Power Supply Filters
Power supply filters and regulators
-The output of the rectifier is a pulsating dc wave.
-We need a constant dc output.
-To do this, we need to filter out the oscillations from the pulsating dc wave.
-This is obtained with a diode capacitor combination.
A capacitor-input filter will charge and discharge such that it fills in the “gaps” between each peak. This reduces variations of voltage. This voltage variation is called ripple voltage.
The advantage of a full-wave rectifier over a half-wave is quite clear. The capacitor can more effectively reduce the ripple when the time between peaks is shorter.
-While charging, the capacitor appears as a short.
-This causes large currents through the diodes.
-To avoid damaging the devices, a surge resistor (Rsurge) is added.
-Rsurge should be small in comparison to the load resistor (RL).
-To most effectively reduce the ripple occurring after filtering we use and IC voltage regulator.
-A regulator has 3 terminals: input, output and reference (or adjust).
-In general, it is better to add capacitors after (and before) the regulator.
-A large capacitor between the input voltage and the input terminal further filters the signal.
– To improve transient response a smaller capacitor is added after the regulator.
-The 78XX series are examples of positive output regulators.
-The 79XX series are examples of negative output regulators..
| Type Number | Output Voltage |
| 7805 | 5 V |
| 7806 | 6 V |
| 7808 | 8 V |
| 7809 | 9 V |
| 7812 | 12 V |
| 7815 | 15 V |
| 7818 | 18 V |
| 7824 | 24 V |
-The quality of voltage regulation is measured by two means:
-Line regulation.
-How much change occurs in the output voltage for a given change in input voltage.
Line regulation = (DVout/DVin)100%
-Load regulation.
-How much does the output voltage change over a certain range of current values: minimum (no load, NL) to maximum current (full load, FL).
Load regulation = (VNL – VFL)/VFL 100%
