Discussion in 'The Projects Forum' started by Jonesy, Mar 17, 2011.

  1. Jonesy

    Jonesy Thread Starter New Member

    Jul 7, 2010
    What's the difference btw pi-filter(choke replaced as resistor) and an RC filter?

    The pi-filter is used as a filter after a rectifier to a positive linear regulator 7805
    (a 240Vac to 5V DC project)

    Does the pi-filter increases the inrush current as the first cap charging current is not reduced by any resistor while a simple RC filter has the resistor before the cap?

    Using a multisim simulation without the PI-filter, the inrush current is less. using just a RC capacitor input filter, I can use a lower VA transformer for rectification and same max load.
  2. Papabravo

    Papabravo AAC Fanatic!

    Feb 24, 2006
    Michigan, USA (GMT-5)
    The main difference is to reduce the ripple in the output without dissipating any significant amount of power. The impedance of an inductor increases with frequency. The impedance of a capacitor decreases with frequency. So series inductors smooth the ripple and capacitors shunt it to ground. Meanwhile the inductor passes DC without any problems, except a "small" IR drop, and the capacitors have no DC current flow to ground. Exactly what you want at the output of a power supply.

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