why transistor is used?

Discussion in 'General Electronics Chat' started by corefinder, Dec 24, 2012.

  1. corefinder

    Thread Starter Member

    Oct 6, 2011
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    Here in this figure i want to know what role Q3 and Q4 are playing. I could have directly connected Q1 and Q2 directly with the micro controller....
     
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  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Q3/4 are drivers for the Pfets Q1/2
     
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  3. corefinder

    Thread Starter Member

    Oct 6, 2011
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    Do fets always require drivers like this or when the fet needs higher gate voltage than microcontroller could provide
     
  4. corefinder

    Thread Starter Member

    Oct 6, 2011
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    Yes, if the collector current is zero then how Vgs will obtain the desired level
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Your schematic does not have enough contrast. Everything is faint. I can barely see the details. I used to have a program to fix it but now the program is gone.

    It looks like the collectors of Q3 and Q4 have nothing to provide a positive voltage so they won't work then the Mosfets also won't turn off.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I think what you are pointing out is the fact that the gates have no pull-up resistors. It won't work without them. The gates need to be pulled up to +BAT.
    You need drivers like this if the drain voltage is higher than the microcontroller's vcc (which it is in this case).
    What is the purpose of D1?
     
  7. crutschow

    Expert

    Mar 14, 2008
    12,981
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    As noted, that circuit will not work as shown.

    The P-MOSFETS require a negative gate-source voltage to turn-on, which is provided when Q3 and Q4 turn on (source is at plus voltage and gate at ground). But the circuit has no pull-up to turn the MOSFETs off (zero volts Vgs) when Q3 and Q4 turn-off. Thus resistors R3 and R4 need to be between the respective MOSFET gate and source, not to Q3's and Q4's collector.
     
  8. corefinder

    Thread Starter Member

    Oct 6, 2011
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    D1 is schottky diode to prevent reverse flow of current
     
  9. corefinder

    Thread Starter Member

    Oct 6, 2011
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    I am not able to understand how Q3 and Q4 are working. Even there is base current but no collector current can flow. Collector terminal is connected to gate terminal of fet. so Vc is zero always. so this fet is not controlled by microcontroller.
    I give up this circuit :(
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    That's what we've been trying to tell you.
    You don't have to give up. Do it as in the attachment.

    What are the values of R3, R4, R5, and R6?
     
  11. ScottWang

    Moderator

    Aug 23, 2012
    4,852
    767
    If you have ACDSee software then you can use it to adjust the contrast of the schematic.
    1. Open schematic.png with ACDSee, move the mouse cursor to the schematic, press right button and choose Edit/Open in Editor.
    2. Choose the editor menu on levels, and click levels
    3. Press enlarge key until you can see the circuit clearly, and adjust gamma, press OK when you feel adjusting is ok.
    4. The last is press save on the menu, and press ok, then it's done.
    5. Exit the Editor.
    6. Reload the schematic.
     
  12. ScottWang

    Moderator

    Aug 23, 2012
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    I just want to say that about the off resistor Rsg, but Ron H is more quickly, so I adjust contrast of the schematic that Ron H just posted.

    [​IMG]
     
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  13. grahamed

    Member

    Jul 23, 2012
    99
    11
    I don't know why I am seeing this thread at this time, but .....

    I guess I am missing something but it looks like both FETs are switching a positive supply to a ground referenced load, yet they are not wired the same, supply to drain and out to source in one and vv in the other.

    I know that the FET channel will pass current in either direction when turned on, and won't when it's not but what about the body diode?
     
  14. wayneh

    Expert

    Sep 9, 2010
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    Why answer a four year old question, with more questions?!
     
  15. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    it is always nice to see a thread where @Audioguru and @Ron H participated and - what the heck happened to Ron?
     
  16. ScottWang

    Moderator

    Aug 23, 2012
    4,852
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    I think he is ok, just getting older and want to take a rest.
     
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