voltage of capacitor

Discussion in 'General Electronics Chat' started by fugrammer, Apr 1, 2007.

  1. fugrammer

    Thread Starter New Member

    Apr 1, 2007
    hi, i am currently trying to understand how a astable circuit works. The point i don't understand is why when Q1 is ON, the negative terminal of the charged capacitor, C1, will be negative voltage (~-5). The explanation i got from wiki is that the voltage across a capacitor cannot change suddenly. However, i am sure there is other explanation.

    Any explanation will be appreciated.

  2. emad

    New Member

    Mar 26, 2007
    dear fugrammer
    the first thing you must consider in your circuit is the Q1 and Q2 unlike
    so one of them turn on before other and cause turn off the other
    the capacitors in the circuit employ to respons which is ON and start charging
  3. fugrammer

    Thread Starter New Member

    Apr 1, 2007
    okay. I don't understand why does the positive lead of C1 is pulled down to 0.35V? I know that the transistor has a low voltage drop of a bout 0.35V, but why did the postitive lead of C1 drop from +5V to +0.35V?
  4. John Luciani

    AAC Fanatic!

    Apr 3, 2007
    The voltage of the capacitor cannot change instantly. If the +terminal has a voltage
    +5V greater than the -terminal and you lower the +terminal to 0V the +terminal
    is still +5V greater than the -terminal. Since the voltage at +terminal is now 0V
    the voltage at the -terminal has to be 5V less which means the -terminal is now
    at -5V.

    The +teminal of the capacitor drops to 0.35V because that is the saturation voltage
    of the transisitor (Vce(sat) on the datasheet). The transistor is being used as a switch
    but cannot get any closer to a short circuit (0V) than the saturation voltage.

    (* jcl *)


  5. fugrammer

    Thread Starter New Member

    Apr 1, 2007
    when measuring the voltage negative lead of the capcitor, are we finding the potential different between it and the - terminal of the battery? Since it is a negative voltage, does it mean electric charges flow from the - terminal of the battery to the negative lead of the capacitor?