Introduction to Thyristors

The thyristor is a multilayer semiconductor device. It requires a door signal to open the "controlled" part, and once it is "on", it acts like a rectifier diode, which is the "rectifier" part of the name. In fact, the circuit symbol of the ristor indicates that this device acts as a controlled rectifier diode.

Thyristor Symbol

However, unlike the connection diode, which is a two-layer (PN) semiconductor device, or the widely used bipolar transistor, which is a three-layer (PNP or NPN) switching device, the Tristor is a four-layer (PNPN) semiconductor device that includes the following. it is represented in series by three PN connections and the symbol as shown.

Like the diode, the Tristor is a one-way device, that is, it transmits the current in only one direction, but unlike the diode, the tristor can be operated as an open circuit switch or rectifier diode, depending on how it works. The thyristor door is triggered. In other words, thyristors can only work in switching mode and cannot be used for amplification.

The silicon-controlled rectifier SCR is one of several power semiconductor devices, along with Triassices (Triode AC's), Diacs (Diode AC's) and UJTs (Unijunction Transistor), which can act as very fast solid-state AC switches to control large AC voltages.

The thyristor is a three-terminal device labeled "Anod", "Cathode" and "Door" and consists of three PN connections that can be made "ON" and "OFF" or "opened" extremely quickly. On for variable times during half cycles to provide a load with the selected amount of power". The study of the ristor can be best explained by assuming that it consists of two transistors connected in a row as a pair of complementary regenerative switches, as shown.

One Thyristor Two Transistor Analogy

One Thyristor Two Transistor Analogy

The two transistor equivalent circuits show that the collector current of the NPN transistor TR2 feeds directly into the base of the PNP transistor TR1, while the collector current of TR1 is fed into the base of TR2. These two interconnected transistors rely on each other for transmission, as each transistor receives the basic emitter current from the collector-emitter current of the other. Therefore, until one of the transistors is given some base current, nothing can happen even if a voltage is present in the Anode Katoda.

When the Anod terminal of the thyristors is negative compared to Katoda, the central N-P connection is forward-oriented, but the two outer P-N connections are inverted and act like an ordinary diode. Therefore, at some high voltage levels, a thyristor blocks the flow of the reverse current until the fault voltage point of the two external connections is exceeded and the thyristor transmits it without the application of a Door signal.

This is an important negative feature of the triciror because tristors can inadvertently be triggered by reverse overvoltage, as well as rapidly rising dv/dt voltage such as high temperature or sudden rise.

If the anode terminal is made positive according to the Katoda, the two external P-N connections are now polarized forward, but the central N-P connection is inverted. Therefore, forward current is also blocked. If a positive current is injected into the base of the NPN transistor TR2, the resulting collector current flows at the base of the TR1 transistor. This causes a collector current to flow through the PNP transistor, which increases the basic current of TR1 and this increases the basic current of TR2, etc.

Typical Thyristor

Since the two transistors are connected to a regenerative feedback cycle that cannot stop, they force each other to go to saturation very quickly. After triggering for transmission, the current flowing from the device between the Anode and the Cathode is limited only by the resistance of the external circuit. Because the advanced resistance of the device during transmission can be very low at a value of less than 1Ω, and therefore the voltage on it decreases.

Next, we can see that a thrister block the current in both directions of an AC source in the case of "OFF" and can be turned into an "ON" state, allowing the transistor to behave like a normal rectifier diode by applying a positive current to its base. TR2 is called the "Gate" terminal for silicone-controlled rectifier.

For the operation of the Silicon Controlled Rectifier, the operating voltage-current I-V characteristic curves are given as follows:

Thyristor I-V Characteristic Curves


When the thyristor is positioned in the "ON" position and the forward current passes (anode positive), the door signal loses all control due to the regenerative locking movement of the two internal transistors. The application of any door signal or pulse after regeneration is initiated will have no effect since the thyristor is already conductive and completely ON.

Unlike the transistor, SCR cannot be prone to staying in some active areas along a load line between blocking and saturation states. The size and duration of the door "opening" pulse has little effect on the operation of the device, since the transmission is controlled internally. Then applying an instant door pulse to the device is enough to ensure the transmission of the device, and even if the door signal is completely removed, it will remain "ON" continuously.

Therefore, the thyristor can also be considered as a Bistable Latch with two stable states, "OFF" or "ON". This is because, without any gate signal, a silicon-controlled rectifier blocks the current in both directions of an AC waveform, and once triggered by transmission, the regenerative locking action means that it cannot be made "OFF" again using only its Gate. .

