Rectifiers, Voltage Regulation and Smoothing Circuits

 Unlike direct current, alternating current reverses direction of flow periodically. Rectifier is a device which converts alternating current to direct current. It does this by cutting off one of the half cycles of an alternating current so that it becomes a pulsating direct current, flowing only in one direction. In the earlier era, vacuum tube diodes were used as rectifiers. Now there are semiconductor diodes which are commonly used in hobby electronics. Other types of rectifier devices are mercury arc valves, stacks of copper and selenium oxide plates and silicon controlled rectifiers. Rectified output needs smoothing out of ripples using high value capacitors and inductors. Voltage regulation is done by devices like zener diodes, to provide a stable output in the face of potentially fluctuating loads as in amateur radio transceivers. Transceivers need high current during transmit mode and only very low current during reception of radio signals.

One disadvantage of half wave rectifier is that it delivers only half the cycle of alternating current and hence the output is low. Pulsating direct current output of the half wave rectifier needs more filtering or smoothing network to produce a steady direct current output as the waves are intermittent and at the frequency of the alternating current. Conversion ratio (η), sometimes called efficiency, of half wave rectifier is about 40.5%, but a center tapped transformer is not necessary for a half wave rectifier.

The full wave rectifier utilizes both halves of the input alternating current cycle, using two diodes, unlike half wave rectifier which utilizes only one half cycle with a single diode. Hence full wave rectifier yields higher voltage output. Conversion ratio of a full wave rectifier is about 81%, but a center tapped transformer is needed for this type of full wave rectifier using two diodes. Conversion losses occur in the transformer and the rectifier itself. A full wave bridge rectifier using four diodes does not require a center tapped transformer. But there is voltage drop across two diodes instead of one in usual full wave rectifier, during each half cycle. Voltage drop across each silicon diode is about 0.7 Volts and can be significant in low voltage rectifier applications as the diode does not conduct when the voltage in the cycle is below 0.7 Volts.

The output voltage of a rectifier circuit can vary due to multiple reasons. As the load varies, voltage drop in the internal resistance of the power supply also varies, increasing with an increase in load current. Variations in alternating current mains supply cause variations in the direct current output voltage. Output of semiconductor devices can vary with temperature. Hence some feedback arrangement acting as a voltage regulator is employed, to overcome these variations in a regulated power supply. Output voltage of a regulated power supply remains stable in spite of the possible variations mentioned above.

A simple way to regulate the voltage is to connect a series resistor to drop source voltage to the required level and a Zener diode to shunt with reverse voltage equal to the set voltage. The Zener diode dumps current to maintain the set output voltage if the input voltage rises above that. As Zener diodes have both voltage and current limitations, this simple regulator is usually employed only in low voltage, low current circuits. This method is also inefficient as it dumps excess current, which is not available to the load. Zener diode is a special type of diode designed to allow current to flow backwards when a certain set reverse voltage known as the Zener Voltage is reached. Cathode of the Zener diode is connected to the positive side of the supply. Zener diodes are there with wide range of voltage and power ratings, to be chosen as per need.

Another option is to use a three terminal integrated circuit as an active regulator. It contains a precision reference, a high gain error amplifier, temperature compensated voltage sensing resistors and transistors and a pass element. Some devices have thermal shut down, over voltage protection and current foldback. Current foldback is a current limiting feature so that when the load attempts to draw more current from the supply, foldback reduces both the output voltage and current well below the normal operating limits. When there is a short circuit, current is typically limited to a small fraction of the maximum current by the foldback mechanism. That reduces the risk of fire and damage due to heat.