From: Dan Fink <[email protected]>
To: Hugh Piggott <[email protected]>
Cc: [email protected], RE-wrenches <[email protected]>
Date: Sat, 17 Dec 2011 08:59:58 -0700
Subject: Re: [RE-wrenches] DC power for LED lighting
Hullo Hugh;
Perhaps I could state it better, that each string of series LEDs needs
to have its own current limiting device (a resistor or regulator).
The reason I advise that is that all LEDs vary a bit in their
volt/amp/lumen performance curves, especially between batches, but even
within a batch. If you were to put 2 "identical" LEDs in parallel
sharing one current limiting resistor, one LED is *always* going to draw
more current than the other and be brighter. If the difference is too
great, the one drawing more current will burn out first. If you are
lucky with your matching, it might not be problem for a very long time,
but I've also seen burnout happen between batches in just a few minutes.
It's a positive feedback loop (thermal runaway). As an LED heats up,
forward voltage drops and current increases.
So, by current limiting each string you avoid both the variable
brightness problem and the thermal runaway problem.
This is a pretty good article on the topic:
http://www.ledsmagazine.com/features/4/8/1
<http://www.ledsmagazine.com/features/4/8/1>
It's nice that we can all purchase decent LED lighting products these
days and not muck about with breadboards and soldering irons like we had
to a few years ago. A good inexpensive compromise is to regulate all the
strings with a single DC power supply, then current limit each string
with a resistor.
--
Dan Fink,
Executive Director;
Otherpower
Buckville Energy Consulting
Buckville Publications LLC
NABCEP / IREC accredited Continuing Education Providers
970.672.4342 <tel:970.672.4342>(voicemail)
On Fri, Dec 16, 2011 at 11:03 PM, Hugh Piggott<[email protected]
<mailto:[email protected]>>wrote:
H Dan,
On 17 Dec 2011, at 00:02, Dan Fink wrote:
> parallel connections are BAD with LEDs, and the string with the
most voltage will eventually fail first, so best practice is
regulate every string.
This statement puzzles me so I wonder if you could clarify it for
me? If you connect two strings of LEDs (or batteries) together in
parallel then their voltage will be the same. So how come you are
talking about one having more voltage?
If the answer is that it's absolutely impossible to make the
connections and leads have the same resistance then this same logic
applies to two strings that are connected each to its own regulated
supply. If a tiny difference in voltage matters (which I doubt)
then you will have the same issues with a single string.
What am I missing?
thanks
Hugh
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