Basics

Filter

Definitions

 

Most power supplies create a constant output voltage. Even variable supplies are adjusted to some fixed value and then left. The output of a rectifier however is not constant. There is a huge ripple voltage, which must be smoothed or filtered.

Filtering generally requires reactive components. Capacitors placed across the load will tend to reduce the ripple voltage, while inductors placed in series with the load will tend to reduce the ripple current.

 

Capacitive Filter
Schematic
Approximation Formulas
Half-Wave Rectifier (add a capacitor)

Full-Wave Rectifier

 

The required size of the filter capacitor (in mfd) can be approximated by:

Where:

C is in mfd.

ILoad is in ma.

RC discharge is assumed.

A closer approximation can be made by using the transformer Schade curves. In any case, ripple voltage increases as the load current increases.

Care must be exercised when selecting filter capacitors. Power supplies generally use large electrolytic capacitors as filters. It is very important to never exceed the voltage rating or to reverse the polarity of electrolytic capacitors.

Although the voltage across the capacitor is relatively constant, the current through it is not. The ripple currents are typically 2 – 3 times the DC load current. The capacitor supplies the load current when the rectifier circuit current falls. It also accepts the excess rectifier current when it exceeds the load current demands. The current in the filter capacitor is therefore an alternating current (AC) since it reverses direction.

 

5Spice simulation

A free spice program is available from http://www.5spice.com/ . Using this program, the voltage and current waveforms may be observed.

Full Wave Bridge and Filter Schematic
Voltage Waveforms
Current Waveforms

 

Capacitive-Inductive Filter
Ripple Factor The significance of ripple in the output is somewhat dependant on the average output voltage. The lower the average value, the more important it is to reduce ripple.

 

For Further Research

RC Filter

RL Filter

Parallel RLC Circuit

http://www.powerdesigners.com/InfoWeb/design_center/Design_Tips/Electrolytics/Caps.shtm

http://www.uoguelph.ca/~antoon/gadgets/caps/caps.html

 

 

Power

Transformer

Rectifier

Filter

Regulator

 
 

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