# Designing Current circuit for 64 leds

Discussion in 'Electronic Basics' started by Everest, Feb 21, 2007.

1. ### EverestGuest

Hi everyone

Dealing with leds..I have few questions.I am trying to make matrix of
64 leds,common anode and darlington pair are used as current sinks for
individual leds. leds at [email protected] of current. I need to design a power
supply which will be enough to supply 64 leds i.e 25ma x 64 = 1.6A of
current + say 100ma for other circuitary on board .Now the leds are
addressed individually there is no row or column matrix.i.e the leds
are in parallel.

For how much output current should i design the power supply? 1.7A ?
or how much...and I will be using a resistor in series with led to
limit the current to 25ma for each led. and max vf for led is 3.4v.
As leds are in parallel 1.7A will get distributed among 64leds
+circuitrary( i.e led controllers)?

what will happen if i design for say 3A but the leds are limited to
25ma?I am not sure how this current distribution works.for parallel
leds.and what output current should i design my powersupply
circuitary?.can u give me your inputs plz .
Thanks
Everest

2. ### feeboGuest

25mA for each LED is on the upper limit - if you are using red LEDs
you can pretty much halve this and still get a good brightness - play
about with different currents that give acceptable brightness and you
might be able to get the total current under an amp in which case as
1Amp regulator (7805) will do very nicely. If you still need a lot of
current, use an LM323 (3A) power reg or a 5.6v Zener to bias the base
for an emitter follower with a power tranny (nice and simple but
bomb-proof), the ubiquitous 2N3055 works very well in this config ( or
the smaller 2N3054) but will get hot as the current goes up and its a
big case with a big heatsink - consider using something like a TIP122
darlington power tranny - this will sustain 5A and surge 8! It's a
smaller package and due to it's operation and construction will need
only a small clip on heatsink.

3. ### ehsjrGuest

You can buy a regulated output power supply cheaper than you
can build one. CAT# PS-528 (\$5.75) from Allelectronics will
work for you. It's a 5V DC 2 amp regulated wall wart style
switching power supply.
http://www.allelectronics.com/cgi-bin/item/PS-528/480/5_VDC_2_AMP_SWITCHING_POWER_SUPPLY_.html

You certainly don't need darlingtons to control the LEDs
individually. Any jellybean NPN will work. With a Vf of
3.4, put an 82 ohm resistor in series with each LED for
about 19.5 mA, or use 68 ohms for about 24 mA.

Ed