Thyristors are a type of semiconductor device. They are designed to regulate and switch high currents. A thyristor allows you to switch an electrical circuit when a control signal is applied to it. This makes it look like a transistor.
Typically, a thyristor has three terminals, one of which is control, and the other two form a path for current flow. As we know, the transistor opens in proportion to the magnitude of the control current. The larger it is, the more the transistor opens, and vice versa. But with a thyristor everything works differently. It opens completely, abruptly. And what’s most interesting is that it does not close even in the absence of a control signal.
Operating principle
Let's consider the operation of a thyristor according to the following simple circuit.
A light bulb or LED is connected to the anode of the thyristor, and the positive terminal of the power source is connected to it through switch K2. The thyristor cathode is connected to the power supply negative. After turning on the circuit, voltage is applied to the thyristor, but the LED does not light up.
If you press the K1 button, current flows through the resistor to the control electrode, and the LED begins to light. Often on diagrams it is designated by the letter “G”, which means gate, or in Russian shutter (control terminal).
The resistor limits the control pin current. The minimum operating current of this thyristor under consideration is 1 mA, and the maximum permissible current is 15 mA. Taking this into account, a resistor with a resistance of 1 kOhm was selected in our circuit.
If you press the K1 button again, this will not affect the thyristor and nothing will happen. To switch the thyristor to the closed state, you need to turn off the power using switch K2. If power is applied again, the thyristor will return to its original state.
This semiconductor device is essentially a latching electronic key. The transition to the closed state also occurs when the supply voltage at the anode decreases to a certain minimum, approximately 0.7 volts.
Device Features
The on state is fixed thanks to the feature internal structure thyristor. An example diagram looks like this:
It is usually represented as two transistors of different structures connected to each other. You can experimentally check how transistors connected according to this circuit work. However, there are differences in the current-voltage characteristics. And you also need to take into account that the devices were initially designed to withstand high currents and voltages. On the body of most of these devices there is a metal outlet on which a radiator can be attached to dissipate thermal energy.
Thyristors are made in various cases. Low-power devices do not have heat dissipation. Common domestic thyristors look like this. They have a massive metal body and can withstand high currents.
Basic parameters of thyristors
- Maximum permissible forward current .
- This is the maximum value of the open thyristor current. For powerful devices it reaches hundreds of amperes. .
- Maximum permissible reverse current Forward voltage
- . This is the voltage drop at maximum current.
- Reverse voltage .
- This is the maximum permissible voltage on the thyristor in the closed state at which the thyristor can operate without affecting its performance. Turn-on voltage
- . .
- This is the minimum voltage applied to the anode. Here we mean the minimum voltage at which the thyristor can operate at all. .
Minimum control electrode current
. It is necessary to turn on the thyristor.
Maximum permissible control current
Maximum permissible power dissipation
- Dynamic parameter
- Transition time of the thyristor from the closed state to the open state
when a signal arrives.
- Types of thyristors
- According to the control method they are divided into:
Diode thyristors, or otherwise dinistors. They are opened by a high voltage pulse that is applied to the cathode and anode.
- Triode thyristors, or thyristors. They are opened by the electrode control current.
- Triode thyristors, in turn, are divided:
Cathode control - the voltage forming the control current is supplied to the control electrode and the cathode.
- Anode control – control voltage is applied to the electrode and anode.
- The thyristor is locked:
- With a non-standardized reverse voltage value - manufacturers do not determine the value of this value. Such devices are used in places where reverse voltage is excluded.
- Triac – passes currents in two directions.
When using triacs, you need to know that they operate conditionally symmetrically. The main part of triacs opens when a positive voltage is applied to the control electrode compared to the cathode, and the anode can have any polarity. But if a negative voltage comes to the anode, and a positive voltage comes to the control electrode, then the triacs do not open and may fail.
By speed divided by unlocking (on) time and locking (off) time.
Separation of thyristors by power
When the thyristor operates in switch mode, the highest power of the switched load is determined by the voltage on the thyristor in open mode at the highest current and highest power dissipation.
The effective current on the load should not be higher than the highest power dissipation divided by the open voltage.
