[sdiy] non-mechanical switching solutions

Roman Sowa modular at go2.pl
Mon Jun 27 16:45:49 CEST 2016


There are a few TO92 MOSFETs with on resistance less than 1 ohm, maybe 
not especially common or super cheap, but available. For example VN3205 
from Microchip (formerly Supertex) with 0.3 ohm.
If you're OK with SOT23, then you have a lot to choose from, tens of 
miliohms, and dirt cheap.

MOSFETS always have reverse diode, so it may be tricky to use it as a 
switch, maybe if you take 2, back to back, it might work regardless of 
overall polarity.

Or get some fancy MUXes from Maxim, they make lots of switch chips with 
low on resistance and high signal voltage.

yet one crazy idea - solid state relays, but maybe it's too pricey.

BTW,

W dniu 2016-06-27 o 16:15, Pete Hartman pisze:
> 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
>
>
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