Last edited:
georgetwo said:
i like that idea. im still afraid of short circuit on the line.
im using a pare of twisted copper wires for transformers. the worse is that the wires are quite small, I used it to make my connection so hidden. this is a rough diagram
Click to expand...

hi,
I cannot see your diagram.?
The total current for 15, 20mA strings is only 0.3Amps.

The supply to the strings MUST be DC, not AC.
 
Yes I got it sir
one more question, where will the resistors be located?
close to the source or close to the leds
and what wattage of resistors i will be using?
thanks allot
 
georgetwo said:
Yes I got it sir
one more question, where will the resistors be located?
close to the source or close to the leds
and what wattage of resistors i will be using?
thanks allot
Click to expand...

hi,
It dosn't matter which end, source or LED.

Wattage = V * I ..So, 1.5V * .02A = 0.03W,,,, so I would use a 0.125W or higher
 
True. It's taken me three Months to work out a solution.

My project has now gone to my boardhouse and been approved. Eagerly awaiting prototype PCB's.

It's been a long slog. And plenty basic thinking.

KISS has heen the hardest thing to do though.

To make a circuit complicated is easy. To make it both reliable and simple is a huge challenge....that's what good Engineering is all about.

Applies to all Engineering fields

Cheers
 
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