Characteristics and Applications of Diodes

What are the types ofdiodes? There are a wide variety of diode products with different shapes and specifications. According to the semiconductor material used, it can be divided into germanium diodes (Ge tubes) and silicon diodes (Si tubes). According to their different uses, diodes can be divided into detection diodes, rectifier diodes, voltage regulator diodes, switching diodes, etc. According to the chip structure, it can be divided into point contact diodes, surface contact diodes, and planar diodes, and their chip structures are shown in Figure 1.

Point contact diodes are made by pressing a very thin metal wire onto the surface of a smooth semiconductor chip, applying a pulse current to firmly sinter one end of the wire with the chip, forming a "PN junction". Due to its point contact, only small currents (not exceeding tens of milliamps) are allowed to pass through, making it suitable for high-frequency low current circuits such as radio detectors.

The "PN junction" area of surface contact diodiodedes is relatively large, allowing for larger currents (several amps to tens of amps) to pass through. They are mainly used in "rectification" circuits that convert alternating current into direct current.

A planar diode is a specially designed silicon diode that can not only pass large currents, but also has stable and reliable performance. It is commonly used in switching, pulse, and ultra-high frequency circuits. What is the conductivity of a diode? The most important characteristic of a diode is its unidirectional conductivity. If we use water flow as a metaphor for current, then the diode connected to the circuit is like a valve installed on a water pipe that allows water to flow in one direction - a check valve, as shown in Figure 2. When water flows from the A end to the B end of the pipe, the valve opens and the water flow is unobstructed; When water flows from end B to end A of the water pipe, the valve closes and the water does not flow. The circuit symbol of the diode is shown in Figure 3. The side with an arrow on the diagram is the positive electrode (anode) of the diode, and the side with a short line is its negative electrode (cathode). In a circuit, current can only flow from the positive pole to the negative pole of a diode. Below, we will demonstrate the forward and reverse characteristics of a diode through a simple experiment.

1. Positive characteristics

Take a 2.5V small light bulb (used for flashlights), a surface contacdiode, and two dry batteries, and connect them according to the circuit in Figure 4. At this point, the positive terminal of the diode is connected to the positive terminal of the power supply, and the negative terminal is connected to the negative terminal of the power supply through a small bulb. The small bulb will light up normally, indicating that the diode is in a conducting state. In electronic circuits, as long as the positive terminal of a diode is connected to the high potential terminal and the negative terminal is connected to the low potential terminal, the diode will conduct. This connection method is called forward bias. It must be noted that when the forward voltage applied across the diode is very small, the diode still cannot conduct, and the forward current flowing through the diode is very weak. Only when the forward voltage reaches a certain value (this value is called the "threshold voltage", which is about 0.2V for germanium tubes and about 0.6V for silicon tubes), can the diode truly conduct. After conduction, the voltage across the diode remains basically unchanged (about 0.3V for germanium tubes and about 0.7V for silicon tubes), which is called the "forward voltage drop" of the diode.

2. Reverse characteristics

Reverse the polarity of the diode in Figure 4, as shown in Figure 5. At this point, the negative pole of the diode is connected to the positive pole of the power supply, and the positive pole is connected to the negative pole of the power supply through a small light bulb. If the small light bulb does not light up, it indicates that the diode cannot conduct. In electronic circuits, the positive terminal of a diode is connected to the low potential terminal, and the negative terminal is connected to the high potential terminal. The diode is in a cut-off state, and this connection method is called reverse bias. It must be noted that even when a reverse voltage is applied to a diode and it is in the cutoff state, there will still be a weak reverse current flowing through the diode, commonly referred to as leakage current. When the reverse voltage across the diode increases to a certain value, the reverse current will sharply increase, and the diode will lose its unidirectional conductivity characteristics. This state is called diode breakdown. The main parameters of a diode are technical indicators used to represent its performance and scope of application, and are called diode parameters. Different types of diodes have different characteristic parameters. For beginners, it is necessary to understand the following main parameters:

