Types of Thyristors; Difference Between Thyristors and SCRs
Thyristors can be classified in various ways:
(1) Classification by turn-off, conduction, and control methods
Based on their turn-off, conduction, and control characteristics, thyristors can be classified into types such as standard thyristors, bidirectional thyristors (TRIACs), reverse-conducting thyristors, gate turn-off thyristors (GTOs), BTG thyristors, temperature-controlled thyristors, and light-controlled thyristors.
(2) Classification by pins and polarity
Thyristors can be classified into two-terminal, three-terminal, and four-terminal types based on their pins and polarity.
(3) Classification by package type
Thyristors can be classified into three types based on their packaging: metal-packaged, plastic-packaged, and ceramic-packaged. Metal-packaged thyristors include stud-mount, flat-pack, and round-case varieties; plastic-packaged thyristors come in versions with or without heat sinks.
(4) Classification by current capacity
Thyristors can be classified into high-power, medium-power, and low-power types based on current capacity. Generally, high-power thyristors utilize metal-case packaging, whereas medium- and low-power thyristors typically use plastic or ceramic packaging.
(5) Classification by turn-off speed
Thyristors can be classified into standard thyristors and high-frequency (fast-switching) thyristors based on their turn-off speed.
Difference between Thyristors and SCRs
The thyristor (also known as an SCR, or Silicon Controlled Rectifier) belongs to the category of power devices and functions as a power semiconductor switching component. "SCR" is simply an abbreviation for the device. Based on operating characteristics, these devices can be classified into unidirectional SCRs (Silicon Controlled Rectifiers) and bidirectional SCRs (TRIACs). A silicon-controlled rectifier (SCR), also known as a thyristor, is a component composed of four semiconductor layers (PNPN) and features three terminals: the anode (A), the cathode (K), and the gate (G).
In circuits, SCRs enable contactless control of alternating current (AC), allowing a small current to control a large current. Unlike relays, they generate no sparks during operation and offer fast switching, long service life, and high reliability. They are widely used in applications such as speed regulation, dimming, voltage regulation, temperature control, and various other control circuits. SCRs are categorized into unidirectional and bidirectional types, each with distinct schematic symbols. A unidirectional SCR contains three PN junctions; two terminals—the anode and the cathode—are connected to the outermost P and N layers, respectively, while a gate terminal is connected to the intermediate P layer.
Unidirectional SCRs possess unique characteristics: they do not conduct when the anode is subjected to reverse voltage, nor do they conduct when the anode has forward voltage but no voltage is applied to the gate; however, they enter a conducting state when forward voltage is applied to both the anode and the gate simultaneously. Once conduction begins, the gate voltage loses control over the device; it remains in the conducting state regardless of the presence or polarity of the gate voltage. To turn it off, the anode voltage must be reduced below a specific critical threshold or reversed.
For bidirectional SCRs (often called TRIACs), the terminals are typically arranged from left to right in the order T1, T2, and G (when viewed from the side with markings, with the terminals facing downward). Adjusting the magnitude or duration of the trigger pulse applied to the gate (G) allows for the control of the conduction current's magnitude.
The key difference from unidirectional SCRs is that in a bidirectional SCR, changing the polarity of the trigger pulse at the gate (G) alters the direction of conduction accordingly, thereby enabling control of AC loads. In contrast, a unidirectional SCR conducts only from anode to cathode after being triggered; hence the distinction between unidirectional and bidirectional types. Thyristors are commonly used in electronic projects; examples include unidirectional types like the MCR-100 and bidirectional types like the TLC336.
Bidirectional thyristors (TRIACs) can be classified in several ways:
By quadrant: Four-quadrant three-terminal TRIACs and three-quadrant TRIACs;
By package type: Standard partial-plastic packages and fully insulated all-plastic packages;
By trigger current: Micro-trigger, high-sensitivity, and standard-trigger types;
By voltage rating: Standard voltage and high-voltage varieties.
