From: Watson 'Atto Parsec' Name
Subject: Re: Best way to power array of LEDs?
References: <0001HW.B9C26E7400A7161B165FEAC0@news.covad.net> <3DC86222.BD639CE8@fanwap.com>
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Date: Thu, 7 Nov 2002 18:12:31 -0800
NNTP-Posting-Date: Thu, 07 Nov 2002 19:13:44 PST
Organization: InReach Internet
In article <3DC86222.BD639CE8@fanwap.com>, firstname.lastname@example.org talked about...
> Is it easy to buy "current limiting diode" there ?
Honestly, I've never seen a circuit on a PC board that uses one. They're
just not found, perhaps because it's easy to make a current limiting
circuit with a few standard parts. One can even be made from a JFET with
the gate tied to the source, as has already been mentioned.
Here is a constant current circuit using the TL431, which can be found in
just about every PC switching power supply. Just take one out of a defunct
AT supply. Check out figure 14 on page 6.
One problem with this is that the transistor has about 2.5V at the emitter,
so the current setting resistor, which for 30 mA is 83 ohms, has to drop a
relatively large amount of voltage and dissipates 75 milliwatts of power.
But, by changing the transistor to a 2N7000 enhancement mode MOSFET, the
voltage across the resistor goes down to about .4V because the gate turnon
voltage of the 2N7000 is about 2.1V. So I think that I can get away with
using the 10 to 15 ohm resistors with a lot less power loss. And since the
gate of the 2N7000s draw no current, I can parallel a dozen or more of them
across the TL-431 to get as many of the LEDs as I want regulated to 30 mA.
All I need add is another MOSFET, another 10 ohm resistor and another LED.
Come to think of it, a TL431 is not needed for the 2.5V reference. A green
LED has about 2.4V across it, so it would make a cheap and dirty voltage
regulator. And a power on light as well.
> I using 8mA current limiting diode CCLHM080
> Just connect it series with LEDs.
My email address is whitelisted. *All* email sent to it
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Subject: line. alondra101 hotmail.com
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Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html