Simple thyristor based alarm
Based on a thyristor, you can make a simple alarm that will respond to light, producing a sound using a piezo emitter. The control terminal of the thyristor is supplied with current through a photoresistor and a tuning resistor. Light hitting the photoresistor reduces its resistance. And the control output of the thyristor begins to receive an unlocking current sufficient to open it. After this, the beeper turns on.
The trimming resistor is designed to adjust the sensitivity of the device, that is, the response threshold when irradiated with light. The most interesting thing is that even in the absence of light, the thyristor continues to remain open, and the signaling does not stop.
If you install a light beam opposite the photosensitive element so that it shines slightly below the window, you will get a simple smoke sensor. Smoke getting between the light source and the light receiver will scatter the light, which will trigger the alarm. This device requires a housing so that the light receiver does not receive light from the sun or artificial light sources.
You can open the thyristor in another way. To do this, it is enough to briefly apply a small voltage between the control terminal and the cathode.
Thyristor power regulator
Now let's look at using a thyristor for its intended purpose. Let's consider the circuit of a simple thyristor power regulator that will operate from an alternating current network of 220 volts. The circuit is simple and contains only five parts.
- Semiconductor diode VD.
- Variable resistor R1.
- Fixed resistor R2.
- Capacitor C.
- Thyristor VS.
Their recommended nominal values are shown in the diagram. As a diode, you can use KD209, thyristor KU103V or more powerful. It is advisable to use resistors with a power of at least 2 watts, an electrolytic capacitor with a voltage of at least 50 volts.
This circuit regulates only one half-cycle of the mains voltage. If we imagine that we have removed all elements from the circuit except the diode, then it will pass only half a wave of alternating current, and only half the power will flow to the load, for example, a soldering iron or an incandescent lamp.
The thyristor allows you to pass additional, relatively speaking, pieces of the half-cycle cut off by the diode. When changing the position of the variable resistor R1, the output voltage will change.
The control terminal of the thyristor is connected to the positive terminal of the capacitor. When the voltage on the capacitor increases to the turn-on voltage of the thyristor, it opens and passes a certain part of the positive half-cycle. The variable resistor will determine the charging rate of the capacitor. And the faster it charges, the sooner the thyristor will open, and will have time to skip part of the positive half-cycle before the polarity changes.
The negative half-wave does not enter the capacitor, and the voltage across it is of the same polarity, so it is not scary that it has polarity. The circuit allows you to change the power from 50 to 100%. This is just right for a soldering iron.
A thyristor passes current in one direction from the anode to the cathode. But there are varieties that pass current in both directions. They are called symmetrical thyristors or triacs. They are used to control loads in AC circuits. Exists a large number of power regulator circuits based on them.
A thyristor is an electronic power partially controlled switch. This device, with the help of a control signal, can only be in a conducting state, that is, be turned on. In order to turn it off, it is necessary to carry out special measures that ensure that the forward current drops to zero. The operating principle of a thyristor is one-way conduction; when closed, it can withstand not only forward but also reverse voltage.
Thyristor properties
According to their qualities, thyristors belong to semiconductor devices. Their semiconductor wafer contains adjacent layers that have various types conductivity. Thus, each thyristor is a device having a four-layer p-p-p-p structure.
The positive pole of the voltage source is connected to the extreme region of the p-structure. That's why, this area called the anode. The opposite region of the n-type, where the negative pole is connected, is called the cathode. Output from the internal region is carried out using a p-control electrode.
The classic thyristor model consists of two having different degrees of conductivity. In accordance with this circuit, the base and collector of both transistors are connected. As a result of this connection, the base of each transistor is powered using the collector current of the other transistor. Thus, a circuit with positive feedback is obtained.
If there is no current in the control electrode, then the transistors are in the closed position. No current flows through the load and the thyristor remains closed. When a current is supplied above a certain level, the positive Feedback. The process becomes an avalanche, after which both transistors open. Ultimately, after the thyristor opens, its stable state occurs, even if the current supply is interrupted.
Thyristor operation at constant current
Considering an electronic thyristor whose operating principle is based on the one-way flow of current, it should be noted that it operates at constant current.
