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Re: 19,008 LEDs on the wall, 19,008 LEDs

Eric Smith wrote:

Michael J. Mahon wrote:


(Scribbling on back of envelope...) If a "lit" LED has an average of
10mA of current flowing, and is driven from 5v with resistive losses,
that's 50mW of power per lit LED, or 50W per 1000 lit LEDs.  So half
lit would be an average power of almost 500W, and all on (which must
be handled) a kilowatt, for a 200 amp 5v supply!



I think you would want the average current of a lit LED to be substantially
less than 5 mA.  Were I doing such a thing, I'd probably try for a
duty cycle of around 5%, and about 20 mA active current, for an average
of 1 mA.


That would save power, but would operate the LEDs at substantially
less than their design brightness.  Anyone building a wall of LEDs
is probably going to want them to be "all that they can be", at least
when showing it off.  ;-)


And I'd probably avoid the resistive losses by eliminating the series
resistors and using switching regulators configured to regulate the current
rather than voltage.

The HP-35 calculator, introduced in 1972 as the world's first handheld
scientific calculator, used inductive LED drive for the same reason.
All the energy went into the LEDs rather than having a significant
fraction burned up in current limiting resistors.


That would be a good plan, and would reduce the power by about a
factor of two.  A more complex design, but worth it with this many
LEDs.

Of course, saving power in a battery-operated device is not just
a good idea, it's a requirement.

-michael

Music synthesis for 8-bit Apple II's!
Home page:  http://members.aol.com/MJMahon/

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