From: firstname.lastname@example.org (budgie)
Subject: Re: Best way to power array of LEDs?
Date: Sat, 05 Oct 2002 15:10:15 GMT
Organization: CanTech Online
References: <0001HW.B9C26E7400A7161B165FEAC0@news.covad.net> <email@example.com> <firstname.lastname@example.org>
NNTP-Posting-Date: Sat, 5 Oct 2002 15:08:41 +0000 (UTC)
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On Fri, 04 Oct 2002 17:19:23 -0700, Lizard Blizzard
>> In the auto situation, the simplest mitigation is to feed the
>> resistor/LED chain thru an LM317 configured as a constant current
>> source. Just the IC and one resistor added, and no voltage variation
>The LM317 has a minimum current which may be greater than a single LED
>can handle. It will work good for a hundred mA or more.
Say wha? If you use an LM317 as a current source, and open circuit it
(which is what you do with the brake circuit idle) then pray tell HOW
there is a a minimum current. I think you are confused with the
standard configs for constant voltage, where there is an adjustment
pin bias current of typically 50uA. Different config, different
>the biggest objection to using the LM317 is that they cost a buck or two
>each, so if you use one for each series string of LEDs, you can end up
>with more cost for the regulator than the LEDs.
>If you're going to spend the money on an LM317, change it to an LM334,
>which is a true current regulator. Even cheaper might be a JFET with
>the gate connected to the source. Problem with that is that the JFETs
>have such a wide range of Idss that you might pick a dozen of them and
>they might all have a wide variation in current.
Problem with JFETs is firstly finding a family/type which has an Idss
figure matching the LED load requirement. If you can do that, the
next issue is (as you mentioned) the parameter spread in N devices.
That's why I would lean to the 317T - a whopping $A1.32 in singles.
>Here's a URL for a LED current regulator using the grain of wheat 1.5V
>25 mA bulbs from Rat Shack. I don't particularly like using lamps for
Nor would I contemplate incandescents in this application.