[sdiy] non-mechanical switching solutions

[Ove Ridé]

You might consider using P FETs for the top stage(s) so you're pulling down
to turn them on. Downside, harder finding devices with a matching low Rdson
due to different doping characteristics in P FETs compared to N FETs.

On Monday, 27 June 2016, Pete Hartman <pete.hartman at gmail.com> wrote:

> I have a problem I'm trying to solve....
>
> ...and let's just assume the problem has to be solved, I can't just
> redesign around it.  In a perfect world ... but that's not where this sits.
>
> I'm working with a ladder filter, switching 3 different capacitor ranges
> (Moog 904A, yes?).  But I don't have room for a multipole rotary switch of
> the necessary size.  What electronic solutions might I use for this?   (and
> note that while step one is to solve four poles for the 904A, step 2 will
> be to solve for the 904C which has a *9 pole* switch, which hopefully can
> use the same solution)
>
> If I use an on-off-on switch, pull ups and ground connected to the common,
> I have three positions giving me 01, 11, and 10 which can be decoded to 3
> out of 4 positions on a mux or a decoder.  A Mux acting as a switch to put
> in or out the various capacitor values....
>
> First thought: CD4052, but that doesn't work well in practice at all.
>
> Second thought: DG409, better characteristics, but that still doesn't work
> sufficiently well.
>
> Dig around a while for why.... aha, the on resistance for both is
> significant, especially for the capacitive stages of a ladder filter.  The
> DG409 has on the order of 100R - 120R.  Too much, and confirmed to be the
> issue by comparing physical connections with wire to physical connections
> with 100R resistors.
>
> Third thought: MOSFETs have low on resistance....  And an initial test of
> VN0104's (on resistance on the order of 3R - 5R, much better) shows
> somewhat more promise.  A little concerned about on capacitance and also
> whether I can get an appropriate Vgs at the top stage of the ladder (where
> the source voltage will be on the order of 10V), but so far so good; may
> need a MOSFET with a lower range for Vgs-on, but the VN0104 is pretty darn
> good with a max of 2.4V.... that's just at the edge of the spec though, I
> think, something better would be nice.  Don't want to be in the position of
> having to "select" working transistors for the top stage.
>
> So the questions for the assembled brains bigger than mine:
>
> 1) can anyone suggest a better (yet still reasonably common) mosfet with
> low Vgs and low rds-on?
>
> 2) some other (electronic, not mechanical) solution that would work in
> this specific instance?  Note that while the above description assumes an
> SPDT switch, I can go up to 2 poles, but not beyond.  An on-on-on would
> also be an option.  Toggle switch is a requirement though.
>
> Just to repeat; hopefully this isn't necessary in this audience, but some
> other fora where I occasionally ask questions, I often get offered answers
> that ignore the constraints I'm trying to fit within (in this case, no
> mechanical solution, redesigning to use a physical rotary switch is not an
> option).  "Your constraints are stupid" is not a helpful suggestion, yet
> you might be surprised how often it comes up :).
>
> Thanks...
>
> Pete
>


-- 
/Ove

Blog: <http://blog.gg8.se/>

"Here is Evergreen City. Evergreen is the color of green forever."
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