Thanks to advantages such as high circuit efficiency, excellent control characteristics, long service life, compact size, and powerful functionality, thyristor products have seen rapid development since the 1960s, eventually giving rise to an independent discipline known as "Thyristor AC Technology."
Today, thyristor technology has matured significantly; product quality has improved, manufacturing yields have risen sharply, and the technology is evolving toward high-voltage, high-current applications.
The roles of thyristors in application circuits include:
Controlled rectification: Similar to diode rectification, this converts AC to DC; however, it allows for the effective control of the DC output voltage level while the AC input voltage remains constant, thereby achieving an AC-to-variable-DC conversion;
Contactless static power switching (solid-state switching): Acting as power switching components, thyristors can replace contactors and relays in applications requiring high switching frequencies. Consequently, thyristor components are widely used in the circuits of various electronic devices and products for functions such as controlled rectification, inversion, frequency conversion, voltage regulation, and contactless switching. Thyristor devices are widely used in a vast array of applications, ranging from household appliances—such as dimmable lights, variable-speed fans, HVAC units, water heaters, televisions, refrigerators, washing machines, cameras, audio systems, voice-activated circuits, timers, sensor-activated lights, Christmas light controllers, and automatic door circuits—to toys, power tools, radio remote control circuits, video cameras, and various industrial control systems.
In these circuits, thyristors typically function as components for controlled rectification, inversion, frequency conversion, voltage regulation, and contactless switching.
As the industrial application of thyristors expands, their functionality has become increasingly versatile. However, thyristors can sustain damage during operation. How can we effectively protect them to ensure a long service life?
1. Overvoltage Protection
Thyristors are highly sensitive to overvoltage. If the forward voltage exceeds the repetitive peak off-state voltage ($U_{DRM}$) by a certain margin, the thyristor may trigger erroneously, causing circuit failure; if the applied reverse voltage exceeds the repetitive peak reverse voltage ($U_{RRM}$) by a certain margin, the device can suffer immediate damage. Therefore, it is essential to understand the causes of overvoltage and the methods to suppress it.
Overvoltage is primarily caused by abrupt changes in supplied electrical power or system energy storage—where the system cannot adjust quickly enough—or by the inability of accumulated electromagnetic energy to dissipate in time. The two main types are overvoltage caused by external shocks (such as lightning strikes) and surge voltages caused by switching operations. Overvoltages resulting from events like lightning strikes or high-voltage circuit breaker operations manifest as voltage spikes lasting from a few microseconds to a few milliseconds, posing a significant risk to thyristors. Surge voltages caused by switching operations can be categorized as follows:
(1) Overvoltages caused by connecting or disconnecting the AC power supply
Examples include overvoltages resulting from the operation of AC switches or the blowing of AC-side fuses. Due to factors such as resonant circuits formed by transformer winding distributed capacitance and leakage reactance, as well as capacitive voltage division, the magnitude of these overvoltages can reach 2 to over 10 times the normal operating voltage. Generally, the faster the switching speed, the higher the overvoltage; disconnecting a circuit under no-load conditions tends to produce higher overvoltages.
(2) Overvoltages generated on the DC side
Significant overvoltages occur when interrupting a circuit with high inductance or high current. Such situations often arise during load disconnection, the opening of a circuit containing a conducting thyristor, or sudden current changes caused by the blowing of a fast-acting fuse element.
(3) Commutation surge voltages
These include commutation overvoltages and commutation oscillation overvoltages. Commutation overvoltage arises from the recombination of residual carriers within the thyristor's internal junctions when the current drops to zero; hence, it is also known as overvoltage caused by the carrier storage effect. Following the commutation overvoltage, a commutation oscillation overvoltage occurs; this is an oscillatory voltage resulting from resonance between inductance and capacitance, and its magnitude depends on the reverse voltage present after commutation concludes. The higher the reverse voltage, the greater the commutation oscillation overvoltage.Various suppression methods can be employed depending on the cause of the overvoltage. These include minimizing the overvoltage source and attenuating the overvoltage amplitude; limiting the rate of rise of overvoltage energy; slowing the dissipation of generated energy while increasing dissipation pathways; and utilizing electronic protection circuits. Currently, the most common approach is to insert energy-absorbing components into the circuit to dissipate energy; these are typically referred to as absorption circuits or snubber circuits.(4) Resistor-Capacitor (RC) Snubber Circuit
Overvoltages typically feature high frequencies; therefore, capacitors are commonly used as the absorption element. To prevent oscillation, a damping resistor is often added, forming an RC snubber circuit. This circuit can be connected to the AC or DC side of the system, or placed in parallel across the anode and cathode of the thyristor. Ideally, non-inductive capacitors should be selected for the snubber circuit, and wiring connections should be kept as short as possible.
