Diode Is an element with different conductivity. This property is used in various electrical and electronic circuits. On its basis, devices are created that have applications in various fields.
Content
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Vacuum diodes
- Principle of operation
- Volt-ampere characteristic (VAC)
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Semiconductor diodes
- Volt-ampere characteristics
- Limit values of parameters
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Types of semiconductor diodes
- Rectifiers and their properties
- Semiconductor Zener Diodes
- How the detectors work
- Features of LEDs
Types of diodes: vacuum and semiconductor. The latter type is currently used in the overwhelming majority of cases. It will never be superfluous to know about how a diode works, what it is needed for, how it is indicated in the diagram, what types of diodes exist, the use of different types of diodes.
Vacuum diodes
Devices of this type are made in the form of electronic tubes. The lamp looks like a glass balloon with two electrodes inside. One is the anode, the other is the cathode. They are in a vacuum. Structurally, the anode is made in the form of a thin-walled cylinder. The cathode is located inside. It is usually cylindrical in shape. An insulated filament is laid inside the cathode. All elements have leads that are connected to the pins (legs) of the lamp. The lamp legs are brought out.
Principle of operation
When an electric current passes along a spiral, it heats up and heats up the cathode, inside which it is located. From the surface of the heated cathode, the electrons that left it, without an additional accelerating field, accumulate in the immediate vicinity of it. Some of them are then returned back to the cathode.
When a positive voltage is applied to the anode, the electrons emitted from the cathode rush to it, creating an anode current of electrons.
The cathode has an electron emission limit. When this limit is reached, the anode current stabilizes. If a small negative voltage is applied to the anode with respect to the cathode, then the electrons will stop moving.
The material of the cathode from which it is made has a high degree of emission.
Volt-ampere characteristic (VAC)
The I - V characteristic of diodes of this type graphically shows the dependence of the anode current on the forward voltage applied to the cathode and anode terminals. It consists of three sections:
- Slow non-linear current rise;
- Working part of the characteristic;
- Anode current saturation region.
The nonlinear section begins after the cutoff region of the anode current. Its nonlinearity is associated with a small positive potential of the cathode, which was left by electrons when it was heated by the filament.
The active area defines an almost vertical line. It characterizes the dependence of the anode current on the increasing voltage.
The saturation area is a line of constant anode current with an increasing voltage between the lamp electrodes. A vacuum tube in this area can be compared to a conductor of electric current. The emission from the cathode has reached its highest value.
Semiconductor diodes
The property of the p - n junction to pass an electric current in one direction has found application in the creation of devices of this type. Direct inclusion is a supply of negative potential to the n -area of the transition, in relation to the p -area, the potential of which is positive. When turned on, the device is in an open state. When the polarity of the applied voltage changes, it will be in the locked state, and the current does not pass through it.
The classification of diodes can be carried out according to their purpose, according to the peculiarities of manufacture, according to the type of material used in its manufacture.
Basically, for the manufacture of semiconductor devices, silicon or germanium plates are used, which have an n-type electrical conductivity. They contain an excess of negatively charged electrons.
Using different manufacturing technologies, it is possible to obtain point or plate diodes at the output.
In the manufacture of point devices, a pointed conductor (needle) is welded to the n-type plate. A certain impurity is applied to its surface. For germanium plates, the needle contains indium; for silicon plates, the needle is coated with aluminum. In both cases, a p - n junction region is created. Its shape resembles a hemisphere (point).
For planar devices, the diffusion or fusion method is used. The area of transitions obtained by this method varies widely. The purpose of the product depends on its value in the future. Wires are soldered to the areas of the p - n junction, which are used in the form of leads from the body of the finished product when installing various electrical circuits.
In the diagrams, semiconductor diodes are indicated in the form of an equilateral triangle, to the upper corner of which a vertical line is attached parallel to its base. The pin of the bar is called the cathode, and the pin of the base of the triangle is the anode.
Direct is called such an inclusion in which the positive pole of the power source is connected to the anode. When turned back on, the "plus" of the source is connected to the cathode.
Volt-ampere characteristics
The I - V characteristic determines the dependence of the current flowing through the semiconductor element on the magnitude and polarity of the voltage that is applied to its terminals.
In the area of forward voltages, three areas are distinguished: a small forward current and a forward working current through the diode. The transition from one region to another occurs when the direct voltage reaches the conductivity threshold. This value is in the order of 0.3 volts for germanium diodes and 0.7 volts for silicon based diodes.
