Vactrols (was: New Stuff)

[harry bissell]

There is a sort of "rule of thumb" for the various materials... if you want
high speed you get a smaller resistance change... if you want wide resistance
change you get increased "dark effect" and much slower speed. Check out the
curves of individual devices... Voltage rating has an effect also (this is
very important if you are playing with a <gasp> tube circuit). Guitar amps
use Vactrols a lot for channel and efffect switching...

There are many different dopings available with different characteristics and
temperature drifts (another big problem).

Early vactrols may have used incandescent lamps or neons in place of LEDs.
These can be very slow (ind) or quite fast (neon).

National Semi has a neat "analog multiplier" app note (AN-20) which may be
used if you use a dual vactrol. Lose the incandescent lamp and use an led
type of it will be unstable at low input voltages.

I think Arthur C. Clarke was the one who suggested a "flashdark"... just the
thing for removing carriers... Flip the switch and projects darkness...  :^)
Harry

Martin Czech wrote:

> :::I suspect it is not just a combination of a LED and a LDR (CdS?), but
> :::a some kind of special photocells. I remember there were some points
>
> And I believe up to now that's exactly what they are ;->
>
> The specific properties of the CdS-crystal should influence carrier life
> time and thus resistance rise time.  Generating carriers with light "out
> of nothing" is no problem, given that the light has enough energy, but
> eating up the carriers get's more and more difficult when the resistance
> rises, because the R*I drop makes it difficult to suck all carriers away,
> they feel not much of the external field.
>
> Perhaps these vactrols have optimised crystals with high recombination
> rate, thus better rise/fall ratios.
>
> ???
>
> I don't know.
>
> m.c.