The LM337 would reference to and regulate *down* from the +5 rail. As if +5 were ground. Use a pot to set the LED brightness, unless you are one of those people with an irrational hatred of pots.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Anyone want to draw this up? For those of us who are mental-schematically challenged...
You're doing this to save seven resistors about the size of a gnat? You're gonna make it less reliable, harder to service, cost more, be later to market...I could go on...
I appreciate creativity, but at some point, you have to ship something. If this is the best use of your design creativity, you should have shipped it long ago. You wonder why managers have ulcers? This is a big part of it.
In a word...PRIORITIZE.
It turns out I had the TL431 connected improperly so it was sinking current through the protection diode...oops.
It's wired up correctly now and the circuit is working well. To check what thermals do to it I've been alternately blasting the fully illuminated display + TL431 with cold spray until they're covered in frost, and then hitting it with a heat gun (held some distance away so as not to melt stuff.)
The apparent illumination doesn't budge. The bench supply shows that the current changes a few tens of uA over the range.
Better would be any of a number of LED driver chips. Not as cheap as resistors, but certainly easier than a bunch of discrete current limiters.
-- Rick C
Something like this, maybe. This is always hard to think about for some reason.
The upper resistor could be a pot.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Thanks John. A bit easier to ?see? now.
Is the 431 really cheaper than 7 resistors?
NT
TL431s in SOT-89 I can get for about 12 cents each in small quantities. They're really tiny - about the same size as a single SMT resistor I would feel comfortable hand-soldering.
CNY 52 for 1K in China, about 0.8 cent each.
--sp
-- Best regards, Spehro Pefhany Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8
afaict not significantly better than a resistor to a microcrontroller pin this application.
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Vref (and emitter) at 2.5V means the collector is somewhere above 2.5V which doen't leave much headroom when the approx 2.2V for the LEDs is added, especially if the drive is from a microcontroller pin, and worse if the supply is 4.75-5.25V USB
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Wow!
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