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Make this Amplifier Short/Overload Protection Circuit

While working with high power amplifier designs, two things become crucial, the protection of the amplifier and the protection of the speakers from an accidental over current influx.


Especially when the amplifier design involves costly mosfets, the design becomes specifically vulnerable to short circuits at the outputs.

A short circuit at the output may be caused due to mishandling or ignorance from the part of the user.

Whatever might be the reason, the end results in the destruction of the precious mosfets inside the amplifier box.

The above mishap can be prevented by adding a small circuit for detecting a short circuit conditions at the outputs of an amplifier.

The given amplifier short/overload protection circuit diagram, shows an inexpensive design using just a single transistor for implementing the intended feature.

Normally a low value resistor is usually employed at the output of mosfet amplifiers, the current developed across this resistor can be well exploited for tripping a relay in case it exceeds the safe maximum current value.

The current threshold across the above resistor is sensed by an LED inside an optocoupler, which lights up the moment a short or overload conditions is sensed.

This instantly triggers the opto transistor which in turn switches ON the transistor driver and the associated relay mechanism.

Since the relay coils support the amplifier connection with the speaker output, disconnects the amplifier from the output connection, preventing the amplifier devices from a possible damage.

The capacitor at the base of the transistor keeps the transistor switched for a few seconds so that the relay does not oscillate randomly.







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  • 31 comments:

    1. see, talk of ingenuity this is one of them, it obviously agreed with my slogan "simple and functional" keep it on.

      ReplyDelete
    2. HI
      this diagram seems not complete how does the relay power up?where is the source of its power?

      ReplyDelete
      Replies
      1. Hi,
        yes by mistake the positive connection is not shown in the diagram, a separate +12V should be connected to the link that connects the relay coil and the opto transistor.

        Delete
    3. have you built and tested this project/the project is impressive does this protection also include d.c currents against the speakers?

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      Replies
      1. not tested, but i am sure about it.

        yes it would also handle DC shorts across the speaker

        Delete
    4. Useful but I want the number of opto couple

      ReplyDelete
    5. OK it is useful circuit I want to know the number of opto couple
      Thank-you

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    6. Say the value of resistance 0.5 ohm iam not able to get in india

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    7. Sir thank you for reply but in India only 1 omhs is available can I use that one
      Thanks again

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      Replies
      1. saravanan, please tell what is the max wattage limit do you prefer to implement? the resistor value will depend on this

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    8. 100 Watts 5.1 amplifier system it is designed by MOSFET IC I have changed my speaker for more than 8 times in 2 years
      Thankyou

      ReplyDelete
      Replies
      1. for 100 watt you can use the following formula

        100/V = current

        therefore R = 1.5/current

        here V is your amp supply voltage

        Delete
    9. Sorry I can't understand which voltage
      Input to transformer is 220 volt and the output of the transformer is 24 volt the transformer current rateing is 5 amps
      Mean while I get only 1 ohmes in India
      Thank-you

      ReplyDelete
      Replies
      1. the 0.5 ohm shown in the diagram will need to be calculated as explained using the formula in the previous comment.

        If your amplifier operating volatge is 24V then

        100/24 = 4.16

        R = 1.5/4.16 = 0.36 ohms

        wattage of resistor will be = 1.5 x 4.16 = 6.24 or a 10 watt will do.

        if you are getting 1 ohm then use two in parallel, in place of the indicated 0.5 ohm

        Delete
      2. .....two 1 ohm in parallel will do in place of 0.36 ohm

        Delete
    10. Thank you sir so I want to ready 6 circuit for 5.1 amplifier

      ReplyDelete
    11. This circuit is new it OK can we use in4148 diode

      ReplyDelete
    12. This comment has been removed by the author.

      ReplyDelete
    13. Does anyone have a COMPLETE working circuit (including missing +12V to relay and opto) with all parts IDENTIFIED? Has anyone tested or built this circuit?

      ReplyDelete
    14. Sir,

      This circuit also prevent DC from amplifiers O/P? means, this circuit act as a speaker protection circuit?..

      How can calculate The resistor value in +75v -0- -75v 250W amplifier?


      ReplyDelete
      Replies
      1. Rahul, yes you are correct, it will protect speaker from burning.

        for 75V 250 watt, the current limiter resistor can be calculated as

        250/75 = 3.33 amps

        R = 2/3.33 = 0.6 ohms

        watt = 2 x 3.33 = 6.66 or 10 watts

        Delete

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