On Wed, 02 Apr 2014 13:37:51 +0100, Martin Brown
wrote:

On 02/04/2014 11:59, John Fields wrote:
On Tue, 1 Apr 2014 18:39:27 -0500, "Maynard A. Philbrook Jr."
wrote:

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Look at all the sims I've posted - or even just one - and it should
be apparent, even to you, that you're talking out of your ass.
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Did you figure out that bloggs screw up yet?

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Here's his post:

"The apparent battery capacity increases dramatically at lower drain
rates. So if you want to increase battery life beyond belief, the
thing to do is wire the LEDs in series and then use a "boost"
converter to step-up the 6V voltage to 24-30 V as required."

Where's the screw-up?

John Fields

It does work as advertised

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Bloggs' claim, yes
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his claim is not at all believable.

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Surely you mean Jamie's...
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Putting red LEDs in pairs with a basic resistor ballast hung off 6v will
easily match or beat his suggested method. The buck converter on that
step up ratio would be lucky to get 80% efficiency and is a lot more
complex to implement.

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Of course, but you seem to have glossed over Bloggs' scheme, which
was to place many LEDs in series and then to use a _boost_ converter
to power the string, now bereft of all except one series current
limiting resistor.
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PWM regulating of the current would be another choice.

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Sure, but with a 6V supply - not a good one.
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The losses could then be as low as Vsat.Imax plus dissipation in a smaller series
resistor to ensure chains share current more or less equally.

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With a 6V supply and two 2.2V diodes in series that only leaves you
with 1.6 volts of headroom for the series resistors _and_ Vsat of
the PWM driver at its output.

Not only that, but you've got the LEDs' Vf high-side tolerance to
contend with, so PWM probably isn't the best idea.
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Most LEDs can survive a higher pulsed current provided that their power
dissipation is not exceeded. More of your supply voltage gets converted
into light this way since Vf rises with increasing current.

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LED's, being diodes, follow a curve where very small Vf changes
result in large If changes but, with a current limiting resistor in
there, changes in Vf will have little effect on the LED's output, I
think.