I. Zener diode
A Zener diode, also known as a voltage regulator diode, is a diode that utilizes the phenomenon that the current can vary over a wide range while the voltage remains essentially constant during the reverse breakdown state of a PN junction. This diode is a semiconductor device with high resistance until it reaches a critical reverse breakdown voltage. At this critical breakdown point, the reverse resistance drops to a very small value. In this low-resistance region, the current increases while the voltage remains constant. Zener diodes are classified according to their breakdown voltage. Because of this characteristic, Zener diodes are mainly used as voltage regulators or voltage reference elements. Zener diodes can be connected in series to operate at higher voltages; by connecting them in series, an even higher stable voltage can be obtained.
The forward characteristic of a Zener diode is similar to that of a regular diode. Its reverse characteristic is characterized by a large reverse resistance and extremely small reverse leakage current when the reverse voltage is below the reverse breakdown voltage. However, when the reverse voltage approaches its critical value, the reverse current increases sharply, a phenomenon known as breakdown. At this critical breakdown point, the reverse resistance drops sharply to a very small value. Although the current varies over a wide range, the voltage across the diode remains essentially stable near the breakdown voltage, thus achieving the diode's voltage regulation function.
II. Common Zener Diode Application Circuits
1. Classic DC voltage regulator circuit
This schematic diagram can be said to be the most classic and original Zener diode application circuit. Its structure is simple and its performance is average. D2 is a Zener diode, and R1 is a current-limiting resistor to protect the Zener diode. The unstable 12V power supply voltage is applied to the Zener diode D2 through resistor R1. Since the Zener diode D2's Zener voltage is 5V, the power supply voltage is much greater than this value. The Zener diode D2 is in working state and conducts, stabilizing the voltage at about 5V for output.
An important point to note is that if the power supply voltage is less than 5V, the Zener diode cannot be activated and therefore will not work. When the power supply voltage is greater than 5V, the Zener diode will operate and stabilize the voltage at its regulated value until it can no longer withstand the high voltage and is damaged.
2. Series-connected voltage regulator circuit
In this circuit, a 13V Zener diode D3 is connected in series to stabilize the base voltage of the transistor, thus achieving a constant output voltage. The specific analysis is as follows: The power supply voltage on the left is 25V. Through resistor R4, the Zener diode D3 is opened, forming a loop. At this point, the Zener diode D3 will stabilize at approximately 13V. Then, the base of transistor Q1 is stabilized at 13V by the Zener diode. Next, due to the diode clamping effect between the base and emitter of transistor Q1, the emitter of transistor Q1 is clamped at approximately 13 - 0.7V = 12.3V. Therefore, the output of this circuit will continuously output a stable 12V voltage.
3 Overvoltage protection
This circuit was commonly used in older televisions and other household appliances. When the DC power supply voltage is normally 115V, the voltage divider between the two resistors will not activate the Zener diode, thus preventing the television from entering standby mode. When the DC power supply voltage is greater than 115V, the result is the opposite. Here's a detailed analysis: When the DC power supply voltage is 115V, the voltage divided by the voltage divider circuit of resistors R6 and R7 is applied to the right side of the Zener diode D4. At this time, the Zener diode will not activate, so transistor Q2 will not operate, the standby protection circuit will not activate, and the television will work normally.
When the DC power supply voltage is greater than 115V, the voltage after voltage division by resistors R6 and R7 is sufficient to turn on the Zener diode D4. At this time, the voltage flows through the Zener diode D4 to the base and emitter of transistor Q2 and enters GND, forming a circuit. This drives transistor Q2 to turn on, causing the standby control IO to be pulled to GND, and the standby control puts the TV into standby mode.
