LED lighting strings

["Matt" <automation@xxxxxxx>]

Hi All

After reading the thread on LED lighting, I'd like to add some LED lighting
to my driveway.

Everything was pretty straight forward to work out, but I couldn't find out
how to calculate the
voltage drop for the run(websites with differing info/cable constants etc).

I've got a regulated 12v 0.85amp transformer and plan to wire the leds as a
series of strings in parallel.

-----xxx------led----led----led---

12v------+----xxx------led----led----led--+---------gnd

-----xxx------led-----led---led---
-etc

The leds are blue 5000mcd 3.2v 20ma and the resistors will be 120ohm
1/4watt, which means I can have 10 strings with 3 leds in a cluster.
The total run,(in a straight line) is 25m, but it will be longer depending
on how its run, I thought it may be better to run it as a ring, maybe a
star, or feed it 12v at 2/3 points??.

Can anyone point me in the right direction for calculating the size of the
wire and volt drop so I can figure out the best way to run the cable.

Matt.


If it helps, I calculated the above.....
0.85a / 20ma = 42.5 total leds
12v / 3.2v = 3.75, round down, so 3 leds per string
42 / 3 = 14 strings, use 75% of the capacity = 10 strings
(12 - (3.2 * 3) ) / 20ma = 120 for the resistor
((12 - (3.2 * 3)) * (12 - (3.2 * 3)) ) / 120 = 0.048 , so 1/4 watt resistor
should be fine.

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