Things to note about Zener diodes

What is a Zener diode? What does it do? Things to note when using Zener diodes Zener diodes are widely used and used in many ways. It seems easy to apply, but it can be easily damaged if you don't pay attention. The following are some points to note when selecting: Multiple Zener diodes can be used in series, but due to the relatively large discrete nature of the diode parameters, they cannot be used in parallel.

Temperature has a great influence on the characteristics of semiconductor devices. When the ambient temperature exceeds 50°C, the maximum power dissipation should be reduced by 1% for every 1°C increase in temperature. The Zener diode pins must be welded more than 5mm away from the tube case. It is best to use a soldering iron below 30W for welding. If you use a 40-75W electric soldering iron, the welding time should not exceed 8-10 seconds. Try to use solder wire with built-in solder instead of using large pieces of solder and adding rosin.

In order to compensate the voltage temperature coefficient of the Zener diode, the Zener diode can be used in series with a silicon diode (including a silicon Zener diode). The number of forward diodes in series should not exceed three. It can also be connected in series with a special temperature compensation tube. use. In order to obtain a lower stable voltage, appropriate Zener diodes can be selected and connected in series with opposite polarity directions, and then an appropriate operating current can be added. That is to say, the Zener diode is used in the forward direction.

The characteristics of ordinary diodes are that they are conductive under forward voltage and cut off under reverse voltage. This reverse voltage is what we usually call withstand voltage. For example, an ordinary diode with a withstand voltage of 100V will cause leakage when the reverse voltage applied to both ends reaches more than 100V. If the reverse voltage continues to increase, the diode will be scrapped due to reverse breakdown.

Ordinary diodes are used in the forward conduction area, while Zener diodes are used in the reverse voltage area. Under the forward connection method, the characteristics of the Zener diode are similar to those of ordinary diodes. The voltage stabilization value of the voltage regulator tube is also its reverse voltage breakdown value. When the reverse voltage exceeds the rated voltage stabilization value of the Zener diode, reverse leakage current will also occur. Different from ordinary diodes, when the Zener diode undergoes reverse breakdown, its leakage current will rise sharply in a nearly straight line. , but as long as the current does not exceed the maximum rated current of the voltage regulator tube, when the reverse voltage is lower than the voltage stabilization value of the voltage regulator tube, the voltage regulator tube will be in the reverse cut-off state without damage.

The four pictures above are different regulated outputs obtained by flexibly using the forward and reverse characteristics of diodes. The ordinary diodes in the picture are silicon diodes. Therefore, during forward conduction, the forward voltage drop across both ends is approximately 0.7V. The resistor R can limit the maximum reverse current and load current of the voltage regulator tube. In the upper left picture, two voltage regulator tubes are connected in reverse series. The output voltage regulator value is the sum of the voltage regulator values ​​of the two tubes. So it is 5.6+3.6=9.2V. The upper right picture and the lower left picture both use an ordinary diode and a voltage regulator tube connected in series. The output voltage is the voltage regulator value of the voltage regulator tube plus 0.7V. So the upper right picture is 5.6+0.7=6.3V; the lower left picture is 3.6+0.7=4.3V. In the picture on the lower right, two ordinary diodes are connected in series in the forward direction, so the output voltage is clamped to about 0.7V+0.7V=1.4V.

What I want to explain here is. The forward voltage of the PN junction of diodes made of germanium and silicon materials is different. The forward voltage of germanium diodes is about 0.2 to 0.3 V. The forward voltage drop of small current silicon diodes is about 0.6 to 0.8 V at medium current levels. Therefore, using germanium diodes in the design of radio remote receiving circuits can improve detection sensitivity. Since the temperature stability characteristics of silicon materials are much higher than those of germanium materials, they have been widely used.

With the above concepts, we can use it flexibly when we encounter that we do not have suitable accessories on hand and need to adjust the voltage regulator output. For example, by using the forward voltage drop of a silicon diode and inserting a rectifier diode in forward series at the output end of a 5V regulated power supply, an output of 4.3V can be obtained. By connecting two in series, you can approximately get an output voltage of 3.6V... But please note that the voltage stabilizing circuit made by using the forward voltage drop of the diode, because the forward voltage drop changes slightly with the current, so if the load current changes When the voltage is larger, the voltage fluctuation is also larger.