A conventional thyristor is turned on by applying a current pulse to the control circuit. This supply is carried out from the side of positive polarity, opposite to the cathode.
During switching on, the duration of the transient process is determined by the nature of the load, the amplitude and speed with which the control current pulse increases. In addition, this process depends on the temperature of the internal structure of the thyristor, the load current and the applied voltage. In the circuit where the thyristor is installed, there should not be an unacceptable rate of voltage increase, which could lead to its spontaneous activation.
Reverse locking mode
Rice. 3. Thyristor reverse blocking mode
Two main factors limit the regime of reverse breakdown and forward breakdown:
- Puncture of the depleted area.
In the reverse blocking mode, a voltage is applied to the anode of the device, negative with respect to the cathode; junctions J1 and J3 are reverse biased, and junction J2 is forward biased (see Fig. 3). In this case, most of the applied voltage drops at one of the junctions J1 or J3 (depending on the degree of doping of the various regions). Let this be transition J1. Depending on the thickness W n1 of the n1 layer, the breakdown is caused by avalanche multiplication (the thickness of the depletion region during breakdown is less than W n1) or puncture (the depletion layer spreads over the entire n1 region, and the junctions J1 and J2 are closed).
Direct locking mode
With direct blocking, the voltage at the anode is positive with respect to the cathode and only junction J2 is reverse biased. Junctions J1 and J3 are forward biased. Most of the applied voltage drops at junction J2. Through junctions J1 and J3, minority carriers are injected into the regions adjacent to junction J2, which reduce the resistance of junction J2, increase the current through it and reduce the voltage drop across it. As the forward voltage increases, the current through the thyristor initially increases slowly, which corresponds to the 0-1 section on the current-voltage characteristic. In this mode, the thyristor can be considered locked, since the resistance of junction J2 is still very high. As the voltage across the thyristor increases, the proportion of voltage dropped across J2 decreases, and the voltages across J1 and J3 increase faster, which causes further increase current through the thyristor and increasing the injection of minority carriers into the J2 region. At a certain voltage value (of the order of tens or hundreds of volts), it is called the switching voltage V BF(point 1 on the current-voltage characteristic), the process acquires an avalanche-like character, the thyristor goes into a state with high conductivity (turns on), and a current is established in it, determined by the source voltage and the resistance of the external circuit.
Two-transistor model
To explain the characteristics of the device in direct blocking mode, a two-transistor model is used. The thyristor can be considered as pnp connection transistor with an n-p-n transistor, with the collector of each of them connected to the base of the other, as shown in Fig. 4 for triode thyristor. The central junction acts as a collector of holes injected by junction J1 and electrons injected by junction J3. Relationship between emitter currents I E, collector I C and bases I B and the static current gain α 1 p-n-p transistor is also shown in Fig. 4, where I Co is the reverse saturation current of the collector-base junction.
Rice. 4. Two-transistor model of a triode thyristor, connection of transistors and current ratio in a pnp transistor.
Similar relations can be obtained for npn transistor when the direction of currents changes to the opposite. From Fig. 4 it follows that the collector current of the n-p-n transistor is at the same time the base current of the p-n-p transistor. Similarly collector p-n-p current transistor and control current I g flow into the base of the n-p-n transistor. As a result, when the total gain in the closed loop exceeds 1, a regenerative process becomes possible.
Current p-n-p bases transistor is equal I B1= (1 - α 1) I A - I Co1. This current also flows through the collector of the npn transistor. The collector current of an n-p-n transistor with gain α 2 is equal to I C2= α 2 I K + ICo2.
Equating I B1 And I C2, we get (1 - α 1) I A - I Co1= α 2 I K + ICo2. Because I K = I A + I g, That
Rice. 5. Energy band diagram in forward bias mode: equilibrium state, forward blocking mode and forward conduction mode.
This equation describes the static characteristics of the device in the voltage range up to breakdown. After breakdown, the device operates as a p-i-n diode. Note that all terms in the numerator of the right side of the equation are small, therefore, while the term α 1 + α 2< 1, ток I A small (Coefficients α1 and α2 themselves depend on I A and usually grow with increasing current) If α1 + α2 = 1, then the denominator of the fraction goes to zero and a direct breakdown occurs (or the thyristor is turned on). It should be noted that if the polarity of the voltage between the anode and cathode is reversed, then junctions J1 and J3 will be reverse biased, and J2 forward biased. Under such conditions, breakdown does not occur, since only the central junction acts as an emitter and the regenerative process becomes impossible.
