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From: "Boris Nogoodnik"
References: <email@example.com> <firstname.lastname@example.org> <3D8222E7.F121BEEA@texas.net> <email@example.com> <3D823F64.36514EE5@texas.net> <firstname.lastname@example.org>
Subject: Re: How to make/solder a circuit fast?
Organization: KGB, Ltd.
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Sat, 14 Sep 2002 04:52:50 -0400
NNTP-Posting-Date: Sat, 14 Sep 2002 04:52:38 EDT
"The little lost angel" wrote in message
> On Fri, 13 Sep 2002 19:39:00 GMT, John Fields
> >If you want to run LED's in parallel what you need to do is make sure
> >the current limiting resistor is large enough to only allow the maximum
> >rated current for the lowest current rated LED to pass. That means that
> >if you have three 20mA LED's in parallel you have to size the resistor
> >to allow only 20mA to pass. if you have a 20mA LED in parallel with a
> >2mA LED, the resistor should only allow 2mA to pass. This approach will
> >give you severe illumination and illumination difference problems.
> Since these are all super bright LED, I think they are all rated for
> 20mA typical and max 30mA? At least the only specs I can find says
> that :(
> >The proper way to run an array of LED's is to run them in series with a
> >single current limiting resistor or to run each of them with its own
> >current limiting resistor with all the Resistor-LED circuits in
> The problem why I didn't do it this way is I have no idea what LED
> will be plugged in or not. I'm only making the widget, the boys get to
> plug in either 1,2 or 3 LEDs of any colour depending on their fancy.
> And I'm trying to keep this thing under US$3 if possible.
> >should also determine the wattage the resistor will be required to
> >dissipate. P = I*E, so in this case P = 0.02A*3V = 0.06W, so a common
> >quarter watt resistor would be fine. You also need to consider the
> Yup, I did this part.
> >regulator's dissipation, and in this case with an input-output
> >differential of 7V and a 60mA load (three 20mA LED's) the regulator will
> >be dissipating 420mW. I don't have a data sheet in front of me, but it
> >wouldn't hurt if you checked to see whether that amount of power would
> >drive the thing's temperature into never-never land.
> I forgot about this though :(
> Accordingly to the LM2931 To92 datasheet, at 0.4W, ambient temp max is
> around 50~60C. Since I expect the ambient temp to be up to 60~70C, I
> guess that means I can't use the To92 package but must up to the
> To-220 which can do like 1W at 100C without a heatsink. *sighz*
> The small T-92 cost me like US$1.5, the To-220 will probably be at
> least $2.50 :(
Specs are very conservative. Exceeding ambient temp by 10 degrees will not make
your TO92 to melt. After all it's not a critical application. And you can glue
TO92 to a little piece of aluminum if you want. I've done that. And I took the
idea from one of the commercial electronics circuits I dissected. I also
overloaded many components in my hobby projects by up to 3 times and nothing
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