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[on:]

Hello, this is a (i think) very simple question.

I want to power 80 simultaneous leds (3.2v ~ 20mA each).
In previous smaller projects I was using a 7805 to regulate the voltage from an external 9v power supply to 5v(because i'm using them in stop motion it is very important to me that they stay at the same brightness over time).

Now for 80 leds the votage drop, current (and thermal dissipation) of the votage regulator is way over the specifications (I calculate over 400ºC of temperature).
So, what could I use instead? I was thinking to make three arrays of leds in series, so give 3v to each array, but i would lose the regulation that the 7805 provided me. Or use several 7805, but the total temperature gain would still be very high.
Is there a way to more efficiently regulate such a high total current (1600 mA).

Also I intend to PWM drive the leds with the arduino... correct me if i'm wrong, but it's seems that a single TIP122 transistor will do.

Thanks in advance!

[Reply #1 on:]

If constant brightness is a key goal then you a constant current driver would be the obvious choice - you may be able to get high voltage ones that can power a series string of 10 or so each...  A different power supply would of course be needed.

The forward voltage of LEDs varies with temperature so a constant voltage supply feeding a series resistor is not that accurate a constant current source (unless you waste a low of power in the resistor).

[Reply #2 on:]

Thanks!, i was reading about constant current drivers (i didn't knew much about them)...
It's seems that i'll have to do something like this:

http://www.reuk.co.uk/Using-The-LM317T-With-LED-Lighting.htm

Is it correct to ditch the 7805 for a LM317T and a resistor?
(of course i'm considering to use a couple of this and a heatsink...)

Now.. How would this solution work with the TIP122 Transistor connected between the current regulator and the leds?

Also this means I don't have to connect series resistor with the leds, given that the current is already regulated?