DC loudspeaker protection

Discussion in 'The Projects Forum' started by andrew24, Aug 20, 2008.

  1. andrew24

    Thread Starter Active Member

    Aug 20, 2008
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    Hallo everyone!:) I've made an amp, and also decided to make this loudspeaker protection circuit with softstart:
    http://users.swing.be/edwinpaij/module_de_protection_cc_pour_hp.htm
    It should interrupt the loudspeaker line for ~2 seconds when amp is turned on (to prevent loudspeakers from undesirable bangs), and also interrupt loudspeaker line when there is DC voltage in the amp's output(usually when amp is broken).
    But my circuit doesn't work correctly.
    When i turn on the amp, output is connected to the speakers for ~2 seconds(so i can hear the music only for 2 sec.), and then interrupted. I think it should be conversely.I believe that all things with relay contacts are OK.
    Also, i noticed that the relay coil gets voltage only when there is a 2 sec. delay, and all the other time it has 0V between the coil contacts.
    ps. this circuit requires +-12V symetrical power supply, but when i was buying a transformer i forgot that after a diode bridge voltage becomes larger, so the final result is +-17V. Maybe is not very important for a circuit operation?
    so i think that's all. It would bve very great if you could help me! :)
    thanks for reding
     
    Last edited: Aug 20, 2008
  2. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    The TL071 has +18 and - 18 Volts as absolute maximum power supply.
    You are probably driving the TL071 to the edge.
    Perhaps you could place a 7812 in th positive power line and a 7912 in the negativ power line.
    In my opinion it serves also for greater stability of the circuit.
    (don't forget the caps on the input and output of the 78XX and 79XX, take a look in their datasheets).

    Greetings,
    Bertus
     
  3. andrew24

    Thread Starter Active Member

    Aug 20, 2008
    76
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    so if i place those voltage regulators, or replace the transformer, the circuit would come back to normal operation?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    It is not obvious from the schematic (as a matter of fact, the relay portion is not quite drawn correctly), but the board layout gives a hint.

    The relay is a SPDT (single-pole double-throw) type, and only the NC (normally closed) and COM (common) contact connections are used, the NO (normally open) contact is connected to an isolated pad. When the coil is energized, the NC contact to COM connection will open, and then connect again when the coil is de-energized. See the relay in the attached modified schematic.

    This confirms my above assertion.
    It's not that the voltage becomes larger - it's because AC is measured in RMS (root means squared) rather than P-P (peak to peak) voltage. A 12v transformer is actually putting out nearly 17v peak to peak.

    If your components are rated for operation at such voltages, then you might be OK. It would be better if you used regulators to obtain the specified voltages. One 7812 and one 7912 with some capacitors should work quite well for you.
    [eta]
    Also, in the circuit diagram, the TL071 opamp is being used as a voltage comparator; ie: in open-loop mode. This is very hard on an opamp, as the output circuit will be in constant saturation, and will overheat. Operational amplifiers are not designed to be used in such a manner. A comparator should have been used instead. While the circuit may work for a period of time, the life of the opamp will be much shorter than normal.
     
    Last edited: Aug 20, 2008
  5. andrew24

    Thread Starter Active Member

    Aug 20, 2008
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    thanks for a corrected shematic :) i am using the NO and COM contact connections and NC left isolated, but if i switch round these contacts, then after i would turn off the amp, loudspeaker line would left open,and while the power supply capacitors still have a charge, the music would be still heart, is it good?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Remove the connection from the NO terminal and connect it to the NC terminal.

    The loudspeaker will be connected as long as the relay coil is not energized.
     
  7. andrew24

    Thread Starter Active Member

    Aug 20, 2008
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    I tried to connect it to a NC and COM, and NO left isolated. Everything seems okay, but there is a few disadvantages:

    1) As NC and COM are always connected when relay coil is off, when I turn off my amp, I still hear the music, because the main power supply capacitors still holds the charge :)

    2) the green LED turn on only when there is a 2 sec delay, and after that red LED is always turned on.
    That’s why I started to think that the problem is not the relay contacts(I think the previous connection was correct), but the control of the relay: in my circuit relay coil gets current only during delay, I think it should be opposite right?

    Andrew
     
    Last edited: Aug 21, 2008
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
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    If you want I once made a delay circuit that has cca 2sec delay for On and immediate disconnection for Off, but it doesn´t have any DC protection.
     
  9. andrew24

    Thread Starter Active Member

    Aug 20, 2008
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    it would be great if you shared it, i would use it if i didn't solve this problem :)
    post it here, or send to andrew@statyba.lt
     
  10. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    I think the original schematic is correct and that the normally open contacts should be used.

    When T4 is off, the relay is off. C1 provides the power-on time constant and when charged switches on T1 which switches on T4 pulling the inverting output of IC1 low which sets the output high switching on T5 which switches on the relay.

    T2 and T3 remove base drive to T4 when DC is detected on the amplifier output. The LEDs indicate green (Ok) when the relay is energised and red (fault/delay) when it is not.

    The power supply capacitor C4 is 100uF, Which is quite small so should discharge fairly quickly when the circuit is switched off causing the relay to drop-out and disconnect the loudspeaker.

    Check T1 and T4 are correct. If either were accidentally substituted for PNP transistors the circuit would behave incorrectly, as described in your first post.
     
    Last edited: Aug 21, 2008
  11. andrew24

    Thread Starter Active Member

    Aug 20, 2008
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    thanks for a detailed description, blocco a spirale!
    everything seems to be fine, i checked all transistors in the circuit, all are NPN except T2,all diodes are placed in the correct way, all components are in their places.
    I don't realise why the circuit inverts signal for a relays. Maybe i connected outputs wrongly?

    The amplifier output(+) is connected to NO, speaker cable(+) is on the relay COM contact, amplifier ground and speaker cable(-) are connected together and connected to protection circuit's ground.
    Do i connected everything correctly?
     
  12. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    With no amplifier or speaker connected, create a fault condition by connecting the input (HP in) to +12V or -12V. what happens?

    Alternatively, if you have a meter available, connect HP in and the black meter probe to 0V, apply power to the circuit, allow a few seconds for everything to stabilise and record on a copy of the schematic the voltages at every node of the circuit. Scan the marked up schematic and post it here.
     
    Last edited: Aug 21, 2008
  13. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I will start a new thread with the circuit.
     
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