Pi-Filter

[Pagedown]

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.

 

[MikeMl]

You always have some source resistance to limit the inrush. In the case of a power transformer, the inrush current is limited by winding resistance, core saturation, and to a lesser extent the forward conduction of the rectifier(s). Nobody purposely puts a series resistor upsteam of the filter capacitor (power wasting). Almost nobody puts a choke (inductor) there either (due to cost), although this would reduce ripple, allowing you to reduce heat losses in the regulator because you could lower the headroom needed to prevent dropout.

 

[JimB]

MikeMl wrote:

Almost nobody puts a choke (inductor) there either (due to cost), although this would reduce ripple,

This is quite correct.
However in the good old days when electronics glowed in the dark (thermionic valves, or electron toobs), it was normal to have Pi LC filters on the HT line to get the ripple down to manageable levels.
Also in those days large value electrolytic capacitors were not common, large, very expensive.

JimB

 

[MikeMl]

JimB,

Preaching to the choir. I still have a kilowatt RF amplifier with a "swinging choke" in its 2500V power supply.

Now show me a 5Vdc 7805 type power supply with a choke in it????

 

[JimB]

Now show me a 5Vdc 7805 type power supply with a choke in it????

No problem, as long as you can show me some hens teeth first!

JimB