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<div id=":27n">From: Dan Fink <a class="moz-txt-link-rfc2396E" href="mailto:danbob88@gmail.com"><danbob88@gmail.com></a><br>
To: Hugh Piggott <a class="moz-txt-link-rfc2396E" href="mailto:hugh@scoraigwind.co.uk"><hugh@scoraigwind.co.uk></a><br>
Cc: <a class="moz-txt-link-abbreviated" href="mailto:danbob@otherpower.com">danbob@otherpower.com</a>, RE-wrenches
<a class="moz-txt-link-rfc2396E" href="mailto:re-wrenches@lists.re-wrenches.org"><re-wrenches@lists.re-wrenches.org></a><br>
Date: Sat, 17 Dec 2011 08:59:58 -0700<br>
Subject: Re: [RE-wrenches] DC power for LED lighting<br>
Hullo Hugh;
<div><br>
</div>
<div>Perhaps I could state it better, that each string of
series LEDs needs to have its own current limiting device
(a resistor or regulator). </div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>It's a positive feedback loop (thermal runaway). As an
LED heats up, forward voltage drops and current increases.</div>
<div><br>
</div>
<div>So, by current limiting each string you avoid both the
variable brightness problem and the thermal runaway
problem.</div>
<div><br>
</div>
<div>This is a pretty good article on the topic:</div>
<div><a href="http://www.ledsmagazine.com/features/4/8/1"
target="_blank" style="color: rgb(17, 85, 204);">http://www.ledsmagazine.com/<wbr>features/4/8/1</a></div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>-- <br>
Dan Fink,<br>
Executive Director;<br>
Otherpower<br>
Buckville Energy Consulting<br>
Buckville Publications LLC<br>
NABCEP / IREC accredited Continuing Education Providers<br>
<a href="tel:970.672.4342" value="+19706724342"
target="_blank" style="color: rgb(17, 85, 204);">970.672.4342</a><span
class="Apple-converted-space"> </span>(voicemail)<br>
<br>
<br>
<div class="gmail_quote">On Fri, Dec 16, 2011 at 11:03 PM,
Hugh Piggott<span class="Apple-converted-space"> </span><span
dir="ltr"><<a href="mailto:hugh@scoraigwind.co.uk"
target="_blank" style="color: rgb(17, 85, 204);">hugh@scoraigwind.co.uk</a>></span><span
class="Apple-converted-space"> </span>wrote:<br>
<blockquote class="gmail_quote" style="margin: 0px 0px
0px 0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;">H Dan,<br>
<div><br>
On 17 Dec 2011, at 00:02, Dan Fink wrote:<br>
<br>
> parallel connections are BAD with LEDs, and
the string with the most voltage will eventually
fail first, so best practice is regulate every
string.<br>
<br>
</div>
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?<br>
<br>
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.<br>
<br>
What am I missing?<br>
<br>
thanks<br>
<span><font color="#888888"><br>
Hugh<br>
<br>
</font></span></blockquote>
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