<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div></div><div>Miniature reed switches? Sort of mechanical, but hey.</div><div><br></div><div>rick</div><div><br>On 27 Jun 2016, at 16:15, Pete Hartman <<a href="mailto:pete.hartman@gmail.com">pete.hartman@gmail.com</a>> wrote:<br><br></div><blockquote type="cite"><div><div dir="ltr">I have a problem I'm trying to solve....<div><br></div><div>...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.<br><br>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)<br><br>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....<br><br>First thought: CD4052, but that doesn't work well in practice at all.<br><br>Second thought: DG409, better characteristics, but that still doesn't work sufficiently well. </div><div><br></div><div>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.<br><br>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.<br><br>So the questions for the assembled brains bigger than mine:<br><br>1) can anyone suggest a better (yet still reasonably common) mosfet with low Vgs and low rds-on?<br><br>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.<br><br>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 :).<br><br></div><div><span style="font-size:12.8px">Thanks...<br><br>Pete</span><div class="" style="margin:2px 0px 0px;font-size:12.8px"></div></div></div>
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