(5) Snubber Circuits Composed of Nonlinear Components (e.g., Selenium Stacks and Varistors)
The time constant (RC) of the aforementioned RC snubber circuit is fixed; consequently, it may fail to discharge rapidly enough to suppress overvoltages characterized by short duration, high peak values, and high energy, resulting in poor suppression performance. Therefore, nonlinear components such as selenium stacks or varistors are typically connected in parallel at the input and output terminals of the converter device. Selenium stacks are characterized by an operating voltage that depends on temperature—specifically, the lower the temperature, the higher the voltage withstand capability. Additionally, selenium stacks possess self-healing properties allowing for repeated use; following an overvoltage event, the burn holes on the selenium substrate are re-covered by melted selenium, thereby restoring the component's operational characteristics. A varistor is a nonlinear metal-oxide resistor based on zinc oxide (ZnO). Its structure consists of two electrodes with a filling of irregular ZnO micro-crystals (10–50 μm in size) between them; the grain boundaries between these crystals consist of a bismuth oxide layer approximately 1 μm thick. Under normal voltage conditions, this grain boundary layer exhibits high resistance, allowing only a minimal leakage current (less than 100 μA). When voltage is applied, an electron avalanche is triggered, causing the grain boundary layer to rapidly transition to a low-impedance state; current surges, dissipating energy and suppressing the overvoltage, thereby protecting the thyristor. Once the surge passes, the grain boundary layer reverts to a high-resistance state. The characteristics of a varistor are primarily defined by the following parameter:
Nominal voltage: The voltage across the varistor when a DC current of 1 mA flows through it. Surge Current Capacity: This is defined as the maximum surge current value—using an impulse waveform with an 8-microsecond rise time and a 20-microsecond pulse width, applied ten times at five-minute intervals—that results in a nominal voltage change of no more than 10%.
Because the grain boundary layer of a standard varistor has a finite discharge capacity and cycle limit, the nominal voltage decreases as the number of discharges and the amplitude of the discharge current increase. If the current exceeds a certain threshold, the nominal voltage drops to zero, causing the varistor to puncture or even explode; therefore, the surge current capacity must be limited.
Leakage Current: This refers to the current flowing through the varistor when half of its nominal DC voltage is applied.
Varistors are widely used due to their high surge current capacity, low residual voltage, and strong overvoltage suppression capabilities. They exhibit low leakage current during normal operation and do not allow follow-through current after discharge. They are available in a wide range of nominal voltage ratings, facilitating user selection, and possess symmetrical volt-ampere characteristics, making them suitable for AC/DC applications as well as positive and negative surges.
Overcurrent Protection
Semiconductor devices—particularly high-voltage, high-current power devices like thyristors—are compact and have low thermal capacity; consequently, their junction temperature must be strictly controlled to prevent catastrophic failure. When a current exceeding the rated value flows through a thyristor, heat cannot dissipate quickly enough, causing the junction temperature to rise rapidly and ultimately leading to the burnout of the junction layer. Overcurrent conditions can arise from various causes, such as thyristor failure within the converter unit, malfunctions in the triggering circuit or control system, AC power supply issues (excessively high or low voltage, or phase loss), load overloads or short circuits, and interference from faults in adjacent equipment.