So how do we make the tristor "OFF"? After the thyristor is self-locking to the "ON" state and a current passes, it can be made "OFF" again only by completely removing the feed voltage and therefore the Anode (IA) current or by lowering the Anod in the Cathode. It is very clear that in order for a ristator to transmit in the first place, the anode current, which is the load current, must be larger than the IL holding current value. This is IL > IH.

Since the thyristor is capable of being in a "OFF" position when the Anode current falls below this minimum holding value, when used in a sinusoidal AC feed, the SCR will automatically be "OFF" at a value close to the cross. Above each half-cycle point and as we now know, the next Door will remain "OFF" until the trigger pulse is applied.

Since an AC sinusoidal voltage continuously shifts from positive to negative in polarity every half cycle, this allows the positive waveform of the thyristor to be "OFF" at zero point 180o. This effect is known as "natural commutation" and is a very important feature of the silicone-controlled rectifier.

Thyristors used in circuits fed from DC sources cannot form this natural commutation condition because the DC supply voltage is continuous, so another way to make the triciror "OFF" must be provided at the appropriate time, as it will remain conductive when triggered.

However, natural commutation in AC sinusoidal circuits occurs every half cycle. Then, during the positive half cycle of an AC sinusoidal waveform, the thyristor is polarized forward (anode positive) and can be triggered as "ON" using a Door signal or pulse. During a negative half-cycle, The Anode is negative when the cathode is positive. The thyristor is inverted by this voltage and cannot transmit even if a Gate signal is present.

Therefore, by applying a Gate signal at the appropriate time during the positive half of an AC waveform, the thyristor can be triggered to the transmission until the end of the positive half cycle. Therefore, phase control (as it is said) can be used to trigger the tristor at any point along the positive half of the AC waveform, and one of the many use of the Silicon Controlled Rectifier is in the power control of AC systems, as shown.

Thyristor Phase Control


ScR is "OFF" at the beginning of each positive half-cycle. Upon the application of the door impact, it triggers the SCR to the transmission and remains fully locked "ON" for the duration of the positive cycle. If the thyristor is triggered at the beginning of the half-cycle ( ε = 0o ), the load (a lamp) will be "ON" for the full positive cycle of the AC waveform (half-wave pointed AC) at a high average. 0.318 x Vp voltage.

As the application of the door trigger pulse increases over half a cycle ( ε = 0o to 90o ), the lamp lights up for a shorter time, and the average voltage given to the lamp also reduces its brightness proportionally.

We can then use a silicone-controlled rectifier as an AC light dimmer, as well as in various other AC power applications such as AC engine speed control, temperature control systems and power regulator circuits, etc.

Until now, we have seen that a tuber is essentially a half-wave device that transmits only positive half of the cycle when anod is positive, and when the Anot is negative, it blocks the current flow like a diode, regardless of the Gate signal.

However, there are more semiconductor devices under the heading "Tristor", which can be transmitted in both directions, fully waved devices or "OFF" by door signal.

Such devices include "Gate Turn-OFF Tristors" (GTO), "Static Induction Thyristors" (SITH), "MOS Controlled Thyristors" (MCT), "Silicon Controlled Switch" (SCS), "Triode Tristors" (TRIAC) and " Light Activated Tristors" (LASCR), all of which are available in various voltage and current degrees, makes them attractive for use in applications with very high power levels.

Tristor Summary

Silicon Controlled Redrestors, commonly known as Thyristors, are three-joint PNPN semiconductor devices that can be used to switch heavy electrical charges, which can be considered two interconnected transistors. They can be locked with a single positive current pulse applied to the door terminals – "ON" and "ON" indefinitely until the anode cathode current falls below the minimum locking levels.

Static Properties of the Ristator

  • Thyristors are semiconductor devices that can only operate in switching mode.
  • The thyristor is current-powered devices, a small Door current controls a larger Anode current.
  • Transmits the current only forward and when the trigger current is applied to the door.
  • The thyristor acts as a rectifier diode when triggered as "ON".
  • To maintain transmission, the anode current must be larger than the holding current.
  • Regardless of whether the door current is applied, it prevents the flow of current in reverse polarization.
  • When triggered as "ON", a gate current will be made conductively "ON" even when it is no longer applied, provided that the Anode current is above the latching current.

Thyristors are high-speed switches that can be used to replace electromechanical relays in many circuits, as they do not have moving parts, have no contact sparks or suffer from corrosion or dirt. But in addition to changing large currents to "ON" and "OFF", thyristors can be made to control the average value of an AC load current without spending a large amount of power. A good example of tristor power control is electric lighting, heaters and control of engine speed.