Lighting up a crap-load of LEDs

[MikeH]

I ordered this slope-style enclosure by hammond, not realizind the back of it was vented.  So I had this idea to put plexi glass over the back and have a line of LEDs across the back that would light up and shine through the slats when the pedal was on.  Except I didn't really think about it at the time, but lighting up 25 or so LEDs off of a 9V power supply doesn't really work.  Eventually the voltage drop becomes too much and they don't light up (right?).  And I'm sure that if they're sucking up all that voltage too it can affect the circuit as well?  So would I need some sort of step-up to increase the voltage to light up all of those LEDs?  Is there a way to make this work?

[jakenold]

Hey Mike.

What I would do, is find out the voltage drop and put the LED's in pairs and connect them in parallel with the power supply. Most power supply's should be able to feed a LOT of LED's. You might also want to measure how many amperes are drawn when the LED's are on, so you get a clue as to how much power the effect will consume.

Kind regards, Jake

[MikeH]

Is voltage drop consistent across an LED no matter what voltage it's being fed?  In other words, of it has a voltage drop of 1v, at 3v it will drop to 2, and at 1.5v it will drop to 0.5?  Or does it depend on the amount of voltage being put into it?

[jakenold]

That's how I understood it, yes. 

[MikeH]

So, since voltage gets divided in series, but not parallel, and amperage gets divided in parallel but not series (Is that right?), I could string 5 or 6 in series, (basically until they wont light; ~1.5V = 6 LEDs to reach 9V), and then put another couple groups in parallel, and that should work as long as I have enough amperage? 

[AL]

I wouldn't light that many LED's with a battery. Try a power supply.

AL

[MikeH]

Oh yeah, for sure.  I'm using a wall wart.

[spudulike]

Use a wizard to design the array you need :-

http://led.linear1.org/led.wiz

[Sir H C]

I like to use current mirrors to do this.  YOu have one PNP transistor with the base tied to the collector, and a pot to set the bias current off the collector.  Put about 1k on the emitter to the top rail.  Now you stick PNP transistors with the bases tied to the base-collector junction of the first transistor and 1k resistors off the emitters to V+.  Off these collectors you can string 3 or 4 LEDs (can handle about 7-8 volts on there).  So add transistors as you need more LEDs and if you need only 2 off there, no problem it will work just the same with 1 or 2 in series as they are current driven not voltage driven.

[MikeH]

Use a wizard to design the array you need :-

http://led.linear1.org/led.wiz

Hey, that's just what I needed.  Too cool!