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Old 10th June 2005, 09:20 AM   #1
wrl is offline wrl  United States
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Join Date: Jan 2003
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Default Relays for speaker protection

Hello,

I am building Rod Elliots Speaker protection circuit to use in my leach amp and I am trying to choose a relay.

Below are two choices:

NTE R25-1D16-12
http://www.nteinc.com/relay_web/pdf/R25.pdf

or

Tyco's OZ series
http://relays.tycoelectronics.com/datasheets/OZ-OZF.pdf

Does anyone have any experience with this. Why is the NTE relay 8 dollars while the tyco is only 1? Are there different characteristics that I am not noticing (build quality?)

One final question for a different but related project, Mouser Electronics claims that this relay below is rated at 10A but the data sheet seems to say that its 5A at both 240V and 120V. Is it 10A or 5A at 120V? Can anybody tell?

http://relays.tycoelectronics.com/datasheets/OMI-2P.pdf


Any input on either topic would be greatly appreciated

Thanks in advance

Wes
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Old 27th June 2005, 09:55 AM   #2
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Default Relays for speaker protection

In my opinion the Tyco OZ series seems to be quite suitable.

I would go for the single make type OZ-SH-112LM1,294 (12V) or the 124LM1 (24V).

These have large QC tab connect terminals for attaching heavy gauge speaker wires and the contact area is large enough to handle the current while having low contact resistance. The only problem I have found using sealed relays for this application is that over time the contact resistance can become intermittently high. I have drilled a small hole through the case and sprayed in some good contact cleaner lubricant and this has fixed the problem. Preferably, a drop of Stabilant 22A on each contact is the way to go if you can get at the contact and can afford the stuff.
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Old 28th June 2005, 12:49 AM   #3
hermanv is offline hermanv  United States
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Default Contact resistance

Only Tyco specs the contact resistance at 100 milliohms, too high for a speaker relay IMHO. Normally speaker relays would be mechanicaly large open frame units.

Also the NTE relay uses Cadmium over silver contacts, depending on how much you believe in exotic cables, this is probably not a good metal choice.

If the amplifier output is capacitor coupled the relay will not stop speaker thumps unless a resistor is wired to precharge the capacitor (to the other speaker leg) through the back contact of the relay.

A 100 watt amplifier can source 5 amps into 4 Ohms, probably double that during peaks. My home amplifier can source up to 40 amps so a bigger relay might be a wiser choice.

(rherber1) A relay that shows high contact resistance after a few uses (less than 1,000) is most likely underated, pits are being burned into the contacts. I think that after a while the cleaner trick will stop working. You may already be giving up some sound quality.
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Old 28th June 2005, 01:29 AM   #4
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Default Re: Contact resistance

Quote:
Originally posted by hermanv
Only Tyco specs the contact resistance at 100 milliohms, too high for a speaker relay IMHO. Normally speaker relays would be mechanicaly large open frame units.

0.1 ohms added to the speaker cable resistance is neither here not there in the scheme of things audio. This is also a nominal rating which the manufacturers must provide to sound reliable and in most cases if the relay tension is correctly set, the resistance will be much lower than 0.1 ohms.

I have used large open frame relays for this application and even with very large contact areas and even these have developed intermittent high resistance. The problem is caused by the lack of any "wetting" current during normal operation in this role, a problem well known to telephone exchange engineers familiar with relay equipment.

Also the NTE relay uses Cadmium over silver contacts, depending on how much you believe in exotic cables, this is probably not a good metal choice.

If the amplifier output is capacitor coupled the relay will not stop speaker thumps unless a resistor is wired to precharge the capacitor (to the other speaker leg) through the back contact of the relay.

One would assume that the OP was intending to drive the relay with a properly designed delay-on and dc fault protection circuit. If he does this then there will be no thumps at all.

A 100 watt amplifier can source 5 amps into 4 Ohms, probably double that during peaks. My home amplifier can source up to 40 amps so a bigger relay might be a wiser choice.

(rherber1) A relay that shows high contact resistance after a few uses (less than 1,000) is most likely underated, pits are being burned into the contacts. I think that after a while the cleaner trick will stop working. You may already be giving up some sound quality.
As I said above, lack of contact wetting current can lead to high contact resistance. The relays used where this symptom was evident were rated for 16 amps and had large area contacts made of low oxidation material. There was absolutely no sign of pitting. During operation as a speaker protection relay there is no dc current through the contacts and because they are only operated during switch-on and switch-off they don't actually get a lot of action to maintain a very low resistance.

I have mil spec reed relay elements, which as you know are hermetically sealed units, and after sitting unused for many years they have developed high contact resistance (usually less than 5 ohms). It usually takes a number of operations to get the resistance back to normal. All relay contacts rely upon frequent operation to produce the necessary rubbing action at the contact points to maintain low resistance.
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Old 28th June 2005, 01:32 AM   #5
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Default Contact resistance

Sorry, I inserted some comments inside your quoted text. Hope you can sort it out.
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Old 28th June 2005, 08:29 PM   #6
cpemma is offline cpemma  United Kingdom
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Do the relay switches carry much current at the time they're switched? If not, you should be looking at gold contacts.
Quote:
Dry circuit loads: No current is switched. The contacts carry current only after they are closed or before they are opened. The currents may be high, as long as they are not switched. Since there is no arcing, contact resistance is kept low by using gold plating or gold alloy contacts.
http://www.leachintl2.com/english/en...rties/how4.htm
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Old 1st July 2005, 06:35 AM   #7
wrl is offline wrl  United States
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Wow, thanks for all the replies. I actually have had a lot on my plate lately so I just went with this relay:

G8P-1A4P-DC24 ( price around $3)
http://www.mouser.com/index.cfm?han...e_pcodeid=65309

based on a recommendation on another thread. Let me know what your oppinions are on this relay, and hopefully sometime soon I'll be able to let you know how it works out.

I figure I can upgrade some other time since these were only 4 bucks.

Thanks again,

Wes
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