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Old 3rd September 2008, 03:31 PM   #1
h_a is offline h_a  Europe
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Default loudspeaker protection relays?

Hi,

I'm currently looking for high quality relays for my dc-protection circuit, so high current, not low-level parts.

Availability at Digikey would be nice!

My current favourite is the Omron G6B, maybe somebody has an opinion on this one. Seems very reliable and is even recommended by Shure.

Omron G5Z would be nice, but nobody seems to carry that part, must be pretty expensive as well.

However if one knows a shop that has the Omron G5Z that would be appreciated!

Have fun, Hannes

PS: Jan, I already know your Amplimo-tungsten/gold relay However I would prefer a sealed relay and I would need also a 12V variant.
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Old 3rd September 2008, 04:11 PM   #2
AndrewT is online now AndrewT  Scotland
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Hi,
to minimise deterioration in the relay's contacts, is it necessary to minimise the current at the time of switch over?

If so, then an input mute should also be timed to shut off the output while the relay is switching.

Comments please.
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Old 3rd September 2008, 04:17 PM   #3
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Correct -- also the DC output offset should have settled to zero volts when the relay contacts close.
What is often overlooked, in the very unfortunate event they are called on to disconnect the load for real, is whether the contacts can actually break the flow of current cleanly without arcing and welding together.
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Old 3rd September 2008, 05:40 PM   #4
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I'm sure reducing unwanted DC before switching prolongs lifetime as it obviously reduces the switched load.

However the Omron G6B is rated at 5A for 30V DC, so arcing should be no issue within this rating.

Or you do it like me - combine the DC-protector with a delayed turn on So DC is settled when the switching happens.

Have fun, Hannes

PS: for the curious: schematic will be published here and I would say pretty soon.
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Old 3rd September 2008, 05:59 PM   #5
AndrewT is online now AndrewT  Scotland
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I was suggesting we discuss whether either or both the AC and DC muting condition has an effect on the life of the contacts, i.e. input signal mute to remove the AC signal across the relay's contacts before it switches either to make or to break.
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Old 3rd September 2008, 06:15 PM   #6
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Well, the datasheet shows no difference in the rated current for either AC or DC*. It does however specify largely different maximum voltages at which 5A may be switched: 30V for DC, 250V for AC.

My guess is that this voltage difference is related to the gas filling where the speed of charged gas molecules is rather slow and would so allow higher AC voltages.

Have fun, Hannes

*The datasheet does not specify wether it's RMS or peak current, but I assume it's peak.
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Old 3rd September 2008, 06:46 PM   #7
AndrewT is online now AndrewT  Scotland
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the relay current rating is for rms.

Now, let's take an amp that has been connected to a load well below it's rated minimum.
Suppose for argument it has +-50Vdc supply rails and the load is 2r0.
The current flowing will be about 20A and the voltage will be 50V and falling.
Open the 5A, 30Vdc relay and tell me what chance it has of breaking that fault current?
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Old 4th September 2008, 06:51 AM   #8
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A DC arc is "self sustaining". Thats why the relay is derated for DC switching. The relay has to at least attempt to protect the speaker in the event of an output fault.
Imagine connecting a large inductive load across a battery and then drawing one wire away. The current doesn't break cleanly, the arc remains as the distance between the terminal and wire increases. If this were a relay (an unsuitable relay that is) the contacts would weld together.
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Old 4th September 2008, 07:43 AM   #9
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Andrew, I'm a bit surprised that just you talk of using a part outside of its specification

But I guess you're just interested in what would happen in that case.

Thanks also for pointing out that it's RMS current, I didn't find that in the datasheet.

I think especially the high-capacity variant of the G6B - 8A! - is interesting and not that expensive, about 3 Euros per piece.

Have fun, Hannes
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