[sdiy] pots with centre detent?

Neil Johnson neil.johnson97 at ntlworld.com
Sat Jun 23 00:32:13 CEST 2012

Hi Dave,

> Has anyone tried and can comment on the CGS81 scheme, where a pair of BAT48s
> are used to create a "dead zone" in the centre of the offset pot's travel?
> http://www.cgs.synth.net/modules/cgs81_proc.html
> It seems like a neat way to solve the problem of "zeroing", if linear pots
> are being used.

I've looked at very similar schemes in the past.  Some issues I see with it are:

1/ It only really works if you're driving the ends of the pots from
the rails, so you get the maximum voltage range.  But doing this is
just dumb (see my previous comments on using the rails as references
:) )  So lets say you're using 5V references, and bipolar.  So you can
put a maximum of 10V across the pot (-5 to +5).  Those two diodes
steal 0.6 to 0.7V, or about 7% of the pot's electrical rotation.  That
is quite a large chunk of the available rotation (about 8-9% of the
mechanical rotation).

2/ Those diodes will be conducting at less than 0.3V across them, but
only a small amount of current.  Depending on the load resistance that
tiny current might still cause some variation in output in the now
"not-so-dead" zone.

Depending on your needs these issues might be perfectly acceptable.
As always, "it depends" :)

Modules and more: http://www.cesyg.com
Homepage: http://www.njohnson.co.uk

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