To replace IRF 540 for 2N3055

Discussion in 'The Projects Forum' started by mhtplsh, Jul 29, 2007.

  1. mhtplsh

    Thread Starter Member

    Jul 29, 2007
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    Kindly suggest the changes i hv to do for replacing IRF540 for 2N3055.
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
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    The IRF540 will only conduct when the gate voltage is at least 2 - 4 volts higher than the source. Source voltage with the MOSFETs conducting will be very close to Vcc, so the gate voltage must be at least 21v.

    Try using a voltage doubler to drive the gates. Protect the MOSFETs with an 14v (or thereabouts) Zener diode across source & gate - keep the gate-to-source voltage below 80% of the 20v limit!
     
  3. hgmjr

    Moderator

    Jan 28, 2005
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    The schematic is a little worrisome to me since the collector of each of the two 2N3055 NPN transistors is more negative than the emitter.

    hgmjr
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hgmjr is correct - the 3055's are reverse biased, so the original circuit can't work. Keep in mind that transistors are current controlled, while FET's are voltage controlled. One can't sunstitute for the other very easily.
     
  5. mhtplsh

    Thread Starter Member

    Jul 29, 2007
    27
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    The circuit is from Elektor. It is a tweeter protector/saver. It is used in parallel to tweeter. I just reversed the polarity of power supply the transistors. i.e. used pnp in place of npn & npn in place of pnp after reversing the polarity of power supply.
    Hope this info will be better to judge the circuit. As the voltage across the tweeter increases, the circuit comes in to action. Here the tweeter voltage is fed to the rectifier.
    I want to use mosfet in this circuit because i feel that they will be better after reading the remarks in some project in this forum.
     
  6. mhtplsh

    Thread Starter Member

    Jul 29, 2007
    27
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    Sorry!
    There was schematics mistake. The 2N 3055 was not proper in diagram.
    Here is the one which is proper.
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    Now the circuit makes a bit more sense.

    As thingmaker3 has already mentioned, the 2N3055 BJTs are current controlled and the mosfets that you propose to change over to are voltage controlled.

    The trick in converting over to mosfets will be to supply the mosfets with the voltage level needed to turn them on under just the right conditions to protect the speakers from overdrive.

    hgmjr
     
  8. mhtplsh

    Thread Starter Member

    Jul 29, 2007
    27
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    Do u think it will be better to use Mosfet?
     
  9. mrmeval

    Distinguished Member

    Jun 30, 2006
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  10. mhtplsh

    Thread Starter Member

    Jul 29, 2007
    27
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    Thanks mrmeval
     
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