The most common method for thyristor overcurrent protection is the use of a fast-acting fuse. Standard fuses operate too slowly; the thyristor would burn out before the fuse could blow, making them unsuitable for thyristor protection. A fast-acting fuse consists of a silver fuse element embedded in quartz sand; it features an extremely short clearing time and is used to protect thyristors. The performance of fast-acting fuses is primarily characterized by the following parameters.
Thyristor Replacement
If a thyristor fails and an identical model is unavailable for replacement, a different model with similar performance parameters may be used instead.
Application circuits are generally designed with significant safety margins. When replacing a thyristor, one need only pay attention to the rated peak voltage (repetitive peak voltage), rated current (average on-state current), gate trigger voltage, and gate trigger current—paying particular attention to the rated peak voltage and rated current.
The replacement thyristor should match the switching speed of the damaged unit. For instance, if a high-speed thyristor used in a pulse circuit or high-speed inverter circuit fails, it must be replaced with another fast-acting thyristor of the same type; a standard thyristor cannot be used as a substitute.
When selecting a replacement thyristor, there is no need to allow for excessive margins on any parameter; the specifications should be as close as possible to those of the original component. Excessive margins not only result in waste but can also cause adverse effects, such as failure to trigger or sluggish triggering response.
Additionally, the physical form factor of the two thyristors should be identical; otherwise, installation may prove difficult.
Thyristor Operating Principle
During operation, the thyristor's anode (A) and cathode (K) are connected to the power supply and load to form the main circuit, while the gate (G) and cathode (K) are connected to the control device to form the control circuit.
Thyristor Operating Conditions:
1. When the thyristor is subjected to a reverse anode voltage, it remains in the off state regardless of the voltage applied to the gate.2. When a thyristor is subjected to a forward anode voltage, it conducts only if a forward voltage is also applied to the gate.
3. Once the thyristor is conducting, it remains in the conducting state as long as a certain forward anode voltage is maintained, regardless of the gate voltage; in other words, the gate loses control after conduction begins.
4. When the thyristor is conducting, it turns off if the main circuit voltage (or current) drops to near zero.
(1) Classification by turn-off, conduction, and control methods
Based on their turn-off, conduction, and control characteristics, thyristors can be classified into types such as standard thyristors, bidirectional thyristors (TRIACs), reverse-conducting thyristors, gate turn-off thyristors (GTOs), BTG thyristors, temperature-controlled thyristors, and light-controlled thyristors.
(2) Classification by pins and polarity
Thyristors can be classified into two-terminal, three-terminal, and four-terminal types based on their pins and polarity.
(3) Classification by package type
Thyristors can be classified into three types based on their packaging: metal-packaged, plastic-packaged, and ceramic-packaged. Metal-packaged thyristors include stud-mount, flat-pack, and round-case varieties; plastic-packaged thyristors come in versions with or without heat sinks.
(4) Classification by current capacity
Thyristors can be classified into high-power, medium-power, and low-power types based on current capacity. Generally, high-power thyristors utilize metal-case packaging, whereas medium- and low-power thyristors typically use plastic or ceramic packaging.
(5) Classification by turn-off speed
Thyristors can be classified into standard thyristors and high-frequency (fast-switching) thyristors based on their turn-off speed.
Difference between Thyristors and SCRs
The thyristor (also known as an SCR, or Silicon Controlled Rectifier) belongs to the category of power devices and functions as a power semiconductor switching component. "SCR" is simply an abbreviation for the device. Based on operating characteristics, these devices can be classified into unidirectional SCRs (Silicon Controlled Rectifiers) and bidirectional SCRs (TRIACs). A silicon-controlled rectifier (SCR), also known as a thyristor, is a component composed of four semiconductor layers (PNPN) and features three terminals: the anode (A), the cathode (K), and the gate (G).
