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Another simple DC protection
Another simple DC protection
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Old 1st October 2011, 07:49 PM   #71
RCruz is offline RCruz  Switzerland
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Another simple DC protection
Hi Christophe

Would you point me a simple comparator ?
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Old 1st October 2011, 11:18 PM   #72
Esperado is offline Esperado  France
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Quote:
Originally Posted by RCruz View Post
Hi Christophe
Would you point me a simple comparator ?
few posts before in this thread, an other example :
http://www.diyaudio.com/forums/elekt...crescendo.html
As i said, i will publish mine as soon i get time to recopy my own (old) solution on the same principle.
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Old 2nd October 2011, 12:43 AM   #73
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by RCruz View Post
Hi Christophe

Would you point me a simple comparator ?
LM311, LM393,
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Old 2nd October 2011, 12:46 AM   #74
Esperado is offline Esperado  France
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Originally Posted by nigelwright7557 View Post
LM311, LM393,
Here you need just an OP amp used as a comparator.
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Old 2nd October 2011, 12:48 AM   #75
nigelwright7557 is offline nigelwright7557  United Kingdom
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Originally Posted by Esperado View Post
Here you need just an OP amp used as a comparator.
Be careful which op amp you use, some invert the output if you go anywhere near the supply rails !!!
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Old 2nd October 2011, 07:25 AM   #76
Esperado is offline Esperado  France
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Originally Posted by nigelwright7557 View Post
Be careful which op amp you use, some invert the output if you go anywhere near the supply rails !!!
Here my schematic. Their is no risk the signal can go near the power rails because it is just atthe level of the input (near 1V). On my amp, as an example, if the amp goes to his rail level (70V) the signal will be reduced by a factor of 30, means 2V, far away from the 12v of the Op amps supplies.
http://www.diyaudio.com/forums/digit...ml#post2731758
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Old 2nd October 2011, 09:22 AM   #77
CopperTop is offline CopperTop  United Kingdom
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So is the preferred approach to break the connection to the speakers, or to short them out? I am not convinced by the series relay approach.

Looking in detail at a relay data sheet, DC breaking capacity is specified for a resistive load. It seems to fall off rapidly as the voltage exceeds 30V. For a typical relay, at 50V DC, breaking capacity is <10% of the nominal current rating (so a 20A relay is rated for <2A at 50V DC).

Just what voltage and current will be present if the amp goes wrong? I'm presuming that it could be 50-60V for a powerful amp, and >10A through the speaker coil. Can you be really confident that you're not going 'tack weld' the contacts even with a large relay and snubber across the contacts?
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Old 2nd October 2011, 10:56 AM   #78
AndrewT is offline AndrewT  Scotland
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A 100W into 8ohm amplifier runs from ~+-50Vdc supply rails.
If there is a serious output fault that sends DC to the speaker terminal, then expect most of that 50V to appear at the terminal and expect the DC resistance of the 8ohm speaker to be ~ 6r0.
The resistances between the PSU and output terminal and the return lead could be ~0r5
The current flowing till the smoothing caps start to discharge (just a us, 0.001ms or so) is ~7A @ 50Vdc.
It takes a mighty Power relay to reliably break that value of DC current at that voltage.

The more powerful the PSU the longer the caps stay recharged to a high (damaging) voltage. Result:- Relay contacts supremely welded or completely burnt out during that one incident.
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Old 2nd October 2011, 11:29 AM   #79
audiofan is offline audiofan  Canada
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If you get DC at speaker you mus have DC in center tap of power transformer ..... protection could be buib at center tap ....... see QUAD 306 schematic
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Old 2nd October 2011, 07:10 PM   #80
Bigun is offline Bigun  Canada
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Another simple DC protection
Quote:
Originally Posted by CopperTop View Post
So is the preferred approach to break the connection to the speakers, or to short them out? I am not convinced by the series relay approach.
The belt n braces approach would be to have something in series - either at the speaker output or the power rails to the output power devices. And have something that shorts the output to ground (e.g. simple Triac circuit) - which will destroy the amplifier if no series breaker is used.

And instead of relay, use power MOSFETs.
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