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
    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.