# Led circuit help using parallel array of leds

Discussion in 'General Electronics Chat' started by scott2la, Dec 29, 2008.

1. ### scott2la Thread Starter New Member

Dec 28, 2008
1
0
I am having trouble designing a schematic for a led setup I am working on.

I will be using 56 white leds. Here are the specs.

Source voltage I would like to have it at 12 volts.....
diode forward voltage
diode forward current (mA)
number of LEDs in your array

3 x 19 array uses 57 LEDs exactly

• each 68 ohm resistor dissipates 61.2 mW
• together, all resistors dissipate 1162.8 mW
• together, the diodes dissipate 5814 mW
• total power dissipated by the array is 6976.8 mW
• the array draws current of 570 mA from the source
I need to be able to dim the leds by use of a pwm.

Is it possible to use transistors to achieve 12 volts from 2 AA rechargeable batteries which together equal 2.4 volts or is it better to use a 9 volt battery" rechargeable 9 volt would be 8.4 volts.

I need to keep the battery source small to fit in the enclosure. I am not sure how long the batteries will last although i have seen lights using only 2 aa's powering similiar lights.

Any help designing a schematic with the pwm and any other info would be greatly appreciated.

I have some basic knowledge of electronics but I am struggling with the power source.

Thanks so much.

2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540

At a transistor going to ground and use it to switch the base of either to ground for PWM.

The current mirror isn't really recommended, it is for illustration only.

3. ### MikeD_72 Active Member

Nov 11, 2008
46
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I don't mean to hijack this thread, but the circuit Bill posted caught my interest. What is the purpose of the two diodes going to ground?

4. ### bertus Administrator

Apr 5, 2008
15,799
2,386
Hello,

The two diodes to the ground are keeping the bases of the transistor at ca. 1.4 Volts.
The base- emittor diode is ca. 0.7 volts.
The current through the resistor will be 0.7 / 36 = 0.01944 A ( 19.4 mA ).
The leds will get about 19 mA through their bodies.

Greetings,
Bertus

5. ### MikeD_72 Active Member

Nov 11, 2008
46
0
Thanks, that was a great explanation