On Sun, 17 Jan 2010 11:56:32 -0800, Archimedes' Lever
wrote:

On Sun, 17 Jan 2010 11:43:13 -0800, John Larkin
wrote:

On Sun, 17 Jan 2010 11:22:23 -0800, Archimedes' Lever
wrote:

On Sun, 17 Jan 2010 10:23:00 -0800, John Larkin
wrote:

On 17 Jan 2010 16:54:58 GMT, mick wrote:

On Sun, 17 Jan 2010 08:35:12 -0800, John Larkin wrote:

On 17 Jan 2010 16:01:35 GMT, mick wrote:

On Fri, 15 Jan 2010 23:02:08 -0800, John Larkin wrote:

snip

There's a pot on the linear regulator; it's in plain sight. And that's
all this thing needs. The PWM accomplishes nothing.



PWM dimming is more efficient than linear dimming - by a long way. It
all depends on how much heat the OP is willing to let the dimmer
dissipate.

PWM is no more efficient than resistive dimming the way he did it. There
are no inductors in his circuit. All this sort of PWM can do is move the
heat around.



I couldn't see his circuit on my server, but I assumed that he was
varying the mark/space ratio. In that case power dissipation in the
output device is always low, depending on Vsat.

That just moves the heat into the series resistors. Overall efficiency
is always the same for a dissipative (inductor-free) regulator.

With a linear regulator and properly chosen series resistors, you can
balance the regulator heat distribution versus dimming level.
Regulator power dissipation versus output voltage is sort of
parabolic... low at low illumination, low at max illumination, peaking
somewhere between.

It's easier to heatsink a voltage regulator than a lot of small
resistors.

Another advantage of a voltage regulator is that it regulates.

John


You take a few of these and drive four sets of LED pairs per chip.

That's $0.50 per LED for constant current, precise control.

Might as well go all out. Beter than any use specific "LED driver" chip
out there.

http://www.edn.com/article/CA6702709.html

http://a330.g.akamai.net/7/330/2540/...6702709XLG.jpg

Yikes! It's horrendously complex, needs an I2C interface to set
brightness, and is constant-voltage, not constant current.


You did not look close enough. It will work in either mode.

And it will
generate lots of EMI, bad news in an airplane.

All he needs is an LM317, a pot, and a few more passives.

I know how to make a simple feedback controlled driver loop, Johnny. I
don't need a primer. That wasn't the point.

The point is that you're AlwaysWrong.

John