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Old 10th May 2013, 01:06 PM   #751
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Bob: according to my own test-rig, the matching of the LDRs cannot be done better.

Best regards - Rudi_Ratlos
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Old 10th May 2013, 01:31 PM   #752
bcmbob is offline bcmbob  United States
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From reading your posts I have no doubt if there was a better way, you would have found it.
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Old 23rd May 2013, 07:19 AM   #753
atupi is offline atupi  Romania
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For speaker protection modules i used following relays: RTE44024F.. - TE CONNECTIVITY / POTTER & BRUMFIELD - POWER RELAY, DPST-NO, 24VDC, 8A | Farnell United Kingdom

Could someone tell me if 2x8A (16A) would be enough ?
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Old 23rd May 2013, 10:19 AM   #754
AndrewT is offline AndrewT  Scotland
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this is the only DC rating for the contacts that I can see.
Quote:
EN60947-5-1
RTE24 DC coil A/B (NO/NC) AC15, 250VAC, 3A 6.050
RTE24 DC coil A/B (NO/NC) DC13, 24VDC, 2A 6.050
RTE24 DC coil A/B (NO/NC) DC13, 250VDC, 0.2A 6.050
I think the 6.050 is 6 thousand and 50 cycles.
But note the very low voltage for DC relative to the 250Vac rating.
As with most mechanical relays they rely on the no voltage of AC to break the arc #.
When there is no "no voltage" period as in breaking D C current then the spacing of the opening contacts severely limits the rating.
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Old 23rd May 2013, 09:13 PM   #755
atupi is offline atupi  Romania
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I should look for a better DC specs of contacts ?
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Old 24th May 2013, 01:02 AM   #756
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by atupi View Post
I should look for a better DC specs of contacts ?
Hi, I understand that you have to be careful, I think that 99% of relays you see in commercial gear used for speaker protection are not safe for your speakers. For stopping the odd noises from power up and power off a 10A relay is fine in my view. But if you want to protect an expensive speaker from failure of the amplifier which might feed your speaker from a high dc voltage then forget most relays you can find as they will likely fail.

You can read about it on Rod Elliot's website. The issue is that the speaker is inductive and when the relay opens the contacts any sudden drop in current will result in a high emf voltage from the speaker which often produces an arc inside the relay. So the current keeps flowing. You can arrange for the relay to switch the speaker from the amplifier output to ground which will extinguish the arc but the switching time of a relay is slow. The diodes usually placed across the relay coil to prevent back emf from the relay coil damaging the control circuit actually slow down the relay.

Using two relays in parallel - could happen that they won't open at exactly the same time. One will open first, fairly easily since the other will still be closed. Then the last one will try to open.....

A far better approach, in my opinion, is to use a pair of MOSFETs with a opto-isolator to form a solid state relay. There are some threads on this topic on this forum. There are no contacts to get dirty and affect the sound and there are modern MOSFETs with negligible on-resistance so there will be less impact on sound than most relays. There's an example here

TGM5 - all-BJT Simple Symmetric Amplifier

In my thread on my TGM5 amplifier I made a simple dc protection circuit. It may not the the best but it shows you another path you can take. You might be interested to note that the output stage of my TGM5 amplifier was inspired by the Roender amplifier of this thread. I like this output stage very much.
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Last edited by Bigun; 24th May 2013 at 01:08 AM.
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Old 24th May 2013, 04:27 AM   #757
roender is offline roender  Romania
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I would not put an relay/mosfet in series between amplifier output and speaker. You should use instead a crowbar type circuit, either mechanical or mosfet/diac based .
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Old 24th May 2013, 04:34 AM   #758
roender is offline roender  Romania
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Simple as this one:
Crowbar Speaker Protection - Electronic Circuits|Circuit Diagram|Electronic Circuit Project Design
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Old 24th May 2013, 09:41 AM   #759
AndrewT is offline AndrewT  Scotland
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If one uses an output crow bar then one MUST use some form of current limiting in the supplies. Otherwise the mains will try to drive the crowbar for ever.
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Old 24th May 2013, 06:38 PM   #760
Sheldon is offline Sheldon  United States
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Originally Posted by AndrewT View Post
If one uses an output crow bar then one MUST use some form of current limiting in the supplies. Otherwise the mains will try to drive the crowbar for ever.
I would think it universal practice to fuse the mains and power rails. I've blown lots of fuses playing around on the bench and shorting this or that, but never with music, and rarely resulting in component failures.

Sheldon
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