When a reverse voltage is applied to the terminals of a diode, the current through it is very small and is called reverse current or leakage current. This dependence is observed up to a certain value of the magnitude of the reverse voltage. This is called the breakdown voltage. When it is exceeded, the reverse current grows like an avalanche.
Limit values of parameters
For semiconductor diodes, there are values for their parameters that cannot be exceeded. These include:
- Maximum forward current;
- Maximum reverse breakdown voltage;
- Maximum power dissipation.
A semiconductor element can withstand a limited amount of direct current through it. When it is exceeded, the p-n junction overheats and fails. Plane power devices have the greatest margin for this parameter. The magnitude of the forward current through them can reach tens of amperes.
Exceeding the maximum value of the breakdown voltage can turn a diode with unidirectional properties into a normal conductor of electric current. Breakdown can be irreversible and varies widely depending on the specific instrument used.
Power Is a quantity that directly depends on the current and voltage, which is applied at the same time to the terminals of the diode. As well as exceeding the maximum forward current, exceeding the limiting power dissipation leads to irreversible consequences. The diode simply burns out and ceases to fulfill its purpose. To prevent such a situation, power devices install devices on radiators that remove (dissipate) excess heat into the environment.
Types of semiconductor diodes
The property of a diode to pass current in the forward direction and not to pass it in the opposite direction has found application in electrical and radio engineering. Special types of diodes have also been developed for performing a narrow range of tasks.
Rectifiers and their properties
Their application is based on the rectifier properties of these devices. They are used to obtain a constant voltage by rectifying an input alternating signal.
A single rectifier diode allows a pulsating voltage of positive polarity to be obtained at its output. Using a combination of these, you can get the output voltage waveform that resembles a wave. When using additional elements in rectifier circuits, such as high-capacity electrolytic capacitors and inductors with electromagnetic cores (chokes), a constant voltage can be obtained at the output of the device, reminiscent of the voltage of a galvanic battery, which is so necessary for the operation of most equipment consumer.
Semiconductor Zener Diodes
These diodes have a high-slope I-V characteristic with a reverse branch. That is, by applying a voltage to the terminals of the zener diode, the polarity of which is reversed, you can use limiting resistors to enter it into the mode of controlled breakdown avalanches. The voltage at the point of the avalanche breakdown has a constant value with a significant change in the current through the zener diode, the value of which is limited depending on the device used in the circuit. In this way, the effect of stabilizing the output voltage at the desired level is obtained.
Technological operations in the manufacture of zener diodes achieve various values of the breakdown voltage (stabilization voltage). The range of these voltages is (3-15) volts. The specific value depends on the selected device from a large family of zener diodes.
How the detectors work
Diodes made using point technology are used to detect high-frequency signals. The task of the detector is to limit one half of the modulated signal. This allows, subsequently, with the help of a high-pass filter, to leave only the modulating signal at the output of the device. It contains low frequency audio information. This method is used in radio receivers that receive an amplitude modulated signal.
Features of LEDs
These diodes are characterized by the fact that when a forward current flows through them, the crystal emits a stream of photons, which are a light source. Depending on the type of crystal used in the LED, the light spectrum can be in the range visible to the human eye or in the invisible range. Invisible light is infrared or ultraviolet light.
When choosing these elements, it is necessary to imagine the goal to be achieved. The main characteristics of LEDs are:
- Power consumption;
- Rated voltage;
- Consumption current.
The current consumption of the LED used for indication in devices of widespread use is no more than 20 mA. At this current, the LED luminescence is optimal. The beginning of the glow begins at a current exceeding 3 mA.
The nominal voltage is determined by the internal resistance of the junction, which is variable. As the current through the LED increases, the resistance gradually decreases. The voltage of the power source used to power the LED must be applied not less than the voltage specified in the passport for it.
The power consumption is a value that depends on the current consumption and the rated voltage. It increases with an increase in the values that determine it. It should be noted that powerful light diodes can contain 2 or even 4 crystals.
LEDs have undeniable advantages over other lighting devices. They can be enumerated for a long time. The main ones are:
- High efficiency;
- Great durability;
- High level of safety due to low supply voltages.
The disadvantage of their operation is the need for an additional stabilized DC power supply, and this increases the cost.
