555 circuit confusion

Discussion in 'General Electronics Chat' started by xxxyyyba, May 16, 2016.

  1. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
    249
    2
    Here is block diagram of 555 timer:

    [​IMG]

    I wouldn't analyse it, but under some circumstances (for some voltages at THRESH and TRIG pins), complemented output Q will be at high voltage level, with respect to ground (pin 1). And that's problem. For BJT to conduct, VBE is aprox. 0.7V, but in this case it will be much higher than 0.7V. What would happen in this case?
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,771
    1,103
    Your diagram only illustrates the principle of operation and isn't intended to be an accurate portrayal of the inner workings of the IC. The not-Q output in practice would provide limited base current to avoid stressing the Vbe junction of the discharge transistor.
     
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  3. wayneh

    Expert

    Sep 9, 2010
    12,089
    3,027
    The discharge pin 7 gives a path to ground. Current sinking is limited by the power dissipation rating of the particular 555 IC you are using. 200mA is typical.

    Or perhaps you were asking about the base current, which Alec_t has answered.
     
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  4. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    In real there is a resistor on the base:

    NE555_internal_schematic.png

    This is taken from the SGS datasheet.

    Bertus
     
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  5. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
    249
    2
    Hello, thanks for replies. It's clear :)
     
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