Transistor help

Discussion in 'General Electronics Chat' started by Robin Mitchell, Dec 8, 2010.

  1. Robin Mitchell

    Thread Starter Well-Known Member

    Oct 25, 2009
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    Hi everyone,

    I am learning electronic theory so i can give my self a head start when i do electronic engineering at uni :)

    But there is something i dont understand. Lets take for example a BC548 transistor. It has a max base current of 500ma. Ok, so lets say i want a current of 100ma on the base with a voltage of 9v, simple, use V = IR. So i get the value of a resistor needed to be 90 ohms.

    Why do other websites use large resistances with the base?
    Like if they have the output of a 4000 chip connected to the base they have a resistor in series > 5K ohms?
    Whats going on, thanks :)
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The main problem is that the 4000 series can not deliver so much current at its output.
    The current may be a couple of mAmps.
    Take a look at the attached PDF for more genegal specs of the 4000 series.

    Bertus
     
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  3. Robin Mitchell

    Thread Starter Well-Known Member

    Oct 25, 2009
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    Hi bertus,
    Kiss from the isle of wight
    X

    Thanks :)

    But that still does not answer the question to why the high resistors?
    if they only source several ma then surely they dont need a resistor?
     
    Last edited: Dec 8, 2010
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    depends, i believe, on what amount of overall current the circuit is designed to use. A battery operated device will use resistances much higher than a similar mains operated circuit, simply to increase useful battery life in the device.
     
  5. Robin Mitchell

    Thread Starter Well-Known Member

    Oct 25, 2009
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    So a high resistor value is used JUST for power conspumption?
     
  6. Kermit2

    AAC Fanatic!

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    it is ONE reason to use it. Not the only reason
     
  7. Robin Mitchell

    Thread Starter Well-Known Member

    Oct 25, 2009
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    oooh more reasons :D

    Acctaully i did find one thing, if there is a high voltage on a base and no voltage on the collector the voltage seems to go through to the emmiter... is that ment to happen?
     
  8. Kermit2

    AAC Fanatic!

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    It's just a PN junction. A diode.

    :)
     
  9. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    You know. With nothing to do at work, but sit here and add inane comments to a forum. The time just seems to draaagggg on and on and on and on. Will this day ever end??

    ;)
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Where did you get the idea that BC548 max base current is 500mA? And why would you want to put that much current through it?
     
  11. tyblu

    Member

    Nov 29, 2010
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    The BC548 has a maximum collector current of 500mA, not base current.
     
  12. Kermit2

    AAC Fanatic!

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    A guy needs to 'smoke a few' before he gets it down good.

    That certain smell a toasted silicon device makes is never forgotten :)
     
  13. beenthere

    Retired Moderator

    Apr 20, 2004
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    Sure makes you read the spec sheet in more detail. Or learn those resistor color codes.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    Absolutely not!
    The max continuous collector current is only 100mA. It saturates fairly well when its base current is 1/20th its collector current so its max base current is only 5mA.
     
  15. tyblu

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    Nov 29, 2010
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  16. Audioguru

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    Dec 20, 2007
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    Fairchild hired little school kids (I don't know from which country) to make their wrong datasheets.

    I always look at the datasheet from the company that invented a part, not a copycat datasheet that frequently is wrong.
    Philips invented the BC108 (metal case) then used the same chip in a plastic case (BC548).
     
  17. hobbyist

    Distinguished Member

    Aug 10, 2008
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    Now that the data sheet is straightened out, the max. IC = 100mA
    and the base current set at 5mA, would make a single base bias resistor be around [(VCC - Vbe) / Ib] =~ [(9v. - 0.8v.) / 5mA] =~ 1.6K ohms.

    However transistor circuits are not designed to run at max. ratings, so by looking at the data sheet, if you choose 10mA, for IC and use there value of 0.5mA, for Ib, then single bias resistor would be around [(9v. - 0.7v.) / 0.5mA.] =~ 16K ohms.

    Does that answer your question.

    And yes, you can choose whatever IC you want, as long as it is not to close to max. ratings, but then you will need to interpolate to find the closest beta value, so as to know what base current to design for to get that collector current value.

    Thats where a different biasing method is used.
    It will help accomodate not having to rely so much on beta values , but more on base voltages.
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    A CD4xxxx IC output goes as high as the supply voltage only when it has no load current.
    The datasheets from Texas Instruments show the output voltage vs output current vs supply voltage for most of theirs.

    With a 10V supply, an output is 10V without a load and is typically 5V at a load current of 14mA or a minimum of 7mA. The short-circuit output current is typically 19.5mA which might cook the IC or a minimum of 9.7mA.
     
  19. Robin Mitchell

    Thread Starter Well-Known Member

    Oct 25, 2009
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    How did you work out the 5ma?
     
  20. Audioguru

    New Member

    Dec 20, 2007
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    All transistors have datasheets.
    They all have a guaranteed spec called Collector-Emitter Saturation Voltage which is the C-E voltage when the transistor is turned on hard. It has nothing to do with Current-Gain which is when a transistor is linear with plenty of collector to emitter voltage.

    2Nxxxx American transistors have the saturation voltage spec'd when the base current is only 1/10th the collector voltage.
    BCxxx European transistors are spec'd when the base current is 1/20th the collector current like this:
     
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