The width of the depletion layers and energy band diagrams at equilibrium, in the direct blocking and direct conduction modes are shown in Fig. 5. In equilibrium, the depletion region of each transition and the contact potential are determined by the impurity distribution profile. When a positive voltage is applied to the anode, junction J2 tends to be reverse biased, and junctions J1 and J3 tend to be forward biased. The voltage drop between the anode and cathode is equal to the algebraic sum of the voltage drops across the transitions: V AK = V 1 + V 2 + V 3. As the voltage increases, the current through the device increases and, therefore, α1 and α2 increase. Due to the regenerative nature of these processes, the device will eventually go into an open state. Once the thyristor is turned on, the current flowing through it must be limited by the external load resistance, otherwise the thyristor will fail if the voltage is high enough. In the on state, junction J2 is biased in the forward direction (Fig. 5, c), and the voltage drop V AK = (V 1 - | V 2| + V 3) is approximately equal to the sum of the voltage across one forward-biased junction and the voltage across the saturated transistor.
Direct conduction mode
When the thyristor is in the on state, all three junctions are forward biased. Holes are injected from region p1, and electrons are injected from region n2, and the n1-p2-n2 structure behaves similarly to a saturated transistor with the diode contact removed to region n1. Therefore, the device as a whole is similar to a p-i-n (p + -i-n +) diode...
Classification of thyristors
- diode thyristor (additional name "dinistor") - a thyristor with two terminals
- Diode thyristor, non-reverse conducting
- diode thyristor, conducting in the opposite direction
- Diode symmetrical thyristor (additional name "diac")
- triode thyristor (additional name "thyristor") - a thyristor with three terminals
- triode thyristor, not conducting in the opposite direction (additional name "thyristor")
- triode thyristor, conducting in the opposite direction (additional name "thyristor-diode")
- triode symmetrical thyristor (additional name "triac", informal name "triac")
- triode thyristor asymmetric
- switchable thyristor (additional name "triode switchable thyristor")
The difference between a dinistor and a trinistor
There are no fundamental differences between a dinistor and a trinistor, however, if the opening of a dinistor occurs when a certain voltage is reached between the anode and cathode terminals, depending on the type of a given dinistor, then in a trinistor the opening voltage can be specially reduced by applying a current pulse of a certain duration and magnitude to its control electrode with a positive potential difference between the anode and cathode, and the trinistor design differs only in the presence of a control electrode. SCRs are the most common devices from the “thyristor” family.
The difference between a triode thyristor and a turn-off thyristor
Switching to the closed state of conventional thyristors is carried out either by reducing the current through the thyristor to the value Ih, or by changing the voltage polarity between the cathode and anode.
Switchable thyristors, unlike conventional thyristors, under the influence of the control electrode current can transition from a closed state to an open state, and vice versa. To close a turn-off thyristor, it is necessary to pass a current of opposite polarity through the control electrode than the polarity that caused it to open.
Triac
A triac (symmetrical thyristor) is a semiconductor device, its structure is analogous to the back-to-back connection of two thyristors. Capable of passing electric current in both directions.
Characteristics of thyristors
Modern thyristors are manufactured for currents from 1 mA to 10 kA; for voltages from several V to several kV; the rate of increase in the forward current in them reaches 10 9 A/s, voltage - 10 9 V/s, the on time ranges from several tenths to several tens of microseconds, the off time ranges from several units to several hundred microseconds; Efficiency reaches 99%.
Application
- Controlled rectifiers
- Converters (inverters)
- Power regulators (dimmers)
see also
- CDI (Capacitor Discharge Ignition)
Notes
Literature
- GOST 15133-77.
- Kublanovsky. Ya. S. Thyristor devices. - 2nd ed., revised. and additional - M.: Radio and Communications, 1987. - 112 p.: ill. - (Mass Radio Library. Issue 1104).
