For workers engaged in hardware design, the Zener tube should be one of the most commonly used devices in our projects.
Zener diodes, also known as Zener diodes.
It plays the role of stabilizing voltage in the circuit . After the diode is reversely broken down, the reverse voltage does not change with the reverse current within a certain range of reverse current to stabilize the voltage .
The biggest difference from ordinary diodes is that they mainly work in the reverse breakdown state .
However, what is reverse breakdown, and what does reverse breakdown voltage refer to? When understanding diodes, it is necessary to understand these terms.
The term reverse breakdown is actually not only used in the study of diodes, but we will also encounter this term in the study courses of triodes in the future.
Diodes are forward conducting and reverse blocking. When a forward voltage is applied, electrons can pass through the diode. However, when a reverse voltage is applied across it, electrons cannot pass through the diode, and the diode in the circuit is equivalent to an open circuit. But the resulting "open circuit" depends on the reverse voltage applied across the diode. As long as the reverse voltage is large enough to a certain value, the diode will be broken down, and the reverse voltage of the breakdown at this time is the reverse breakdown voltage.
However, Zener diodes mainly use the characteristics that the current changes greatly in the reverse breakdown state, but the voltage remains basically unchanged, and finally plays a role in stabilizing the voltage in the circuit. At the same time, the reverse breakdown of the Zener diode is reversible. That is, after losing the reverse voltage, the voltage regulator tube continues to resume normal work. As long as the reverse current does not exceed the maximum allowable range when subjected to reverse voltage, the Zener tube will not be damaged due to thermal breakdown.
Before and after the reverse breakdown of the Zener diode, according to R=U/I, the resistance will rapidly decrease from a large value to a small value. Therefore, the voltage stabilization value of the Zener diode is its breakdown voltage value, which is mainly related to the doping concentration of the PN junction.
According to the breakdown voltage of the diode, the peak voltage URWM of the reverse voltage applied to the diode is generally half or two-thirds of the reverse breakdown voltage. For example, the reverse working peak voltage of 2CZ52A silicon diode is 25V, and the reverse breakdown voltage is about 50V. When using it, the reverse voltage applied to the diode should be prevented from being too high.
The figure below is the volt-ampere characteristic curve of the Zener diode.
The key parameters of the Zener diode
Stable voltage
The voltage value when the Uz Zener tube works stably after reverse breakdown is called the stable voltage.
Stable current The
reverse current when the Iz Zener tube works stably after reverse breakdown is called the stable current. The maximum reverse current allowed by the Zener tube is called the maximum stable current. When using a Zener tube, the operating current cannot exceed it, and it is generally designed to be greater than twice the output voltage;
When the dynamic resistance Rz
regulator works on the reverse breakdown curve, the ratio of the voltage change △u to the current change △i is called the dynamic resistance. The smaller the dynamic resistance, the better the voltage stabilization performance;
The rated power consumption Pz
is determined by the allowed temperature rise of the chip, and its rated value is the product of the stable voltage Uz and the allowed maximum current Izm.
Temperature coefficient α
The temperature change of the Zener tube will cause a small change in the stable voltage. Therefore, the relative change in the voltage across the tube caused by a temperature change of 1°C is the temperature coefficient. The smaller the temperature coefficient, the better, indicating that the Zener tube is affected by temperature. The effect is minimal.
Since it is a voltage regulator, and the working state is in the reverse breakdown state, when the Zener diode is in use, its negative pole is connected to a high potential, and its positive pole is connected to a low potential.
The picture shows a simple voltage regulator circuit diagram
When the grid voltage fluctuates,
the load changes.
In the voltage stabilizing circuit, it is often used together with the current limiting resistor R.
Working conditions of the Zener diode: 1. In the state of reverse breakdown 2. In the state of reverse breakdown, Iz<Iz max.
Regarding the voltage regulator tube in the circuit, we often see the use of various combinations. The following is a simple schematic diagram. We not only need to understand its function, but also its final voltage value.
In Figure 1, both the 6V and 8V voltage regulator tubes are broken down, and what is the value of VCC for the voltage regulation of the two.
Figure 2. The forward conduction of the 6V regulator tube, the reverse breakdown voltage regulation of the 8V regulator tube, the VCC is 8V regulator plus the forward conduction voltage drop of the 6V regulator tube is 8.7V.
Figure 3. The 8V Zener tube conducts forward, and the 6V Zener tube reverses the breakdown voltage regulation. VCC is a 6V regulator plus the forward conduction voltage drop of the 8V Zener tube is 6.7V.
As shown in Figure 4, both the 6V and 8V voltage regulators are forward-conducting, and the sum of VCC’s forward-conducting voltage drop is 1.4V.
As shown in Figure 1, the 6V regulator tube with a low regulator value breaks down first, the 8V regulator is in an open circuit state, and VCC is 6V.
Figure 2 3 4 At least one Zener tube is forward-conducting, so VCC is 0.7V.
Finally, we also need to understand the difference between positive and negative poles of diodes, especially SMD ones, which are mainly divided into the following methods:
-
Use a magnifying glass or a microscope to
distinguish and identify the color band. The obvious mark with the color band is the cathode of the diode . At the same time, on the PCB pad, the cathode is often marked with a vertical line. -
Multimeter measurement
Turn the multimeter to the diode gear
and touch the red and black test leads to both ends of the diode. When the multimeter displays a value of about 0.3-0.6, the end that the black test lead touches is the negative pole of the diode, and the end that the red test lead touches is the positive pole of the diode. .