Semiconductor Fundamentals Unit 3 – Full-Wave Rectification and Filtering
Exercise 2 – Power Supply Filtering
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to demonstrate how a filter
significantly reduces the ripple of a pulsating dc output to a relatively smooth dc voltage by
using a capacitive input filter circuit. You will verify your results with a multimeter and an
oscilloscope.
DISCUSSION
- Most electronic equipment requires a smooth dc voltage; therefore, filters are required after a
rectifier to reduce ripple. Ripple present in the volt range can be reduced to the millivolt
range. - One example is the capacitive input filter which is implemented by placing an electrolytic
capacitor across the bridge rectifier output and in parallel with the load resistor. - Under no-load conditions the capacitor charges rapidly to the peak full-wave rectifier voltage
output. Since there is no discharge path, the capacitor remains charged, maintaining the
rectifier output when the rectifier input drops to zero. - With a load present, the capacitor discharges through the load, maintaining a near-constant
load voltage. Another output pulse recharges the capacitor before the capacitor can fully
discharge. - Discharge rate of a capacitor is longer than the charge time and depends on the RC (load
Resistance times Capacitance) time constant. - Discharge time, for constant frequencies, will affect the magnitude of the ripple. Increases in
the capacitance and/or load resistance reduces ripple. Ripple is also reduced as frequency
increases. - Other types of filters are configured with inductors or combinations of resistors, capacitors,
and inductors. - Additional filter sections remove a portion of the rectifier ripple content.
- The circuit used in this exercise is not a regulated power supply. Parallel load resistance
reduces the peak output voltage of the rectifier.