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Old 24th June 2011, 07:24 AM   #1
Bonsai is offline Bonsai  Taiwan
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Default Output Relays

Sh1t Happenz.
My 250 watter blew a channel this weekend . . .and took the bass units out on one of my B&W 703’s. As it went out, there were sparks and noises coming out of the of the amp and then a smell like burnt varnish from the speaker.
I was and still am mightily pissed off.
At first I thought it has the Vbe multiplier that had opened up, but after removing the module and checking the components, it seems an MJ21194, MJE15033 and an MJE15032 blew short. The pre-drivers, VAS and all the small signal components were ok, although the driver emitter resistor was badly burnt (this is a 27 Ohm that connects the two driver transistor emitters together – both drivers were short, so this resistor had 140V across it before it too open circuited). The 0.22 Ohm 7W emitter ballast resistors on the output devices made it though – even on the 21194 device that blew s/c. It looks like the output device failed short, put about 75V across the speaker, causing the bass drivers to fail.
Then I wondered why the MCU based protection controller did not kick in and open the output relays as soon as it detected an output DC condition. I took a look at the relays. Well, one set of relay contacts was ‘stuck’ (i.e. welded) in the closed position and the cover was smoked over in the vicinity of the contacts. So, it looks like the other two relays opened correctly (or dropped out when the power was pulled).
In my wisdom, and against everything I was always told about paralleling relay contacts, I had put 3 sets of relays in parallel (Tyco RP3 16A 250 VAC devices) in the interests of low output resistance. The MCU probably tried to do its job, so relays 1 and 3 opened up as the fault protection kicked in, but the last relay to open up was #2. I don’t know at this stage whether or not the failed contact actually did open correctly, only to succumb to subsequent arcing, or whether it simply stuck because it was well outside the DC load capability.
If you look at the relay spec, it shows the DC load breaking curve as displayed below. At 75V, the DC load breaking capability of the relay is about 1.1A, or about 10% of the probable current that flowed during the failure event. This was probably enough to cause the contacts to stick together and blow the speakers.
The lesson to be learned here is that relay DC handling specs really do need to be adhered to. So, I will replace this relay and fix the module up, along with my speakers, but for all new power amp designs, I will go with a different appproach. Many people recommend using an auto relay, but I checked out the DC load switching capability and its not clear that they would reliably switch on a fault condition. I did some research and the Panasonic EP(AEP) series look like they may do the trick - the 10A PCB mount version.

Now, I have to work out why the damn output stage failed in the first place.
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Last edited by Bonsai; 24th June 2011 at 07:32 AM.
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Old 24th June 2011, 08:23 AM   #2
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DC and relays are generaly not friends, you get relays with two contacts that open sequentialy, the one that makes or breaks the contact is made of tungsten or something.
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Old 24th June 2011, 09:45 AM   #3
Bonsai is offline Bonsai  Taiwan
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Yes you are correct. I think they are great for turn on/off muting but if you want to switch under an extreme fault condition like the one I had, a normal relay will not work- you need a device designed to handle DC specifically
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Old 24th June 2011, 11:22 PM   #4
Tajzmaj is offline Tajzmaj  Slovenia
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Hi. Sad story.......There is one another option.....to use triac protection. Such circuit which short the output ....... Quad 405 has such circuit at later models....and of course leave relay protection there as well.
Those Tyco/Schrack has many contacts so you can connect them in series or parallel.....
http://www.farnell.com/datasheets/573461.pdf
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Old 25th June 2011, 12:05 AM   #5
Bigun is offline Bigun  Canada
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I'd also read over Rod Elliot's thoughts on this - I remember he advocates a relay switching the speaker to gnd as the only way to stop the dc arc.
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Old 25th June 2011, 12:33 AM   #6
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I just had an interesting idea. (lol) One could use an ordinary rela, one set of contacts goes to the amp, the other also goes to the amp, but through a highish ohmed resistor, high enough so that the current the full rails can provide to the speakers can not damage them. This should be a more forgiving error condition than a short to earth.
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Old 25th June 2011, 05:08 AM   #7
qusp is offline qusp  Australia
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Quote:
Originally Posted by digits View Post
I just had an interesting idea. (lol) One could use an ordinary rela, one set of contacts goes to the amp, the other also goes to the amp, but through a highish ohmed resistor, high enough so that the current the full rails can provide to the speakers can not damage them. This should be a more forgiving error condition than a short to earth.
wouldnt the current just find another way if you did that?, thought the idea of relay protection was that you provided a path the current would prefer to take than your gear
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Old 25th June 2011, 07:50 AM   #8
Mooly is offline Mooly  United Kingdom
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Hi Bonsai... I can just imagine $%%^

I have the B&W 703's too (have a pdf manual if you want it) so I understand the frustration.

DC protection... we all hope we never need it, when we do will it work ? Nobody ever tests it too (for real) by sticking a screwdriver across an output device and seeing what happens.

Doug Self did a very interesting write up many years ago on this and the opinion was that protection at best may save a potential "fire" due to the coil burning but would not stop damage to the speaker.
Switching (breaking) a high DC current thats flowing in an inductive load is not easy... as you have found out the arc drawn just welds the relay contacts.

There was a very interesting thread (think it was this one) using FET's as a series switch and controlling them with photovoltaic diodes.

http://www.diyaudio.com/forums/solid...et-relais.html

Another issue is the integrator time delay on any DC protection scheme and this together with the relay pull in delay introduces anoher issue. This was something I actually looked into but never developed into an actual circuit as yet. The idea was to compare the input and output of the amplifier rather than detect a DC fault and any difference indicates a problem.

Another simple DC protection

Post #39. Still can't find the original thread...
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Old 25th June 2011, 08:36 AM   #9
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I would go for UPC1237 for simplicity, but sadly the voltage you run is about 10V past its max.
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Old 25th June 2011, 10:38 AM   #10
AndrewT is offline AndrewT  Scotland
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I prefer a belt and braces and hammock approach, Although I'll be truthful and admit I have never (yet) incorporated all 4 protections in the same amplifier.

Fuse protection of the supply rails after the main smoothing caps.
Current, or preferably IV, limiting that allow high transient currents to pass to reactive load, but limits DC output current to suit the power device SOAR.
DC detect and relay protect the speakers.
Input mute on fault detection.

Other possibles could be Crowbar/Thyristor short across output terminals on fault detection.
MosFET fuses in the supply lines.
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Last edited by AndrewT; 25th June 2011 at 10:40 AM.
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