In circuits, SCRs enable contactless control of alternating current (AC), allowing a small current to control a large current. Unlike relays, they generate no sparks during operation and offer fast switching, long service life, and high reliability. They are widely used in applications such as speed regulation, dimming, voltage regulation, temperature control, and various other control circuits. SCRs are categorized into unidirectional and bidirectional types, each with distinct schematic symbols. A unidirectional SCR contains three PN junctions; two terminals—the anode and the cathode—are connected to the outermost P and N layers, respectively, while a gate terminal is connected to the intermediate P layer.
Unidirectional SCRs possess unique characteristics: they do not conduct when the anode is subjected to reverse voltage, nor do they conduct when the anode has forward voltage but no voltage is applied to the gate; however, they enter a conducting state when forward voltage is applied to both the anode and the gate simultaneously. Once conduction begins, the gate voltage loses control over the device; it remains in the conducting state regardless of the presence or polarity of the gate voltage. To turn it off, the anode voltage must be reduced below a specific critical threshold or reversed.
For bidirectional SCRs (often called TRIACs), the terminals are typically arranged from left to right in the order T1, T2, and G (when viewed from the side with markings, with the terminals facing downward). Adjusting the magnitude or duration of the trigger pulse applied to the gate (G) allows for the control of the conduction current's magnitude.
The key difference from unidirectional SCRs is that in a bidirectional SCR, changing the polarity of the trigger pulse at the gate (G) alters the direction of conduction accordingly, thereby enabling control of AC loads. In contrast, a unidirectional SCR conducts only from anode to cathode after being triggered; hence the distinction between unidirectional and bidirectional types. Thyristors are commonly used in electronic projects; examples include unidirectional types like the MCR-100 and bidirectional types like the TLC336.
Bidirectional thyristors (TRIACs) can be classified in several ways:
By quadrant: Four-quadrant three-terminal TRIACs and three-quadrant TRIACs;
By package type: Standard partial-plastic packages and fully insulated all-plastic packages;
By trigger current: Micro-trigger, high-sensitivity, and standard-trigger types;
By voltage rating: Standard voltage and high-voltage varieties.
Thanks to advantages such as high circuit efficiency, excellent control characteristics, long service life, compact size, and powerful functionality, thyristor products have seen rapid development since the 1960s, eventually giving rise to an independent discipline known as "Thyristor AC Technology."
Today, thyristor technology has matured significantly; product quality has improved, manufacturing yields have risen sharply, and the technology is evolving toward high-voltage, high-current applications.
The roles of thyristors in application circuits include:
Controlled rectification: Similar to diode rectification, this converts AC to DC; however, it allows for the effective control of the DC output voltage level while the AC input voltage remains constant, thereby achieving an AC-to-variable-DC conversion;
Contactless static power switching (solid-state switching): Acting as power switching components, thyristors can replace contactors and relays in applications requiring high switching frequencies. Consequently, thyristor components are widely used in the circuits of various electronic devices and products for functions such as controlled rectification, inversion, frequency conversion, voltage regulation, and contactless switching. Thyristor devices are widely used in a vast array of applications, ranging from household appliances—such as dimmable lights, variable-speed fans, HVAC units, water heaters, televisions, refrigerators, washing machines, cameras, audio systems, voice-activated circuits, timers, sensor-activated lights, Christmas light controllers, and automatic door circuits—to toys, power tools, radio remote control circuits, video cameras, and various industrial control systems.
In these circuits, thyristors typically function as components for controlled rectification, inversion, frequency conversion, voltage regulation, and contactless switching.
As the industrial application of thyristors expands, their functionality has become increasingly versatile. However, thyristors can sustain damage during operation. How can we effectively protect them to ensure a long service life?
1. Overvoltage Protection
Thyristors are highly sensitive to overvoltage. If the forward voltage exceeds the repetitive peak off-state voltage ($U_{DRM}$) by a certain margin, the thyristor may trigger erroneously, causing circuit failure; if the applied reverse voltage exceeds the repetitive peak reverse voltage ($U_{RRM}$) by a certain margin, the device can suffer immediate damage. Therefore, it is essential to understand the causes of overvoltage and the methods to suppress it.
