Diodes and their Applications

A diode is a PN structure with unidirectional conductivity.

P-n junction is formed on the basis of impurity of semiconductor, there is a special a qualitative physical structure, is by the p-type and n-type semiconductor through a certain method combined, formed at the interface between the two has the features of physical structure, the PN junction, it is a common physical basis of diode, triode structure, diode unidirectional conductivity is determined by it. In the PN junction two end of the lead and packaging, it becomes a crystal diode, referred to as diode.

There are many types of diodes commonly used, the main applications are: rectifier circuit, clamp circuit, amplitude limiting circuit and component protection circuit.

The character symbol of pole tube is denoted by VD in international standard and D in China.

One, diode voltameter

The volt-ampere characteristic of the diode refers to the relationship between the current passing through the diode and the applied voltage when the voltage is applied at both ends of the diode.

The relationship is represented by a curve, which is called a Voltamine-characteristic curve

(1) When the diode applied forward is very small, the forward current is very weak. Only when the forward voltage exceeds a certain value, the forward current increases significantly, and this voltage is called the conduction voltage or threshold voltage, denoted by UTH. At room temperature, UTH of silicon tube is 0.5 ~ 0.6 V. UTH of germanium tube = 0.1-0.2V. After the diode conduction, with the increase of voltage, the current will increase rapidly;

Diodes and their Applications(2) When reverse voltage is applied to both ends of the diode, only a very weak reverse current passes through. Under a certain temperature, when the reverse voltage value changes, the reverse current IS basically unchanged, so it IS often called the reverse saturation current, denoted as IS. The Is of low-power silicon tubes Is generally less than 0.1 μA, while that of germanium tubes Is about A few microamps.

(3) When the reverse voltage reaches a certain value, about -50V as shown in FIG. 2.4, the diode enters the reverse breakdown state. At this point, as long as the uD changes a little, the current value will quickly increase. This state is called reverse breakdown state, and voltage regulator diodes are used to make use of this characteristic.

Two, the main parameters of the diode

1. Maximum rectifier current IF: maximum current allowed to pass through the diode when it is working continuously for a long time

2. Highest reverse working voltage URM: The maximum reverse working voltage allowed when the diode is working

3. Highest operating frequency fM: fM is the upper cut-off frequency of diode operation. When this value is exceeded, the diode will not reflect the unidirectional conductivity well because of the junction capacitance.

Three, diode application examples

1. Rectification: rectification is to change alternating current into pulsating direct current. The rectifier can be realized by the unidirectional conductivity of the diode.

2. Amplitude limiting: the voltage drop after the diode conduction is small and basically unchanged, can constitute the amplitude limiting circuit, so that the output voltage amplitude is limited within a voltage value.

3. Protection: In electronic circuits, diodes are often used to protect other components from excessive voltage damage.

Diode limiting circuit and waveform Diode limiting circuit and waveform

Because the diode exists conduction voltage drop, so the input sine wave signal, the output signal is the waveform of the top.

Protective effect of diode

When, the switch is closed, the battery to discharge, due to the diode with unidirectional conductivity, electricity flows through the switch S directly to the resistance R and inductance L, and then flow back to the battery cathode, when the switch is off, when because the inductor has the effect of energy storage, the circuit is disconnected, then so at the moment, the current will be through the diode to resistance R and inductance L, and so on, after know power consumption, To protect the circuit.

Due to the unidirectional conductivity of the diode, the positive half-axis signal can pass through the diode, while the negative half-axis signal cannot pass through the diode, resulting in the output waveform is not a complete waveform, but only the pulse of the positive half-axis signal after rectification.

During rectification, the maximum rectifier current of diode parameters should meet :IF≥IL = 0.45U2/RL

Four, light emitting diode (LED) and light emitting digital tube:

Leds are made of special semiconductor materials such as gallium arsenide. Different materials give off different colors of light.

The characteristics of light-emitting diode are: the working voltage is larger than the ordinary diode, its working voltage is different with the different materials, generally 1.7V ~ 2.4V. Ordinary green, yellow, red and orange leds operate at a voltage of about 2V; White leds usually operate at voltages higher than 2.4V; Blue leds generally operate at voltages higher than 3.3V. The working current of leds is generally in the range of 2mA ~ 25mA.

The above content is related to the application of diode knowledge, in the actual design occasion, there will be more applications and diode types, the need for more practical development work to understand in detail.

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