Time constant for RLC circuit?

Discussion in 'General Electronics Chat' started by Sci, Feb 13, 2005.

  1. Sci

    Thread Starter New Member

    Jan 21, 2005
    Whats the time constant for an RLC circuit?
  2. dragan733

    Senior Member

    Dec 12, 2004
    I think that in a RLC circuit one doesn't define time constant. There has two expressions with dimensional unit: time, but that is not time constant.
  3. iamspook


    Aug 6, 2008
    Time Constant for an RC circuit is tor = RC
    for an LC circuit it is tor = L/R

    In a RLC circuit, you have both combined to worry about. So we actually need to calculate what's
    called the Q-factor. (Quality factor) which describes how resistance dampens the peak value at the resonant frequency, and the bandwidth over which oscillation is significant.

    In a series RLC tuned circuit, Q = (1/R )(sqrt(L/C))
    In a parallel: Q = R sqrt(C/L)

    Q is called 'quality' because it is the ratio of 2 * pi *
    (energy stored) / (power lost)

    Pure L and C lose no energy, so it is the resistive component which
    lowers Q (and makes Q finite)

    If you pull a weight on a spring and let it go, then it will exponentially
    decay as resistive forces absorb and dissipate energy. Since the maths
    for this involves an exponential decay, you need some point which describes
    when it falls to a percentage point of the original swing, otherwise theoretically,
    it will be infinitesimally moving for ever. The time constant is the time it takes
    to reach that point. Q factor describes it nicely.

    Depending on the weight and strength of spring, this decay will either be oscillatory
    or an exponential decay. There is a special value of Q which describes the
    boundary between oscillatory and exponentially decaying behaviour. This is called
    'critical damping' and is a desirable target for filter networks. (The calculation of
    Q(crit) varies with the type of filter.