[sdiy] bunch O' LEDS

[Eric Brombaugh]

While there is the potential for current-hogging where several of the 
diodes will conduct more than others it's probably not a big deal. If 
you look at Linear Tech and Intersil product lines you'll see a lot of 
LED drivers designed to efficiently power backlights for mobile 
applications and a lot of these put LEDs in parallel.

You don't need to drive them linearly though - you could use a MOSFET to 
gate full current into the whole string of LEDs and then use PWM to get 
a nice linear brightness variation. Make a nice sawtooth at a fairly 
high frequency to prevent flicker and then use a comparator to set the 
output pulsewidth proportional to the CV input. That would also prevent 
the output transistor from having to dissipate a lot of power and could 
cut down on heat sinks, etc.

Eric

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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