At 04:04 PM 8/5/2005 -0500, you wrote:


> > I suppose you need the low side of the LEDs to be grounded...
>
>Preferably.  One of the ideas I thought of was to put R101 at the common
>ground of the LEDs instead with the transistor bridging that, but then
>realized that a constant-current source would automatically adjust that out.
>Doh!

;-)

> > Do you have enough voltage for the LM317 to work (about 3V on top of the
> > LED forward voltage)?
>
>Per the datasheet, each LED has 1.7V typical, which adds up to 10.8V per
>string of 6.  I was looking at the dropout specs on the LM317 datasheet this
>morning which showed a dropout of ~1.6-1.8V for this current and expected
>temps.

*Plus* Vref (1.25V nominal), in this configuration, yes?

<snip>
>Wow, that actually makes sense to me, in spite of my amateur op-amp 
>knowledge.
>But unnecessarily complex.

< 15 parts counting series diodes and 3 caps, not insane. And very resistant
to crud on the 12V using a 100V transistor and shunt reference/power supply.
And 150mV minimum rather than 3V.

If you go with the obvious LM317s, careful about sneak currents back through
the regulators. The series diodes will prevent it.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


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