Conductive characteristics, functions and main parameters of diodes

Conductive characteristics, functions and main parameters of diodes

A diode has two terminals, an anode and a cathode, and current can only flow in one direction. That is, current can flow from anode to cathode, but not from cathode to anode. The application of this unidirectional characteristic of diodes is usually called "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 the diode, the positive terminal is called the anode, and the negative terminal is called the cathode. Current can only move from the anode to the cathode. Some beginners tend to have such a misconception: "Half" of a semiconductor is a "half" of half; only a "half" of a surface diode flows (this is wrong), and all diodes are semiconductors". In fact, diodes and semiconductors are completely different things. We can only say that a diode is a device composed of semiconductors. Semiconductors can flow current in either direction.

Conductive characteristics of diodes

The important characteristic of diodes is unidirectional conductivity. In the circuit, current can only flow in from the positive terminal of the diode and flow out from the negative terminal. The following is a simple experiment to illustrate the forward and reverse characteristics of the diodes.

1. Positive characteristics

In an electronic circuit, if the anode of a diode is connected to the high potential end and the cathode to the low potential end, the diode will conduct. This connection 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 "threshold voltage", the germanium tube is about 0.2V, and the silicon tube is about 0.6V), the diode can be directly turned on. After the conduction, the voltage across the diode remains basically unchanged (the germanium tube is about 0.3V, and the silicon tube is about 0.7V), which is called the "forward voltage drop" of the diode.

2. Reverse characteristics

In electronic circuits, the anode of a diode is connected to the low potential end, and the cathode is connected to the high potential end. At this time, almost no current flows through the diode, and the diode is in the cut-off state. This connection method is called reverse bias. When the 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 increase sharply, and the diode will lose its unidirectional conductivity. This state is called the breakdown of the diodes.

The role of diodes

1. Rectification: By using the unidirectional conductivity of the diode, the alternating current with alternating directions can be converted into pulsed direct current with a single direction.

2. Switch: The resistance of the diode is very small under the action of forward voltage, and it is in the conduction state, which is equivalent to a switch that is turned on; under the action of reverse voltage, the resistance is very large, and it is in the cut-off state, just like a disconnected switch. switch. Using the switching characteristics of diodes, various logic circuits can be formed.

3. Limiting: After the diode conducts forwardly, its forward voltage drop basically remains unchanged (0.7V for silicon tubes and 0.3V for germanium tubes). Utilizing this characteristic, it can be used as a limiting element in the circuit to limit the signal amplitude within a certain range.

4. Freewheeling: it plays the role of freewheeling in the inductance of the switching power supply and in inductive loads such as relays.

5. Wave detection: It plays the role of wave detection in the radio.

6. Variable capacity: used in the tuner of the TV.