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Why can't I saturate this transistor?

  1. Sep 18, 2014 #1
    Hi,
    I started using Proteus Isis and was playing around just to get used to it. I designed a simple transistor circuit that should work in the saturated region. The LED needs 2.3V. For saturation, the collector and emitter terminals should be shorted or have a tiny voltage drop across it. So, since the LED requires 20mA, I calculated that the series resistance should be around 139. The BC547 transistor has a dc gain around 110 at that current. This means that a base current of 0.2mA or more should should be enough to saturate the transistor. The circuit is shown below. Whatever base current I used, ##V_{CE}## didn't get below 0.2V which indicates a transistor is near saturation. I've cross-checked values from the datasheet. Surely, something is terribly wrong !!

    2zsa051.jpg
     
  2. jcsd
  3. Sep 18, 2014 #2

    davenn

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    hi ya
    welcome to PF

    why have you got all your meters in AC mode when they are in a DC circuit
    Im not familiar with the inner working of Proteus, but maybe that's why you are getting oddball readings ?

    Dave
     
  4. Sep 18, 2014 #3
    Thanks for the welcoming. I actually didn't notice that I was using the meters in the AC mode. I changed them to DC and the values were the same. Is something wrong with the circuit ?

    23jg5xe.png
     
  5. Sep 18, 2014 #4
    Try increasing R1 to about 1kohm
     
  6. Sep 18, 2014 #5
    I've done all that. The current becomes so small that the LED turns OFF.
     
  7. Sep 19, 2014 #6
    Try increasing the collector supply to 12 volts
     
  8. Sep 19, 2014 #7

    davenn

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    yup that sounds about right for a 5V supply and ~ 20mA through the LED with a ~ 2V drop
    200 Ohms give or take a bit is ok

    the BC547 is going to turn on at ~ 0.6 - 0.7V and the 0.54 Vdrop across the C-E is indicating that its currently turned on.


    the definition of saturation as in your common emitter case is ...

    As base current increases the collector current increases to a point where any more base current will not increases the collector current at that point the transistor is saturated whereby current may flow in both direction and the two diodes will essentially be in a parallel mode and the voltage drop from emitter to collector will reflect the status as a low voltage drop.

    as far as I can see, that's what is happening in your circuit
     
    Last edited: Sep 19, 2014
  9. Sep 19, 2014 #8

    davenn

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    You also need to understand that the C-E Vdrop is not a fixed value at saturation ie. is isn't 0.2V and only 0.2V
    it is dependant on the base drive, the higher the drive the higher the C-E Vdrop value at onset of saturation

    cheers
    Dave
     
  10. Sep 19, 2014 #9

    jim hardy

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    I'd suggest building one from real parts. Your simulation's accuracy is at the mercy of a programmer someplace.

    I looked for information on Proteus' parts library, to no avail.
    However the BC547 is allowed (per a Fairchild datasheet at http://www.elektronik-kompendium.de/public/schaerer/FILES/bc550.pdf )
    Vce as much as 600 millivolts at 100ma and 250 millivolts at 10 ma .
    You're operating it in an area between those limits.

    BC547 is a high voltage version of that transistor, 50 volt, so probably has a stout collector.
    Try swapping it for a 30 volt BC548 and see if the program calculates any differently.
    Then try a 65 volt BC546.

    It's really educational to poke at things and learn their nuances..

    old jim
     
    Last edited: Sep 19, 2014
  11. Sep 19, 2014 #10
    Thanks Jim. Tried different NPNs but can't bring VCE down enough.
     
  12. Sep 19, 2014 #11

    jim hardy

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    Hmmmm ... one wonders what the Proteus program assumes about that transistor. Any details provided about that ?

    I do notice Fairchild specifies Vcesat with Ic = 20Ib.
    Fairchild's graph, figure 3 specifies Ic = 10Ib .
    You're at 15.
    Oughta work just fine.
    How's your Vbe look compared to the datasheet ?

    Poke at that simulation program a bit more.... find its limits.
     
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