The conductive characteristics, functions, and main parameters of diodes

The conductive characteristics, functions, and main parameters of diodes

A diode has two terminals, anode and cathode, and current can only flow in a single direction. That is to say, current can flow from the anode to the cathode, but not from the cathode to the anode. The application of the unidirectional characteristic possessed by diodes is commonly referred to as the "rectification" function, which can convert alternating current into pulsating direct current. For example, the modulation of radio signals by radio receivers is accomplished through rectification.

The English word for diode is diode. The positive and negative terminals of a diode, with the positive terminal referred to as the anode and the negative terminal referred to as the cathode. Current can only move from the anode to the cathode direction. Some beginners are prone to the misconception that 'half of a semiconductor is half of a semiconductor', and that only half of a current flows through a surface diode (which is incorrect), so all diodes are semiconductors'. In fact, diodes and semiconductors are completely different things. We can only say that diodes are devices composed of semiconductors. Semiconductors can flow current in any direction.

The conductivity characteristics of diodes

The important characteristic of a diode is its unidirectional conductivity. In a circuit, current can only flow in from the positive pole of a diode and out from the negative pole. Below, we will demonstrate the forward and reverse characteristics of a diode through a simple experiment.

1. Positive characteristics

In electronic circuits, connecting the positive terminal of a diode to the high potential terminal and the negative terminal to the low potential terminal will cause the diode to 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 diodes and about 0.6V for silicon diodes), can the diode conduct directly and positively. After conduction, the voltage across the diode remains basically unchanged (germanium diode is about 0.3V, silicon diode is about 0.7V), which is called the "forward voltage drop" of the diode.

2. Reverse characteristics

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. At this time, there is almost no current flowing through the diode, and the diode is in a cut-off state. This connection method is called reverse bias. When a diode is reverse biased, there will still be a weak reverse current flowing through the diode, which is called 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 function of diodes

1. Rectification: By utilizing the unidirectional conductivity of diodes, alternating alternating alternating current can be converted into pulsed direct current in a single direction

2. Switch: A 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, various logic circuits can be formed.

3. Limiting: After the diode conducts in the forward direction, its forward voltage drop remains basically unchanged (0.7V for silicon diodes and 0.3V for germanium diodes). By utilizing this characteristic, as a limiting element in the circuit, the signal amplitude can be restricted within a certain range.

4. Continuous current: It plays a continuous current role in the inductance of switching power supplies and inductive loads such as relays.

5. Detection: Plays a detection role in radios.

6. Variable capacitance: used in the high-frequency head of a television.

7. Display: Used for VCD DVD、 On calculators and other displays.

8. Voltage regulator: A voltage regulator diode is essentially a surface junction silicon diode that operates in a reverse breakdown state. In the manufacturing process of diodes, make them have low-voltage breakdown characteristics. The reverse breakdown voltage of the voltage regulator diode is constant, and a current limiting resistor is connected in series in the voltage regulator circuit to ensure that the current after the voltage regulator diode breaks down does not exceed the allowable value. Therefore, the breakdown state can be sustained for a long time without damage.

9. Trigger: Trigger diode, also known as bidirectional trigger diode (DIAC), belongs to a three-layer structure and is a symmetrical two terminal semiconductor device. Commonly used to trigger bidirectional thyristors for overvoltage protection and other purposes in circuits.

Main parameters of diodes

The technical indicators used to indicate the performance and applicability of a diode 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 forward current value that a diode is allowed to pass through during long-term continuous operation. Because the current passing through the diode will cause the core to heat up and the temperature to rise. When the temperature exceeds the allowable limit (around 140 for silicon diodes and around 90 for germanium diodes), the core will overheat and be damaged. So, do not exceed the rated forward working current value of the diode during use. For example, the rated forward operating current of the commonly used IN4001-4007 germanium diodes is 1A.

2. Reverse working voltage

When the reverse voltage applied across the diode reaches a certain value, it will break down the diode and lose its unidirectional conductivity. In order to ensure safe use, the reverse working voltage value is specified. For example, the reverse withstand voltage of IN4001 diode is 50V, and the reverse withstand voltage of IN4007 is 1000V.

3. Reverse current

Reverse current refers to the reverse current flowing through a diode at a specified temperature and reverse voltage. The smaller the reverse current, the better the unidirectional conductivity of the diode. It is worth noting that there is a close relationship between reverse current and temperature, with the reverse current doubling for every 10 degrees Celsius increase in temperature. For example, for a 2AP1 germanium diode, if the reverse current is 250uA at 25 o'clock and the temperature rises to 35, the reverse current will increase to 500uA, and so on. At 75 o'clock, its reverse current has reached 8mA, not only losing its unidirectional conductivity, but also causing the diode to overheat and be damaged. For example, the 2CP10 silicon diode has a reverse current of only 5uA at 25 ° C, and when the temperature rises to 75 ° C, the reverse current is only 160uA. Therefore, silicon diodes have better stability than germanium diodes at high temperatures.

In summary, semiconductor diodes are used in almost all electronic circuits and play an important role in many circuits. They are one of the earliest semiconductor devices and have a wide range of applications. The N pole (negative pole) of a low-power diode is mostly marked with a color circle on the outside of the diode. Some diodes also use diode specific symbols to represent the P pole (positive pole) or N pole (negative pole), and some use symbols marked as "P" or "N" to determine the polarity of the diode. The positive and negative terminals of a light-emitting diode can be identified by the length of its pins, with the long pin being positive and the short pin being negative. When using a digital multimeter to measure a diode, connect the red probe to the positive terminal of the diode and the black probe to the negative terminal of the diode. The resistance value measured at this time is the forward conduction resistance value of the diode, which is exactly the opposite of the probe connection method of a pointer multimeter.