Links
- Thyristors: principle of operation, designs, types and methods of inclusion
- Control of thyristors and triacs via a microcontroller or digital circuit
- Converter devices in power supply systems
- Rogachev K.D. Modern power switched thyristors.
- Domestic Analogues of Imported Thyristors
- Directories on thyristors and analogues, Replacing thyristors, replacing diodes. Zener diodes
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Thyristors are widely used in semiconductor devices and converters. Various power supplies, frequency converters, regulators, exciters for synchronous motors and many other devices were built on thyristors, and in Lately they are being replaced by transistor converters. The main task for a thyristor is to turn on the load at the moment the control signal is supplied. In this article we will look at how to control thyristors and triacs.
Definition
A thyristor (thyristor) is a semiconductor semi-controlled switch. Semi-controlled means that you can only turn on the thyristor; it turns off only when the current in the circuit is interrupted or if reverse voltage is applied to it.
It, like a diode, conducts current in only one direction. That is, to be included in an alternating current circuit to control two half-waves, you need two thyristors, one for each, although not always. A thyristor consists of 4 semiconductor regions (p-n-p-n).
Another similar device is called a bidirectional thyristor. Its main difference is that it can conduct current in both directions. In fact, it consists of two thyristors connected in parallel towards each other.
Main characteristics
Like any other electronic components, thyristors have a number of characteristics:
Voltage drop at maximum anode current (VT or Uoc).
Direct voltage in closed state (VD(RM) or Uзс).
Reverse voltage (VR(PM) or Urev).
Direct current (IT or Ipr) is the maximum current in the open state.
The maximum forward current capacity (ITSM) is the maximum peak on-state current.
Reverse current (IR) is the current at a certain reverse voltage.
Direct current in the closed state at a certain forward voltage (ID or Isc).
Constant unlocking control voltage (VGT or UУ).
Control current (IGT).
Maximum control current of the IGM electrode.
Maximum permissible power dissipation at the control electrode (PG or PU)
Principle of operation
When voltage is applied to the thyristor, it does not conduct current. There are two ways to turn it on - apply a voltage between the anode and cathode sufficient to open it, then its operation will be no different from a dinistor.
Another way is to apply a short pulse to the control electrode. The opening current of the thyristor lies in the range of 70-160 mA, although in practice this value, as well as the voltage that needs to be applied to the thyristor, depends on specific model and the type of semiconductor device and even the conditions in which it operates, such as ambient temperature.
In addition to the control current, there is such a parameter as the holding current - this is the minimum anode current to keep the thyristor in the open state.
After opening the thyristor, the control signal can be turned off; the thyristor will be open as long as direct current flows through it and voltage is applied. That is, in an alternating circuit the thyristor will be open during that half-wave the voltage of which biases the thyristor in the forward direction. When the voltage goes to zero, the current will also decrease. When the current in the circuit drops below the holding current of the thyristor, it will close (turn off).
The polarity of the control voltage must match the polarity of the voltage between the anode and cathode, which you observe in the oscillograms above.
The control of a triac is similar, although it has some features. To control a triac in an alternating current circuit, two control voltage pulses are needed - for each half-wave of a sine wave, respectively.
After applying a control pulse in the first half-wave (conditionally positive) of the sinusoidal voltage, the current through the triac will flow until the beginning of the second half-wave, after which it will close, like a regular thyristor. After this, you need to apply another control pulse to open the triac on the negative half-wave. This is clearly illustrated in the following waveforms.
The polarity of the control voltage must match the polarity of the applied voltage between the anode and cathode. Because of this, problems arise when controlling triacs using digital logic circuits or from microcontroller outputs. But this can easily be solved by installing a triac driver, which we will talk about later.
Common thyristor or triac control circuits
The most common circuit is a triac or thyristor regulator.
Here the thyristor opens after there is enough value on the capacitor to open it. The opening moment is adjusted using a potentiometer or a variable resistor. The greater its resistance, the slower the capacitor charges. Resistor R2 limits the current through the control electrode.
This circuit regulates both half cycles, meaning you get full adjustment power from almost 0% to almost 100%. This was achieved by installing a regulator, thus regulating one of the half-waves.
