Capacitor input filter for voltage regulater

[Pagedown]

This question has been boggling my mind. The capacitor input(C2) filter in this schematic helps me filter the output from the rectifier. With combination of Rchoke (is just a resistor and not an inductor) of 2.4ohm, it limits my peak current and filters the output voltage from the rectifier.

The question is why the 120 Hz output from the full wave rectifier can pass through since it is actually an RC filter. 1/2*pi*2.4*1000u = 66Hz (cut-off frequency) ?

Any help is GREATLY appreaciated.

P.S I have tried using a pi-filter which can give me higher output voltage filtering, but absorbs more current.

 

[Perfection]

The cutoff frequency only represents the frequency where the filtering action becomes significant, (generally considered to be where it's reducing by about 3db) it doesn't mean that it will 100% filter out any frequency below (in the low-pass case) it.

I should also point out that because you have a significant load, and nonlinear elements, the way the system is going to behave is going to be significantly different then you LPF calculations.

Here are three things you can do to improve your performance:
1. Increase load resistance
2. Increase C2 capacitance
3. Add additional filtering stages

 

[vk6kro]

The output of a bridge rectifier contains a DC component and an AC component at 120 Hz.

The AC component is reduced by the low pass filter, especially when there is no load.

This leaves the DC component.

When there is a load, the capacitor charges up to the peak voltage of the rectifier output and then discharges slowly into the load when this pulse is reduced.

This tends to fill in the gap between rectified output pulses and bigger capacitors do a better job of filling in this gap.

The result of this is that the output is DC with a "ripple" on it where the capacitor discharges partly before the next rectifier pulse.

 

[Pagedown]

Thank you all for the informative replies.

Adding filtering stages?Do you mean RC-RC stage?
What is the purpose of the resistor actually? Why not just a Cap?

 

[vk6kro]

One effect of the resistor is to reduce the charging current of the capacitor. This can be many times the output current and may cause you to need bigger rectifiers than you would without the resistor.

Sometimes the transformer has enough resistance to limit this current but the series resistor helps.

 

[sophiecentaur]

The ripple on the rectified AC is also affected on load by the winding resistance of the transformer. The source resistance limits how fast the reservoir capacitor charges up on each rising peak of volts from the rectifier.