Fotios need help with input selector. Rotary switch or Relays? - Page 2 - diyAudio
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Old 14th August 2007, 08:05 AM   #11
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While some applications may need relays (electrical control rather than operator, for example) , and adequate relays may exist, a rotary switch (and some push-button types) offers a wiping contact.

For low voltage appications, a wiping contact may be the only way to remove oxides and what ever else forms on the contact metals. Relays, rocker switches, etc. may have contacts that just touch/open.

High voltage applications rely on a contact arc to burn thru any contact impurities upon contact.

Once I asked an engineer why a control signal line had such a large resistor to ground (aircraft input discrete) on a 28 V input, thinking it was a waste of power, and he said since the voltage wasn't high enough to clean the contact, drawing a relatively large current also would draw an arc. 30 years in electronics and that was a new one for me

So the moral of the story is for small signals, a mechanically self-cleaning switch contact is preferred.
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Old 14th August 2007, 08:13 AM   #12
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Fotios,

Nelson Pass is one of the nicest people in audio. There are some real characters too. We can be a very demanding group, yet he keeps giving.

I wrote him a letter in pre-WWW days and he not only provided an answer, he sent me parts to do the experiment. Don't expect that from him, but you can expect a helpful response.
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Old 14th August 2007, 08:17 AM   #13
Nordic is offline Nordic  South Africa
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some of the better relays like those made by Finder has a removeable seethrough cap... so they can be cleaned....
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Old 14th August 2007, 08:28 AM   #14
fotios is offline fotios  Greece
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Quote:
Originally posted by multi-volti
Once I asked an engineer why a control signal line had such a large resistor to ground (aircraft input discrete) on a 28 V input, thinking it was a waste of power, and he said since the voltage wasn't high enough to clean the contact, drawing a relatively large current also would draw an arc. 30 years in electronics and that was a new one for me :bigeyes
Wow! This is an amazing information also for me. Thanks verry much Murray. Also thanks for your exhortation to contact me with "some" persons. You know what i mean. Thank you verry much again.
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Old 14th August 2007, 08:31 AM   #15
fotios is offline fotios  Greece
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Quote:
Originally posted by Nordic
some of the better relays like those made by Finder has a removeable seethrough cap... so they can be cleaned....
Thanks again Nordic. In a such way i am thinking also. Thanks
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Old 14th August 2007, 08:51 AM   #16
fotios is offline fotios  Greece
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From all relay constructors untill now only from the OMRON i have seen noise figures in dB caused from the electromagnetic interference of coil when activated. My previous thought of the inverse way of using relays (deactivated to pass the signal, activated to ground their contacts) seems to be realised in good way but with the use of DPDT instead SPDT relays. I will post the idea later today. Again thanks to all.
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Old 14th August 2007, 10:18 AM   #17
sandyK is offline sandyK  Australia
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Default Rotary switch or relays

Fotios
Either method can work very well. The larger multi deck rotary switches can be configured to have very little interchannel coupling.However good quality suitable switches such as the Danish DACT rotary switches are fairly expensive.
What nobofdy else seems to have mentioned,is the desireability of switching the earth side of the individual inputs too. This is particularly important when you have multiple inputs in use, especially things like TV audio, where there is often an earthed TV antenna mast .This will result in a much better S/N in this situation. BTW, the rotary switch in my preamplifier (I believe that you have seen the photo) has never given any trouble whatsoever, over many years of use.
There have been many good points made by Anatech and others too. I have previous work related experience of mercury wetted Clare relays, and they are not the answer to a maiden's prayer !
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Old 14th August 2007, 10:37 AM   #18
mzzj is offline mzzj  Finland
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Default Re: Rotary switch or relays

Quote:
Originally posted by sandyK
Fotios
I have previous work related experience of mercury wetted Clare relays, and they are not the answer to a maiden's prayer !
SandyK
I am curious to know what kind of problems you encountered with mercury wetted relays? Was it dry-switching or power use?



Anandtech, How horribly expensive? Last time I was looking for mercury wetted reeds the price was usually 1-3usd /pcs
For european diyer's: SHP/Elfa may still have CP-Clare CV60202 merc wetted on discount, 35 eurocents/piece (0.5usd)
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Old 14th August 2007, 10:42 AM   #19
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Default Use sealed reed relays

Reed relays have their contacts sealed in an inert gas filled glass tube.

They are specifically designed for low level switching without any self-cleaning.

I have used miniature reed relays by the tens of thousands switching nanovolt level signals in automatic test systems. They will effectively last for ever unless abused with inductive loads.

With only 2 or 3 pF between contact and coil, any feedthrough of noise in the audio bandwidth and at reasonably low impedences is negligable.

My present pre-amp / selector uses a rotary switch on the front panel driving a small pcb of relays at the rear, close to the input RCAs.
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Old 14th August 2007, 11:01 AM   #20
sandyK is offline sandyK  Australia
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Default Rotary switch or relays

mzzi
It was mainly switching use in Telecommunication relay sets as used in Crossbar Telephone Exchanges, but varied types of use and coctact switching requirements as to current and voltage.
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