EMI leaking down shaft of rotary switch

ggherold · 2009-06-09T02:04:31+00:00

I=92m using Grayhill series 50 rotary switches for low level signals.Every once in a while, the rotating shaft loses contact with thegrounded switch body. When this happens I get electro-static fieldsin the room leaking into the circuit. (The circuit lives inside ametallic box.) It is only the high impedance part of the circuit thatappears susceptible. I was able to measure shaft to groundresistances and correlate large resistance (1k ohm to 10 k ohm) withEM leakage. Typical resistance values were less than 1 ohm. So I=92vecontacted Grayhill and they are preparing a quote on switches with adifferent shaft to body sealing option. But these are special ordermil-spec parts and I=92m afraid the price will match. There is a blurbon the first page about positive shaft grounding for emi shielding,but I found that in a random sample of new switches on hand about 1/2showed 1 k ohm shaft to body resistance with a bit of wiggling of theshaft. And I worry that the rest may =91fail=92 with a bit of wear.These switches will get a lot of work.I started thinking about making my own grounding contact to theshaft. There are probably hundreds of ways to do this. My =91bestidea=92 so far is a conductive knob, perhaps nickel plated, and aconductive washer (phosphor bronze?) that sits under the knob and hassome sort of fingers that make contact with the bottom of the knob.Searching the web proved fruitless, but as always you have to knowwhat to search for. It seems like someone must have already had thisproblem and perhaps has a solution to sell me. Any thoughts, ideas,links or clues will be greatly appreciated.Thanks,George Herold

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Robert Baer 17 years ago

Hopefully my crazy idea of disks to create beyond cutoff waveguides is useable for the band of frequencies of "interest". Otherwise, you might have to "hotFOOT" it by switching the resistors with FETs (bad pun somewhere there), using the switch to control the FETs.

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Jasen Betts 17 years ago

here's an idea drill a small hole (~1mm) through the panel near the shaft hole shuch that the hole will be hidden behind the knob but is not covered by the nut or flange, through this hole put a pice of spring wire bend the underside end at 90 degress and do the same at the top side but twist it so the the spring touches opposite sides of the switch, eg it presses on the left side of the flage below the panel and the right side of the shaft above the panel

Plan

_______________ / \\ / ......... \\ / ............. \\ / ... _______ ... \\ / ... / \\ ... \\ / ...#############... \\ / ...###############... \\ / ...#################...@ \\ + / ...#################..@ \\ + \\ ...#################.@.. / + \\ ...#################@.. / + \\ ...###############@.. / + \\ ...#############@.. / + \\ ...###########@.. / + \\ ............@. / + \\ ..........@ / + \\ @ / + \\_________@_____/ + @ + @ +++++++++(@)@ ++++ spring behind panel (@) hole @@@ spring above panel _ / \\ nut \\_/ _ / \\ ### shaft ... ... bolt/bushing ............ ............ ......@@@@@@@

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John Fields 17 years ago

The space between the shaft and the bushing is tiny, so with a light enough lubricant capillary action would draw it in along with the powdered silver.

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James Arthur 17 years ago

Yep, that's my spin on John's idea, with an ASCII telling.

I think it elegant, affordable, and very robust, but agree that Grayhill's the preferred route.

Cheers, James Arthur

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EMI leaking down shaft of rotary switch

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