RE: LED lighting strings

["Keith Doxey" <ukha@xxxxxxx>]

Hi Matt,

dont quite follow your calculations. You can actually run 126 LED's from
your power supply.

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

LED forward current 20mA
LED forward voltage 3.2V

LED's are in SERIES so the current through the string will be 20mA. You can
have 850/20 = 42 strings of 3 LED's which is 126 LED's

The forward voltage across the three LED's will be 3.2*3 = 9.6V therefore
the resistor needs to drop 12-9.6 = 2.4V. Resistor needs to be V/I =
2.4/0.02 = 120 ohm as you correctly worked out.

If you only run 10 strings then total power consumption will only be 10 *
20ma = 200mA. Buy a reel of 0.75mm Twin flex. Rated at 6A, double sheathed
so quite tough and will not suffer from noticable voltage drop with such
low
currents. Its also quite cheap.

Keith

www.diyha.co.uk
www.kat5.tv


> -----Original Message-----
> From: Matt [mailto:automation@xxxxxxx]
> Sent: 04 November 2003 20:17
> To: ukha_d@xxxxxxx
> Subject: [ukha_d] LED lighting strings
>
>
> 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.
>
>