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From: firstname.lastname@example.org (The little lost angel)
Subject: Re: How to make/solder a circuit fast?
Date: Sat, 14 Sep 2002 03:56:09 GMT
Organization: Yes please, I'm quite disorganized & lost at the moment
References: <email@example.com> <firstname.lastname@example.org> <3D8222E7.F121BEEA@texas.net> <email@example.com> <3D823F64.36514EE5@texas.net>
NNTP-Posting-Date: Sat, 14 Sep 2002 03:54:41 +0000 (UTC)
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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
>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 :(
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