[sdiy] bunch O' LEDS

[harrybissell]

Since the LEDs (diodes) have a soft 'knee' they might both
light if the forward voltages were close.

I would not count on them 'both' lighting... or 'not' lighting
without dropping resistors.

I repaired the backlight from my Alesis D4.
It used 24 LEDS in twelve series pairs... all in
parallel.  NO dropping resistors.

To fix it, I bought 100 SMT LEDs and selected the closest 24
for forward voltage drop, within .02V.   They ALL light and are equally
bright. It took three hours to do

H^) harry

Scott Gravenhorst wrote:
> 
> Well, I always thought that when diodes are connected in parallel (without series
> resistors), they don't "share current" like resistors do.  One diode (of a pair) will
> turn on and it's dynamic resistance will limit the voltage across the other one to a
> value below what is necessary to turn it on.
> 
> But since I'm here to learn, I'm making this statement in hopes that if it's wrong, I'll
> get educated.
> 
> "phil macnutt" <philmacnutt at mac.com> wrote:
> >Ok guys, I need a little help.
> >
> >
> >
> >Sounds weird, but I need to drive a bunch of leds with a pitch CV.
> >
> >For example I'd like the LEDS to see 0-1 amp (50 leds)  corresponding  to a
> >0-2volt input.  The LEDS are all in parallel, and do not have resistors on
> >each one.
> >
> >I poked around and found a really simple circuit that is nothing but an
> >opamp and a single resistor that does this V-C conversion.
> >
> >Works great, but the amp could never drive a whole stack of leds.  So, I
> >guess I need a circuit that I can use a power transistor with at the output
> >and some sort of pot to adjust the sensitivity (based on the CV range).
> >
> >If anybody knows of info already on the web regarding this, just point me in
> >the right direction
> >
> >
> >
> >On another topic, I'm currently experimenting with various paints for
> >silkscreening front panels, and I should have some information on which
> >paint is best if anybody is interested.  I got my screens from Westar, and
> >they look great.
> >
> >
> >
> >Thanks in advance for any assistance!
> >
> >phil
> >
> >
> 
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