A simplified circuit is shown below, here only half of the period is regulated, the second half-wave passes without change through the diode VD1. The operating principle is similar.
A triac regulator without a diode bridge allows you to control two half-waves.
According to the principle of operation, it is almost similar to the previous ones, but it is built on a triac with its help, both half-waves are regulated. The differences are that here the control pulse is supplied using a bidirectional DB3 dinistor after the capacitor is charged to the desired voltage, usually 28-36 Volts. The charging speed is also controlled by a variable resistor or potentiometer. This scheme is implemented in most.
Interesting:
Such voltage regulation circuits are called SIFU - pulsed phase control system.
The figure above shows an option for controlling a triac using a microcontroller, using the example. The triac driver consists of an optosimistor and an LED. Since an optosimistor is installed in the driver output circuit, a voltage of the required polarity is always supplied to the control electrode, but there are some nuances here.
The fact is that to regulate the voltage using a triac or thyristor, you need to apply a control signal at a certain point in time, so that the phase cut occurs to the desired value. If you shoot control pulses at random, the circuit will of course work, but adjustments will not be achieved, so you need to determine the moment when the half-wave crosses zero.
Since the polarity of the half-wave at the current moment in time does not matter to us, it is enough to simply track the moment of transition through zero. Such a node in the circuit is called a zero detector or null detector, and in English-language sources “zero crossing detector circuit” or ZCD. A version of such a circuit with a zero-crossing detector using a transistor optocoupler looks like this:
There are many optodrivers for controlling triacs, the typical ones are the MOC304x, MOC305x, MOC306X line, produced by Motorola and others. Moreover, these drivers provide galvanic isolation, which will protect your microcontroller in the event of a breakdown of the semiconductor key, which is quite possible and probable. This will also increase the safety of working with control circuits by completely dividing the circuit into “power” and “operational”.
Conclusion
We talked about basic information about thyristors and triacs, as well as their control in circuits with “changes”. It is worth noting that we did not touch upon the topic of turn-off thyristors; if you are interested in this issue, write comments and we will consider them in more detail. Also, the nuances of using and controlling thyristors in power inductive circuits were not considered. To control the "constant" it is better to use transistors, since in this case you decide when the key opens and when it closes, obeying the control signal...
Thyristors are solid state electronic devices with high switching speed. These devices can be used to control all kinds of low-power electronic components. However, along with low-power electronics, power equipment is successfully controlled using thyristors. Let's consider the classic circuits for connecting a thyristor to control fairly high loads, for example, electric lamps, electric motors, electric heaters, etc.
Switching the semiconductor into the open state is possible by applying a small inrush current pulse to the control electrode U.
When the thyristor passes load current in the forward direction, the anode electrode A is positive with respect to the cathode electrode K, from the point of view of regenerative clamping.
Typically, the trigger pulse for electrode Y should have a duration of several microseconds. However, the longer the pulse, the faster the internal avalanche breakdown occurs. The opening time of the transition also increases. But the maximum gate current must not be exceeded.
Scheme 1: KN1, KN2 - push buttons without fixation; L1 - load in the form of an incandescent lamp 100 W; R1, R2 - constant resistors 470 Ohm and 1 kOhm This simple circuit on/off switch is used to control an incandescent lamp. Meanwhile, the circuit can be used as a switch for an electric motor, a heater, or any other load designed to be powered by constant voltage.
Here the thyristor has a forward biased transition state and is switched into mode short circuit normally open button KH1.
This button connects the control electrode U to the power source through resistor R1. If the value of R1 is set too high relative to the supply voltage, the device will not work.
One has only to press the KH1 button, the thyristor switches to the direct conductor state and remains in this state regardless of the further position of the KH1 button. In this case, the current component of the load shows a greater value than the clamping current of the thyristor.
Advantages and disadvantages of using a thyristor
One of the main advantages of using these semiconductors as a switch is the very high current gain. A thyristor is a device that is actually controlled by current.
The cathode resistor R2 is usually included to reduce the sensitivity of the electrode Y and increase the potential of the voltage-current ratio, which prevents false operation of the device.
