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Old 26th October 2005, 08:15 PM   #1
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Join Date: Apr 2005
Location: Spain
Default rotary mains switch and snubbers

Hi,

I’m going to connect a rather complex system (a valve amplifier and a home theatre amp both sharing the same main L&R loudspeakers, and a headphone amplifier). The small headphone amp and the relay-switching system for selecting the amp to drive the loudspeakers will be built into the valve amp case (there is room for them), and the neatest arrangement is to use only one front rotary mains switch with enough positions to select which amp to use: amp#1, amp#2 (relay PSU on and coils energised to connect amp#2 output to loudspeakers) and headphone amp.

There are many toggle switches available for mains use, rated at 6A, 15A or even 30A, but with not enough positions (on-off-on max). I’ve found in Mouser a high capacity NKK rotary switch (HS16-2N) that just fits in the available space. It has 2 pole-11 positions (!) phosphor bronze (movable) and Ag-plated brass (stationary) contacts and is rated at 12A/125Vac and 6A/250Vac (this is the relevant value in my case). Is it appropriate?

The original rotary mains switch of valve amp is only 4A rated, but it is protected from the high voltage “on-off” transients by a “snubber” RC circuit (a special cap with “0.022uF+100 Ohm” marked in its body, connected from ac-in to ac-out switch contacts for both live and neutral). Is this “snubber” also necessary for the new higher quality 6A-rated mains switch? (perhaps only for the valve amp contacts, as the mains transformer of headphone amp is small and the PSU for powering the relays is a very small switch-mode type). If I have to use “snubbers”, what is the correct type of capacitor for mains voltage and current? The 100 Ohm resistor wired in series with the cap must be also a special type (and what power rating)? As we are dealing with mains, I’d like to clarify these doubts before any action.

Thank you

Regards
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Old 28th October 2005, 09:35 AM   #2
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Join Date: Sep 2005
My amp uses a 220nF mains cap and 100R 1W resistor in series, between the switched active and switched neutral.

The capacitor you use MUST be mains AC rated. Mains caps are sold as such and normally have a rating on them like "275V AC". Don't use anything else. The resistor can be a standard 1W type (I used 5W because I had one lying around). The leads should of course be suitably insulated so that they can't accidentally touch the chassis or a curious organic unit (human) poking around inside the amp.

I would definitely still use the RC snubber, it protects the switch contacts.
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