contact resistance of rotary switch ?

[Bernhard]

What will be the contact resistance of a very high quality gold or silver contact rotary switch ?

[Bobken]

Hi,

It should be much less than 10 mOhms if it is a well made switch.

The gold-flashed silver contact Elma miniature "01" series is 5 mOhms as a guide, and the NSF "JMK" series is <3 mOhms.

Even a cheap (less than 1 Euro) Lorlin "CK" series with brass/silver-plated contacts claims <10 mOhms.

All these examples are 'initially', so they probably do detereorate with use as a Grayhill version suggests 20 mOhm initially, with a max. of 50 mOhm, but I think that is a poor example of switch contacts.

Regards,

[tedr]

when you have clean metal to metal contact, as intended, the figures are a few milliohms. Note that dirt and oil can affect all metals even gold. Silver Oxide is a conductor (good) but silver sulphide (from atmospheric contamination) is an insulator. Gold is preferred for small signals, low power, silver when there is some current being switched.

Ted

[Bernhard]

thanks

Originally posted by tedr
Gold is preferred for small signals, low power, silver when there is some current being switched.

Can 8mA be regarded as some current ?

I need to switch currents up to 8 mA to ground through resistors that have values as low as 30 milliohms.
For I/V conversion.

[AndrewT]

Originally posted by Bernhard
thanks



Can 8mA be regarded as some current ?

I need to switch currents up to 8 mA to ground through resistors that have values as low as 30 milliohms.
For I/V conversion.

I think the variation in switch resistance (repeatability) is going to make achieving accuracy difficult.

[Bobken]

Originally posted by AndrewT

I think the variation in switch resistance (repeatability) is going to make achieving accuracy difficult.

I see absolutely eye-to-eye with Andrew on this one.

What kind of resistors will you be using which are merely 30 mOhms in value?

Regards,

[AndrewT]

Originally posted by Bobken
I see absolutely eye-to-eye with Andrew on this one.

"great minds"
what's the rest of the proverb?

[Bernhard]

Originally posted by Bobken

What kind of resistors will you be using which are merely 30 mOhms in value?

Copper wire

Higher values will be manganin wire.

My idea is to integrate a volume control into the passive I/V conversion and connect the power amp directly to the low impedance passive output stage.
The whole analog section is already completely free of active devices and standard resistors.

I have rotary switches with 26 steps that gives 0dB to -60dB in 2,5dB steps + muting.

0,03 ohm is -60dB. 30 ohm is 0 dB

[Bobken]

Hi,

That's an interesting concept, and I hope that it goes well for you.

If you are not already aware of it, it would be worthwhile trying some Cramolin-based contact cleaner/enhancer, in my experience.

The most 'popular' being made by Caig Labs (www.caig.com) in the USA, and you will see from their website that this product is used by manufacturers in critical locations like aero-space, & telecoms etc.

Their marketing names have changed over the years (Cramolin, Pro-Gold etc.), but the basic ingredient has remained the same for the 25+ years I have been using Caig products. Several high-end audio manufacturers also use it to provide lower-resistance and more reliable long-term contacts which will enhance the sounds.
It also has some lubricating properties, which helps to prevent plated contacts from detereorating as quickly, and this might be of some help with your rather critical requirements here.

Regards,

[oshifis]

Why do you need extremely low contact resistance? If the resistors are connected in series (their sum making up 30 ohms), the switch contact will not affect them. It will be in series with the load, i.e. with your power amplifier that has high input impedance, I suppose. So the low contact resistance may not be very important.
8 mA x 30 ohms = 240 mV voltage at the input of your power amplifier, if I got it right.