[sdiy] pots with centre detent?

[Dave Kendall]

Hi Neil.

Thanks - that all makes sense. Seems using the largest V refs possible 
fed to the pots would be the way forward when using this method.

cheers,
Dave

On Jun 22, 2012, at 23:32, Neil Johnson wrote:

> 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" :)
>
> Cheers,
> Neil
> -- 
> Modules and more: http://www.cesyg.com
> Homepage: http://www.njohnson.co.uk
>