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Old 03-17-2011, 12:15 PM
Jonesy Jonesy is offline
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Default Pi-FIlter

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.
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Old 03-17-2011, 02:50 PM
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Papabravo Papabravo is offline
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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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