1. Rated forward working current

It refers to the maximum forward current value that a diode is allowed to pass through during long-term continuous operation. Because the current passing through the tube will cause the core to heat up and the temperature to rise. When the temperature exceeds the allowable limit (around 140 ℃ for silicon tubes and around 90 ℃ for germanium tubes), the core will overheat and be damaged. Therefore, do not exceed the rated forward operating current value of each type of diode specified in the manual or instruction manual during the use of the diode. For example, the rated forward working current of the commonly used 2AP1 pig diode is 16mA; the 2CP1O silicon diode is 100mA.

2. Maximum reverse operating voltage

As mentioned above, when the reverse voltage applied across the diode reaches a certain value, it will break down the tube and lose its unidirectional conductivity. In order to ensure safe use, the maximum reverse operating voltage value (referring to its maximum value for AC power) has been specified. For example, the voltage for 2AP1diode is 20V, while for 2CP10 diode it is 25V.

3. Reverse current

Reverse current refers to the reverse current flowing through a diode at a specified temperature and maximum reverse voltage. The smaller the reverse current, the better the unidirectional conductivity of the tube. It is worth noting that there is a close relationship between reverse current and temperature, with the reverse current doubling for every 10 ℃ increase in temperature. For example, if the reverse current of a 2AP1 germanium diode is 250uA at 25 ℃ and the temperature rises to 35 ℃, the reverse current will increase to 500uA. Similarly, at 75 ℃, its reverse current has reached 8mA, not only losing its unidirectional conductivity, but also causing the tube to overheat and be damaged. For example, the 2CP10 silicon diode has a reverse current of only 5uA at 25 ℃, and a reverse current of only 160uA when the temperature rises to 75 ℃. Therefore, silicon diodes have better stability than germanium diodes at high temperatures. How to test the quality of diodes? Under amateur conditions, a multimeter can be used to test the performance of diodes. Before testing, first turn the conversion switch of the multimeter to the Rx1k position of the ohm range (be careful not to use Rx1 to avoid excessive current burning out the diode), and then short-circuit the red and black probes to zero the ohm range.

1. Positive characteristic test

As shown in Figure 6, connect the black probe (positive terminal of the battery inside the meter) of the multimeter to the positive terminal of thediode, and connect the red probe (negative terminal of the battery inside the meter) to the negative terminal of the diode. If the pointer does not swing to zero but stops in the middle of the dial, the resistance value at this point is the forward resistance of the diode, and generally the smaller the forward resistance, the better. If the forward resistance is 0, it indicates that the chip is short circuited and damaged; If the forward resistance approaches the ∞ value, it indicates that the chip is open circuit. Both short-circuit and open circuit pipes cannot be used.

2. Reverse characteristic testing

As shown in Figure 7, touch the red probe of the multimeter to the positive terminal of thediode and the black probe to the negative terminal of the diode. If the pointer is at or near the ∞ value, the tube is qualified.

Example 1 of diode application: The rectifier component utilizes the unidirectional conductivity of the diode to convert alternating alternating current into pulsating direct current in a single direction. Figure 8 depicts the simplest diode half wave rectification circuit.

2. The switching element diode has a small resistance under forward voltage and is in a conducting state, equivalent to a turned on switch; Under the action of reverse voltage, the resistance is very large and in a cut-off state, like a disconnected switch. By utilizing the switching characteristics of diodes (Figure 9), various logic circuits can be formed.

After the forward conduction of the limiting elementdiode, its forward voltage drop remains basically unchanged (0.7V for silicon tubes and 0.3V for germanium tubes). By utilizing this characteristic, as a limiting element in the circuit, the signal amplitude can be restricted within a certain range. The limiting circuit composed of silicon diodes VDI and VD2 in Figure 10 can limit the amplitude of positive and negative signals to around ± 0.7V.