In addition to making a voltage stabilizing circuit, the voltage regulator tube can also be used as a one-time protection circuit. For example, in important parts, to prevent the voltage from rising and damaging valuable components, a voltage regulator tube can be connected in reverse parallel with the power supply there. When the power supply voltage suddenly rises due to a fault and exceeds the voltage stabilization value of the voltage regulator tube, the voltage regulator tube will It quickly switches from the cut-off state to reverse breakdown, directly short-circuiting the power supply, blowing out the fuse or current-limiting protection resistor, and protecting all the loads of the power branch circuit from damage due to overvoltage. Since the voltage stabilizing tube used for this kind of protection usually does not work, it can be temporarily used during emergency repairs. In an emergency, experienced maintenance personnel can even use local materials to disassemble them and use them as spare parts in short supply for maintenance.

Precautions for using Zener diodes:

1. Pay attention to the difference between general diodes and Zener diodes. Many general diodes, especially glass-encapsulated tubes, are similar in appearance and color to Zener diodes. If you are not careful to distinguish them, you will use them incorrectly. The difference is: look at the appearance, many Zener diodes are cylindrical, shorter and thicker, while general diodes that are cylindrical are more slender; look at the logo, the outer surface of the Zener diode is marked with "Stabilizer" Value, such as 5V6, means the regulated voltage value is 5.6V; use a multimeter to measure it. According to the unidirectional conductivity, use the X1K block to first determine the positive and negative polarity of the diode under test, then use the X10K block, and connect the black test lead to the cathode of the diode. , connect the red test lead to the anode of the diode, and the measured resistance value is compared with that when the X1K block is used. If the reverse resistance value appears to be very large, it is most likely a general diode. If the reverse resistance value appears to become very small, then is a Zener diode.

2. Pay attention to the difference between forward use and reverse use of Zener diodes. When a zener diode is used in forward conduction, it is basically the same as when a general diode is used in forward conduction. After forward conduction, the voltage at both ends is basically unchanged, both about 0.7V. Theoretically, a Zener diode can also be used as a voltage regulator in the forward direction, but its voltage stabilization value will be lower than 1V, and its voltage stabilization performance is not good. Generally, the forward conduction characteristics of the Zener diode alone are not used to stabilize the voltage. voltage, but uses reverse breakdown characteristics to stabilize the voltage. The reverse breakdown voltage value is the voltage regulation value. Sometimes two voltage regulator tubes are used in series, one using its forward characteristics and the other using its reverse characteristics, which can both stabilize the voltage and perform temperature compensation to improve the voltage stabilization effect.

3. Pay attention to the role of the current-limiting resistor and the influence of its resistance value. In the Zener diode voltage stabilizing circuit, a resistor R is generally connected in series, as shown in Figure 1 or 2. This resistor plays the role of current limiting and improving voltage stabilization effect in the circuit. If this resistor is not added, that is, when R=0, it is easy to burn out the voltage regulator tube, and the voltage stabilization effect will be extremely poor. The greater the resistance of the current-limiting resistor, the better the voltage stabilization performance of the circuit, but the voltage difference between the input and output will also be too large, and the more power will be consumed.

4. Pay attention to the pressure difference between input and output. During normal use, the output voltage of the Zener diode voltage stabilizing circuit is equal to the voltage stabilization value at both ends of the voltage stabilizing tube after reverse breakdown. If the voltage value input to the voltage stabilizing circuit is less than the voltage stabilization value of the voltage stabilizing tube, the circuit will The voltage stabilizing effect is lost. Only when the voltage is greater than the relationship, the voltage stabilizing effect will be achieved, and the greater the voltage difference, the greater the resistance of the current limiting resistor, otherwise the voltage stabilizing tube will be damaged.

5. Voltage regulator tubes can be used in series. After several voltage regulator tubes are connected in series, multiple different voltage stabilization values ​​can be obtained, so series connection is more common. The following example illustrates how to obtain the voltage stabilization value after two voltage stabilizing tubes are used in series. If the voltage stabilization value of one voltage regulator tube is 5.6V and the voltage stabilization value of the other is 3.6V, assuming that the voltages of the voltage regulator tubes are both 0.7V when they are in forward conduction, there are four different voltage stabilization values ​​after series connection.

6. Voltage regulator tubes are generally not used in parallel. After several voltage regulator tubes are connected in parallel, the voltage regulator value will be determined by the lowest one (including the voltage value after forward conduction). Let’s take the above two voltage stabilizing tubes as an example to illustrate the calculation method of the voltage stabilizing value. There are four situations after the two are connected in parallel, and there are only two voltage stabilization values. Unless there are special circumstances, Zener diodes are not used in parallel. This is an overview of Zener diodes, I hope it can help you.