Overvoltage is primarily caused by abrupt changes in supplied electrical power or system energy storage—where the system cannot adjust quickly enough—or by the inability of accumulated electromagnetic energy to dissipate in time. The two main types are overvoltage caused by external shocks (such as lightning strikes) and surge voltages caused by switching operations. Overvoltages resulting from events like lightning strikes or high-voltage circuit breaker operations manifest as voltage spikes lasting from a few microseconds to a few milliseconds, posing a significant risk to thyristors. Surge voltages caused by switching operations can be categorized as follows:
(1) Overvoltages caused by connecting or disconnecting the AC power supply
Examples include overvoltages resulting from the operation of AC switches or the blowing of AC-side fuses. Due to factors such as resonant circuits formed by transformer winding distributed capacitance and leakage reactance, as well as capacitive voltage division, the magnitude of these overvoltages can reach 2 to over 10 times the normal operating voltage. Generally, the faster the switching speed, the higher the overvoltage; disconnecting a circuit under no-load conditions tends to produce higher overvoltages.
(2) Overvoltages generated on the DC side
Significant overvoltages occur when interrupting a circuit with high inductance or high current. Such situations often arise during load disconnection, the opening of a circuit containing a conducting thyristor, or sudden current changes caused by the blowing of a fast-acting fuse element.
(3) Commutation surge voltages
These include commutation overvoltages and commutation oscillation overvoltages. Commutation overvoltage arises from the recombination of residual carriers within the thyristor's internal junctions when the current drops to zero; hence, it is also known as overvoltage caused by the carrier storage effect. Following the commutation overvoltage, a commutation oscillation overvoltage occurs; this is an oscillatory voltage resulting from resonance between inductance and capacitance, and its magnitude depends on the reverse voltage present after commutation concludes. The higher the reverse voltage, the greater the commutation oscillation overvoltage.Various suppression methods can be employed depending on the cause of the overvoltage. These include minimizing the overvoltage source and attenuating the overvoltage amplitude; limiting the rate of rise of overvoltage energy; slowing the dissipation of generated energy while increasing dissipation pathways; and utilizing electronic protection circuits. Currently, the most common approach is to insert energy-absorbing components into the circuit to dissipate energy; these are typically referred to as absorption circuits or snubber circuits.(4) Resistor-Capacitor (RC) Snubber Circuit
Overvoltages typically feature high frequencies; therefore, capacitors are commonly used as the absorption element. To prevent oscillation, a damping resistor is often added, forming an RC snubber circuit. This circuit can be connected to the AC or DC side of the system, or placed in parallel across the anode and cathode of the thyristor. Ideally, non-inductive capacitors should be selected for the snubber circuit, and wiring connections should be kept as short as possible.
(5) Snubber Circuits Composed of Nonlinear Components (e.g., Selenium Stacks and Varistors)
The time constant (RC) of the aforementioned RC snubber circuit is fixed; consequently, it may fail to discharge rapidly enough to suppress overvoltages characterized by short duration, high peak values, and high energy, resulting in poor suppression performance. Therefore, nonlinear components such as selenium stacks or varistors are typically connected in parallel at the input and output terminals of the converter device. Selenium stacks are characterized by an operating voltage that depends on temperature—specifically, the lower the temperature, the higher the voltage withstand capability. Additionally, selenium stacks possess self-healing properties allowing for repeated use; following an overvoltage event, the burn holes on the selenium substrate are re-covered by melted selenium, thereby restoring the component's operational characteristics. A varistor is a nonlinear metal-oxide resistor based on zinc oxide (ZnO). Its structure consists of two electrodes with a filling of irregular ZnO micro-crystals (10–50 μm in size) between them; the grain boundaries between these crystals consist of a bismuth oxide layer approximately 1 μm thick. Under normal voltage conditions, this grain boundary layer exhibits high resistance, allowing only a minimal leakage current (less than 100 μA). When voltage is applied, an electron avalanche is triggered, causing the grain boundary layer to rapidly transition to a low-impedance state; current surges, dissipating energy and suppressing the overvoltage, thereby protecting the thyristor. Once the surge passes, the grain boundary layer reverts to a high-resistance state. The characteristics of a varistor are primarily defined by the following parameter:
Nominal voltage: The voltage across the varistor when a DC current of 1 mA flows through it. Surge Current Capacity: This is defined as the maximum surge current value—using an impulse waveform with an 8-microsecond rise time and a 20-microsecond pulse width, applied ten times at five-minute intervals—that results in a nominal voltage change of no more than 10%.
