Please Help

Discussion in 'General Electronics Chat' started by gani, May 26, 2005.

  1. gani

    Thread Starter Active Member

    May 15, 2005
    46
    0
    Hi everyone,

    I was just reading Floyd,RC across dc supply. But the author hasn mentioned abt RC parallel across DC supply.It takes 1 timeconstant for the voltage to reach 63% of its full value.Is this applicable to RC parallel too?
    Hmm.....can someone help me?Please....?

    Thanx a lot!
     
  2. David Bridgen

    Senior Member

    Feb 10, 2005
    278
    0
    No.
    A capacitor connected across a d.c. source without any series R will charge immediately to the source voltage.
     
  3. gani

    Thread Starter Active Member

    May 15, 2005
    46
    0

    I really appreciate your immediate reply.Thankyou.
    The capacitor voltage and current still follow the exponential curve?
    i suppose its the same with RL parallel to dc supply.L will
    develop current.......am i right?

    Thanx for all the help.
     
  4. David Bridgen

    Senior Member

    Feb 10, 2005
    278
    0
    Taking into account the extremely small resistance which will inevitably appear in series with the voltage source and the C, then, theoretically, yes.

    But in practice you wont see it. The voltage across the C rises effectively instantaneously. The charging current will be a very narrow pulse.
     
  5. gani

    Thread Starter Active Member

    May 15, 2005
    46
    0
    hmm....ok but i think i am a bit confused...
    it is an R C circuit parallel to dc supply.....so extremely small resistance..?
     
  6. David Bridgen

    Senior Member

    Feb 10, 2005
    278
    0
    The left hand diagram shows a series RC across a d.c. source. The C charges up exponentially through the R, as you already know, reaching 60-odd percent in one CR period.

    The right hand diagram shows a parallel RC across a d.c. source. The only resistance in series with the C is that of the wiring (very small) and that of the source (negligible.) The time constant is therefore very short and the C charges (almost) instantaneously.
     
  7. gani

    Thread Starter Active Member

    May 15, 2005
    46
    0
    Thanx a lot for ur timely help.
    ok so i guess the RL parallel is similar to this case too.

    once again,thankyou!
     
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