When the thyristor latches and remains in the “on” state, this state can only be reset by interrupting the power supply or reducing the anode current to the lower holding value.
Therefore, it is logical to use the normally closed button KH2 to open the circuit, reducing the current flowing through the thyristor to zero, causing the device to go into the “off” state.
However, the scheme also has a drawback. The mechanical normally closed switch KH2 must be powerful enough to match the power of the entire circuit.
In principle, one could simply replace the semiconductor with a high-power mechanical switch. One way to overcome the power problem is to connect a commutator in parallel with the thyristor.
Scheme 2: KN1, KN2 - push buttons without fixation; L1 - incandescent lamp 100 W; R1, R2 - constant resistors 470 Ohm and 1 kOhm Finalization of the circuit - switching on a normally open low-power switch in parallel transition A-K, gives the following effect:
- activation of KH2 creates a “short circuit” between electrodes A and K,
- The clamping current decreases to a minimum value,
- The device goes into the “off” state.
Thyristor in AC circuit
When connected to an AC source, the thyristor works slightly differently. This is due to the periodic change in polarity of the alternating voltage.
Therefore, application in power supply circuits alternating voltage will automatically result in a reverse biased transition state. That is, during half of each cycle the device will be in the “off” state.
For the variant with alternating voltage, the thyristor trigger circuit is similar to the circuit with constant voltage supply. The difference is insignificant - the absence of an additional switch KH2 and the addition of diode D1.
Thanks to diode D1, reverse bias with respect to the control electrode U is prevented.
During the positive half-cycle of the sinusoidal waveform, the device is shifted forward, but when switch KN1 is turned off, zero gate current is supplied to the thyristor and the device remains “off”.
In the negative half-cycle, the device receives a reverse bias and will also remain “off”, regardless of the state of the switch KH1.
Scheme 3: KN1 - latching switch; D1 - any diode for high voltage; R1, R2 - constant resistors 180 Ohm and 1 kOhm, L1 - incandescent lamp 100 W If switch KH1 is closed, at the beginning of each positive half-cycle the semiconductor will remain completely “off”.
But as a result of achieving a sufficient positive trigger voltage (increasing control current) on electrode Y, the thyristor will switch to the “on” state.
The hold state latching remains stable during the positive half-cycle and is automatically reset when the positive half-cycle ends. Obviously, because here the anode current drops below the current value.
During the next negative half-cycle, the device will be completely "off" until the next positive half-cycle. Then the process is repeated again.
It turns out that the load has only half the available power from the power supply. The thyristor acts as and conducts alternating current only during positive half-cycles, when the junction is biased forward.
Half wave control
Thyristor phase control is the most common form of AC power control.
An example of a basic phase control circuit is shown below. Here the thyristor gate voltage is generated by the circuit R1C1 through the trigger diode D1.
During the positive half cycle, when the junction is forward biased, capacitor C1 is charged through resistor R1 by the circuit supply voltage.
The control electrode Y is activated only when the voltage level at point “x” causes diode D1 to operate. Capacitor C1 is discharged to the control electrode U, setting the device to the “on” state.
The duration of the positive half of the cycle, when conduction opens, is controlled by the time constant of the chain R1C1, specified by the variable resistor R1.
Scheme 4: KN1 - latching switch; R1 - variable resistor 1 kOhm; C1 - capacitor 0.1 μF; D1 - any diode for high voltage; L1 - incandescent lamp 100 W; P - conductivity sinusoid Increasing the value of R1 causes a delay in the triggering voltage applied to the thyristor control electrode, which in turn causes a delay in the conduction time of the device.
As a result, the proportion of the half-cycle that the device conducts can be adjusted between 0 -180º. This means that half the power dissipated by the load (lamp) can be adjusted.
There are many ways to achieve full-wave control of thyristors. For example, you can include one semiconductor in a diode bridge rectifier circuit. This method easily converts the alternating component into a unidirectional thyristor current.
However, a more common method is the use of two thyristors connected in inverse parallel.
The most practical approach seems to be the use of one triac. This semiconductor allows transition in both directions, making triacs more suitable for AC switching circuits.
Full technical layout of the thyristor