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On Mon, May 26, 2008 at 01:56:24AM -0400, Spehro Pefhany wrote:

Doh, this ended up in my sent-mail, but never got past that... weird...

> >Isn't the trick with Charlie-plexing that you are exploiting that the
> >combined forward voltage of two leds in series is high enough that a
> >single led in parallel will take all the current? My 12V will be
> >significantly higher than the forward voltage of the diodes used.
> 
> I think with the sneak paths you'd get 12V/3 = 4V across the coils that are
> not supposed to be energized. If that's too much to guarantee *no* flipping
> then it won't work reliably without all those diodes.

I did some quick experimentation, the coils can be reliably flipped with
as little as 3V with a unknown by hand pulse-width, and that same 3V
accross three dots fails to flip them.

That said, in a private email someone found the supplier of my modules.
As far as I can tell they just put their website online within the past
few months:

http://www.flipdots.com/EN/electromagnetic_displays/products/page-5/dot_stripes.html

The 2.7" modules are the exact ones I have, which they are now making
with original tooling bought from the defunct MARK IV FP Electronics.

They specify a 350ma minimum current, achievable with 1ms pulse at 7.5V
given the coils inductance.


Anyway, I may build up a pulse tester and see how well things work in
temperature extremes, and from unit to unit, although diodes would be
safer.


FWIW this is a quite nice work of art using these things:

http://www.troika.uk.com/cloud.htm

> A 3-cent BAV99 each isn't so bad.

Yup, $64.74/1000x and $200 worth of labour. :)

- -- 
http://petertodd.org 'peter'[:-1]@petertodd.org 
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