Because the grain boundary layer of a standard varistor has a finite discharge capacity and cycle limit, the nominal voltage decreases as the number of discharges and the amplitude of the discharge current increase. If the current exceeds a certain threshold, the nominal voltage drops to zero, causing the varistor to puncture or even explode; therefore, the surge current capacity must be limited.
Leakage Current: This refers to the current flowing through the varistor when half of its nominal DC voltage is applied.
Varistors are widely used due to their high surge current capacity, low residual voltage, and strong overvoltage suppression capabilities. They exhibit low leakage current during normal operation and do not allow follow-through current after discharge. They are available in a wide range of nominal voltage ratings, facilitating user selection, and possess symmetrical volt-ampere characteristics, making them suitable for AC/DC applications as well as positive and negative surges.
Overcurrent Protection
Semiconductor devices—particularly high-voltage, high-current power devices like thyristors—are compact and have low thermal capacity; consequently, their junction temperature must be strictly controlled to prevent catastrophic failure. When a current exceeding the rated value flows through a thyristor, heat cannot dissipate quickly enough, causing the junction temperature to rise rapidly and ultimately leading to the burnout of the junction layer. Overcurrent conditions can arise from various causes, such as thyristor failure within the converter unit, malfunctions in the triggering circuit or control system, AC power supply issues (excessively high or low voltage, or phase loss), load overloads or short circuits, and interference from faults in adjacent equipment.
The most common method for thyristor overcurrent protection is the use of a fast-acting fuse. Standard fuses operate too slowly; the thyristor would burn out before the fuse could blow, making them unsuitable for thyristor protection. A fast-acting fuse consists of a silver fuse element embedded in quartz sand; it features an extremely short clearing time and is used to protect thyristors. The performance of fast-acting fuses is primarily characterized by the following parameters.
Thyristor Replacement
If a thyristor fails and an identical model is unavailable for replacement, a different model with similar performance parameters may be used instead.
Application circuits are generally designed with significant safety margins. When replacing a thyristor, one need only pay attention to the rated peak voltage (repetitive peak voltage), rated current (average on-state current), gate trigger voltage, and gate trigger current—paying particular attention to the rated peak voltage and rated current.
The replacement thyristor should match the switching speed of the damaged unit. For instance, if a high-speed thyristor used in a pulse circuit or high-speed inverter circuit fails, it must be replaced with another fast-acting thyristor of the same type; a standard thyristor cannot be used as a substitute.
When selecting a replacement thyristor, there is no need to allow for excessive margins on any parameter; the specifications should be as close as possible to those of the original component. Excessive margins not only result in waste but can also cause adverse effects, such as failure to trigger or sluggish triggering response.
Additionally, the physical form factor of the two thyristors should be identical; otherwise, installation may prove difficult.
Thyristor Operating Principle
During operation, the thyristor's anode (A) and cathode (K) are connected to the power supply and load to form the main circuit, while the gate (G) and cathode (K) are connected to the control device to form the control circuit.
Thyristor Operating Conditions:
1. When the thyristor is subjected to a reverse anode voltage, it remains in the off state regardless of the voltage applied to the gate.2. When a thyristor is subjected to a forward anode voltage, it conducts only if a forward voltage is also applied to the gate.
3. Once the thyristor is conducting, it remains in the conducting state as long as a certain forward anode voltage is maintained, regardless of the gate voltage; in other words, the gate loses control after conduction begins.
4. When the thyristor is conducting, it turns off if the main circuit voltage (or current) drops to near zero.
Sep 09,2026