RC Filters
Pre-1970s: Passive filters (R, L, C) between rectifier and load resistance
Modern use: Rare, except in special applications like audio amplifiers
- Bridge rectifier with capacitor-input filter
Typical ripple: Up to 10 percent peak-to-peak across filter capacitor
Lower ripple: Requires excessively large filter capacitor
Additional filtering: Done with RC sections between capacitor and load resistor
RC sections: Passive filters using R, L, or C components
Design criterion: \(R\) much greater than \(X_C\) at ripple frequency
Ripple reduction: Each section attenuates by a factor of at least 10
Disadvantage: DC voltage loss across each resistor
Suitability: Only for very light loads (small current or large resistance)
LC Filters
- Large load current: LC filters are better than RC filters
LC filters: Reduce ripple by using inductors
Design criterion: \(X_L\) much greater than \(X_C\)
DC voltage drop: Smaller across inductors than resistors
LC filters: Obsolete due to size and cost of inductors
Replacement: Integrated Circuit (IC) voltage regulators
IC Voltage Regulators
Include diodes, transistors, resistors in miniaturized package
- IC voltage regulator between filter capacitor and load resistor
Function: Reduces ripple and holds output voltage constant
- Example regulator: LM7805
LM7805: Provides 5V fixed positive output (requires input 2-3V higher)
Other regulators: 78XX series (e.g., 9V, 12V, 15V), 79XX series (negative outputs)
IC voltage regulators: Standard for ripple reduction due to low cost
