Voltage Regulation

If a special diode called a Zener diode is connected in parallel with the output of a power supply, the diode limits the output voltage. The diode must have an adequate power rating to prevent it from burning out. The limiting voltage depends on the particular Zener diode used. Zener diodes are available for any reasonable power-supply voltage.
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A power supply with a Zener-diode voltage regulator in the output.
Above Figure is a diagram of a full-wave bridge dc power supply including a Zener diode for voltage regulation. Note the direction in which the Zener diode is connected in this application: with the arrow pointing from minus to plus. This is contrary to the polarity used for rectifier diodes. It’s important that the polarity be correct with a Zener diode, or it will burn out.
 
A Zener-diode voltage regulator is inefficient when the supply is used with equipment that draws high current. When a supply must deliver a lot of current, a power transistor is used along with the Zener diode to obtain regulation. Below Figure shows such a circuit.
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A voltage-regulator circuit using a Zener diode and an NPN transistor.
Voltage regulators are available in integrated-circuit (IC) form. The regulator IC, also called a regulator chip, is installed in the power-supply circuit at the output of the filter. In high-voltage power supplies, electron tubes are sometimes used as voltage regulators. These are particularly rugged, and can withstand much higher temporary overloads than Zener diodes, transistors, or chips. However, some engineers consider such regulator tubes archaic.