Help : Simple Power Supply Filter

[kittmaster]

Based on his schematic, the 10K resistor is the LOAD resistor. If a PIE filter is used, you have a capacitor in SERIES with the output load. How is the DC voltage going to get to the 10K resistor when one of the functions of a capacitor is to pass AC and block DC?

Hence a PIE filter is the wrong solution for ANY DC rectification circuit.

😉

[lacy]

Your descriiption of a Pi filter is incorrect. It is arranged as follows: Cap to ground>Series Resitor or Coil>Cap to ground. This is the classic Pi arrangement. If you look at the picture I posted earlier you will see this arrangement. The only different between what he has now and what I an refereing to is the series resistor or coil.

[lacy]

Just as a added note. What he had before in hiis first circuit is what I would refer to as a high pass Pi type filter. The one I use is a low pass Pi type filter. The only difference is that the high pass has the seriies capacitor as you have stated, while the low pass has the capacitors in "parallel" to ground usually with a resitor or coil in between them. I thought I would make that dstinction to maybe clear up what I am calling a Pi filter.

[kittmaster]

My comments were based on his schematic alone, he has a resistor going to ground, then a capacitor in series, and then a resistor to ground. That is not a pi filter, its actually a high pass filter, but since it is not sinusoidal coming in, its pulsating DC, the output follows the input right behind the expontial constant because he as a very small capacitor that is doing a voltage hold charge based on the levels from the secondary.

That becomes evident as your run the simulation.

If he wants a pi filter control, it should be from the rectifier> cap to ground to a a series inductor with another cap to ground to the load resistor.

If I'm in error or not addressing the question correctly, I'm only going by what I was looking at in the power supply.

Again Pi filters are not common anymore due to the voltage drops they create across the inductor and decrease system efficiency.

[lacy]

Thanks Chris. You understood my question. I thank you for the reply and I can understand about why they are not used anymore. I really appreciate that as it helps me in designing PSs. Now maybe I won't have to have as many components to deal with in my designs. This person has his right and I don't thing he was too concerned with designing a Pi filter. I was the one questioning as to why they were not used anymore and you have answered that for me. Thanks a bunch

Message Edited by lacy on 10-06-2007 09:07 AM