Date: Wed, 19 Apr 2000 18:24:40 -0400
Reply-To: David Beierl <dbeierl@IBM.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@IBM.NET>
Subject: Re: Third brake lights (tack-free)
In-Reply-To: <2.2934ad0.262f1d91@aol.com>
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At 10:32 4/19/2000, Rico Sapolich wrote:
>Oh yes, the power of the internet! Sometimes it seems like its true prowess
>is its ability to disseminate inaccurate information in terms of absolutes.
>A CURRENT-LIMITING SERIES RESISTOR IS NOT NEEDED TO OPERATE AN LED AT A
>VOLTAGE NEAR ITS TYPICAL OPERATING VOLTAGE.
Oh all right. I'm sorry I said *absolute* when I should have said "unless
you're reckless." Yes, I'm quite familiar with forward voltage vs. current
curves. I was speaking in practical terms of a working installation that's
expected to operate for some period of time. Also speaking to an audience
who I didn't think wanted to hear an earful about diode
characteristics. And, I might add, I've smoked quite a few LEDs in my
time, not that I'm proud of it. Embarrassed, rather.
>certain that I am not full of bullshit. Hell, if you feel a little frisky,
>connect ONE of them across 2 of the batteries (i.e., about 3.12 V) and watch
>it glow brightly.
Ok. I was feeling frisky. Tried a red one first. Half an amp, got a
brightish flash which quickly dimmed and then continued to dim until it
went out. Current gradually sagged to 400 ma, and the device was sizzling hot.
Another red (similar), same results.
Orange, lit up nice and bright at 50 milliamps (twice its rated absolute
maximum current). Gradually got hot and slightly dimmer. Current steady.
Yellow, bright at 60 ma, three times its absolute maximum rating. Quickly
got hot and moderately dimmer. Current sagged to 58 ma.
Another yellow (high-brightness type) lit up very bright at 220 ma. Got
slightly dimmer and current sagged to 190 ma. Leads were hot.
So what's your point? That if you're really lucky you can get a device to
balance on a usable place on the Vf/If curve? I agree. With the lower
voltage (per LED) you suggested, the orange and one of the yellows might
even have survived for a while. I hope you're not suggesting this is good
engineering practice, or the way to build a reliable piece of equipment....
>*absolute* you come across.
Yes, I do apologize for that *absolute* -- very misleading.
:)
david
David Beierl - Providence, RI
http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
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