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Old 9th November 2005, 09:17 PM   #1
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Default Advice needed for selecting a speaker relay

Hi,

I'm going to build a relay-based switch (one DPDT 5 A relay for each stereo channel) in order to use the same pair of loudspeakers alternatively with two different amplifiers (about 30-40 W each). The switching will be always done with both amps off, with no current passing through the relay contacts when it changes from its "normally closed" to the "normally open" position with the relay coil energized (this is called, I believe, "dry switching"). I've searched in several forums and websites about different contact materials and it seems that "dry switching" requires gold-plating of the contacts because the dirt (oxides, sulphur derivatives, etc) that forms with time a thin film over the silver isn't eliminated by the electric arc generated when switching a real load (at zero or very low level currents there is no arc and no wetting action). The gold-plating protects the silver or silver-nickel alloy below it from contaminants. Moreover, this switch would be rarely used, perhaps only two or three times a week, so the mechanical "wipping" action on the switch contacts when opened and closed is also nearly useless.

However, the relays available with gold-plating are only for small signals, less than 2 A, not for the speaker signals I want to switch. Most of 5 A relays have silver-nickel contacts with no gold, even from top brands like Omron. In fact, in Mouser there is only one gold-plated 5 A rated relay (Fujitsu FTR-F1 series) but unfortunately not in the 24 VDC version I need.

My question is if the gold plating is really needed in this "dry-switching" condition. Can I use a relay with standard "bare" silver-nickel contacts, without worrying about the quality of contact after a period of time? If the relay is a sealed one the problem is minimized? The Omron G2RS would be a good choice? Do you have any suggestions?

Thanks in advance for your help.

Best regards
Jose
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Old 9th November 2005, 09:46 PM   #2
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Hardsilver is good but if you make sure you'll never break with any current small 1-2 A relays will work. You say 30-40 watts but this is music I'll suppose so the average power is pretty low, 1-5 watts I'll imagine. No risk to get the relays warm from losses.

Goldplated relay contacts are only for making a reliable contact only when the time comes. If you plan to switch often this gold is torn away pretty soon.

Personally I have used Schrack RP relays with silver (not usre if it's pure silver or hardsilver, the datasheet said silver) for about more than 15 years, every day almost and I have no problems with them.

8 A relays are specified for > 10 mA so under this level there are no garantees.

My advise is: Normal 5-8 A hardsilver relays might very well work. 1-2 A signal relays will most likely survive also.
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Old 9th November 2005, 11:40 PM   #3
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Hi Jose,

I'm going to agree with peranders on this completely. Relays are designed to wipe the contacts when they close. Normal relays are fine, but silver plated are much better. There is normally no oxide buld up unless the relay is unused for a long time.

-Chris
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Old 12th November 2005, 02:51 PM   #4
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Try a switch?

Preferably a rotary switch, use multiple parallel contacts.

If ur only going to use it once in a while, a relay (making it remote controlled) it rather un-needed.

If you use relay contacts, be 100% certain to use multiple contacts, do not rely upon a single contact - bad, very bad.

I do not like relays in speaker circuits - and I wouldn't even consider a small signal relay in that application. My feeling is that if you wouldn't turn your amp on under full load with the thing, don't use it to run ur speakers. Again, paralleled contacts are essential in this sort of application.

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Old 13th November 2005, 08:17 PM   #5
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Thank you all for your help

I agree than the best switches are those that doesn't exist. In fact, I have soldered the speaker cable directly to the loudspeaker crossover input, but I need some way to easily change the amplifier at the other end of cable. However, your recommendations regarding the possible use of signal relays and the advantages of multiple contacts have made me to think a little more.

Can I use two high quality 2A DPDT signal relays in parallel (two for each stereo channel)? Perhaps this way I would have a combined relay with higher quality gold plated contacts and higher rating (as the signal would pass through both relay contacts in parallel, that 2A rating would increase and the resistance decrease, isn't it?). Remember that the actual switching would be done always with both amplifiers off and no current though the relays, so perhaps this rules out the use of power 5A relays rated at minimum 10mA.

Moreover, this arrangement would provide a kind of "multiple contact" switching, as Bear suggests (many signal relays have bifurcated contacts). What is your opinion about this?

I suppose that the problem in this case would be the power supply to the relay coils. I have a 24 VDC Stontronics switching-mode PSU, rated at 0.5 A at output. Do you think this 0.5A rating would be high enough to power the four relays (two per channel), whose power consumption would be about 0.5 W each?

Again, thanks in advance for your helpful advice

Best regards
Jose
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Old 13th November 2005, 09:57 PM   #6
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Several people have recommended Amplimo relays over the years which have both gold and high current contacts. Do a search on "Amplimo" and "relay" for the relevant threads.

James
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Old 13th November 2005, 10:03 PM   #7
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Be careful you don't over engineer or over agonise this. Otherwise, it'll never get done. You may fine things are not quite as critical as you first believe.

-Chris
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Old 14th November 2005, 04:53 AM   #8
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It's never wrong to ask but the application "speaker relay" isn't the most demanding application.

Use hardsilver, start with this!
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Old 10th December 2005, 10:16 PM   #9
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Quote:
Originally posted by spendorspain
Thank you all for your help

<snip>

Can I use two high quality 2A DPDT signal relays in parallel (two for each stereo channel)? Perhaps this way I would have a combined relay with higher quality gold plated contacts and higher rating (as the signal would pass through both relay contacts in parallel, that 2A rating would increase and the resistance decrease, isn't it?). Remember that the actual switching would be done always with both amplifiers off and no current though the relays, so perhaps this rules out the use of power 5A relays rated at minimum 10mA.

Moreover, this arrangement would provide a kind of "multiple contact" switching, as Bear suggests (many signal relays have bifurcated contacts). What is your opinion about this?

I suppose that the problem in this case would be the power supply to the relay coils. I have a 24 VDC Stontronics switching-mode PSU, rated at 0.5 A at output. Do you think this 0.5A rating would be high enough to power the four relays (two per channel), whose power consumption would be about 0.5 W each?

Again, thanks in advance for your helpful advice

Best regards
Jose
I'd be cautious about using a switching supply anywhere near my signal path. At the very least I'd try to take a close look at the possibility of inductive coupling/capacitive coupling to the speaker lines - no good reason to add hash to the signal, imho.

Lesseee... 24vdc, 0.5A = 12watts? seems enough, but then a simple transformer, bridge, and cap will do the trick jes fine...

Assuming you're not switching under load stringing a number of relays together should be good enough.

But, what about a knife switch?

I prefer larger "power" relays myself - usually FLAT silver contacts. But then I don't use relays on my speaker lines, ever.

Surplus relays are pretty darn cheap... relays from junked equipment/dumpsters, even cheaper... I'd aim for more bigger stuff.
Consider a 3phase "motor contactor"??

Others may or may not agree with this...

as always my opinions are mine. Ymmv.

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