Basic Electronics

Thyristor Introduction, the Four Layer Diode, Comparison between a Thyristor & Transistor

Thyristors Introduction:

The thyristor is a Greek word that means a “door”. The devices which comprise four semiconductors or three PN junctions are called thyristors (these are also sometimes known as silicon-controlled rectifiers or SCR. The term thyristor has been derived from “thy ratron” and transistor. (Thyritron is a gas-filled tube that functions as an SCR). Thyristors are also known as PNPN devices. These devices can be found in different forms e.g. unijunction transistor (UJT), silicon-controlled rectifier (SCR), Triac, diac, and silicon-controlled switches (SCS), etc. These devices might have two, three, or four terminals. The number of terminals depends on the type of device. These devices have two or more junctions and they can be used as a very fast-paced turning ON or OFF switch. Thyristor is such a semiconductor device in which switching operation is produced via using the internal feedback. in other words, thyristors are semiconductor switches that have just two conditions i.e. ON or OFF. In the ON state, the resistance on the switch or thyristor is at its minimum, whereas in the OFF condition, its resistance is maximum and thus its working depends on the feedback. These devices are also called latching devices (a latch is a type of switch, which if once closed, remains in closed mode until it is opened again by someone. That’s a switch which if turned on, remains powered on even if control signals are removed and even then, does not turn on, is called a latch). Basically, a thyristor is a solid-state electronic switch for the control of high values of current and it is commonly used in place of release and mechanical switches.

In diagram 6.1 (a), the construction of a thyristor has been illustrated, in which PNPN four layers and three junctions J1, J2, and J3 are clearly visible. Two terminals on two opposites sides of the thyristor have also been shown, through which they are supplied voltages, as is evident from Figures (b) and (c). the intrinsic junction of thyristor is reverse or forward biased, via providing potential difference parallel to its two terminals. Remember that forward or reverse bias depends on polarity. The flow of forwarding current between terminals stops by providing a reverse voltage on any junction.

Figure 6.1 – the PNPN construction for thyristors (a) four sections with three junctions J1, J2 and J3 (b). reverse bias at J2 with positive voltage on terminal 1 (c). reverse bias at J1 with negative voltage terminal 1.

Figure 6.1

When positive voltages are applied on upper terminal 1, the midpoint junction is reverse biased at that time as has been shown via diagram b, because the junction’s 2 N side is +ive while the P side becomes negative. Thus, due to reverse bias parallel to J2, no forward current flows through the two sides fitted terminals.

When the terminal is provided negative voltages, as shown in diagram c, the junctions J1 and J3 become reverse biased. Thus, due to the internal reverse bias, the flow of current between the two terminals does not occur again.

In fact, an extra trigger voltage is required for forward biasing a thyristor’s internal reverse-biased junction, which is provided via an additional electrode called a gate. With the supply of extra trigger voltage through the gate, the flow of current between the terminals commences. Therefore, remember that so long as a pulse is not given on the thyristor’s gate, it does not conduct at all.

Figure 6.2 – thyristor structure and symbol

Figure 6.2

In the diagram, the construction and symbol of a thyristor have been shown. Like a power JFET, an SCR can turn current ON or OFF extensively. Apart from controlling the quantity of AC power, a thyristor is also used as a lamp dimmer. Moreover, it is also used in voltage protection, heaters, lighting system, motor speed controller, ignition systems, charging circuits, timing circuits, and other heavy current loads.

The Four-Layer Diode

We know that a thyristor is a device that has four layers, this four-layer device is known as a thyristor., which practically contains three junctions. If thyristor is converted into an equivalent circuit of two inter-connected functioning transistors (such a circuit is called a latch) as shown in figure 6.3 (a), the operation of the circuit can be explained through this equivalent circuit. In figure 6.3 (a), the higher transistor Q1 is a PNP device, whereas the lower transistor Q2 is an NPN device. Q1 collector output operates the base of Q2, exactly in a similar manner Q2 output is received on base). Therefore, the base of a thyristor operates as a switch on this extraordinary connection. To clearly understand the operation of a thyristor, it is necessary to go through the following details.

Positive Feedback

In diagram 6.3 (a), positive feedback is applied in the exceptional circuit of the equivalent thyristor, (called transistor latch) as has been shown in the figure. Any change in Q2 base current gets amplified and feedback again via Q1, due to which an increase in actual change occurs. Due to this feedback, the base current of Q2 keeps on changing incessantly, until both the transistors enter their saturated or cut-off region (we know that when a transistor enters its saturation or cut-off region, it works as an open or close switch respectively, that’s the reason a thyristor is also sometimes called an electronic switch)

Figure 6.3 – transistor latch

Figure 6.3

Suppose that Q2 base current increases momentarily, thus Q2 collector current will also increase, as a consequence, Q1 base current will also increase. As a result of an increase in the base current of Q1, its collector will also increase. As Q1 collector output is received by the Q2 base, thus an increase in Q1 collector current will cause an increase in the Q2 base current as well due to a feedback operation. This process of amplification and feedback goes on until both the transistors are saturated. In such a situation, the circuit act as a close switch collectively, as has been demonstrated in figure (b).

Contrary to this, if Q2 base current reduces due to certain reasons, Q2 collector current will also decline. In this way, the Q1 base current will also diminish (because Q2 collector output is received on the Q1 base). As a result of a decrease in Q1 base current, Q1 collector current also decreases, due to which Q2 base current declines further. This process continues on until both the transistor reaches its cut-off condition (or the transistors enter their cut-off region) under such a condition, these transistors react like an open switch (as shown in figure c)

There are two conditions of a switching circuit i.e. open or close as has been displayed in figure (a). it remains on any one of these two conditions until this mode has been changed through some external force (i.e. if the switch is closed, it will remain closed until it is opened through some extrinsic source. On the contrary, if the switch is open, it will continue to remain open until closed by some external force). If the circuit is open, it will remain open until the Q2 base current is increased through some source. If the circuit is closed, it will be closed until the base current Q2 is reduced. As this switching circuit can retain either of its conditions (i.e. if it is closed, it will remain closed and if open, it will stay open), therefore such type of circuit is called a latch.

Closing a Latch

In figure 6.4 (a), a latching circuit fixed alongside a load resistor has been shown, which has been provided supply voltage VCC. it is assumed that the latch is opened, as can be judged from figure (b). As no current passes through the load resistor in such a situation, therefore, voltage value found parallel to the latch equals supply voltage. Thus, an operating point can be found on the lower end of the load line as a result of a latch being open (figured)

The open latch shown in diagram (b) can normally be closed through two methods. According to the first method, which is known as breakover method, the latch is closed through a break over, as shown in figure (b). breakover means breaking down of Q1 collector diode by means of increasing supply voltage value (VCC) intensively. As the Q2 base current increases due to an increase in Q1 collector current, positive feedback kicks off, due to which both transistors saturate. As the saturation mode starts, both the transistors seem to be short-circuited (i.e. both acts just like a typical short circuit). Thus, the latch tends to close as has been illustrated in the figure. Therefore, in its typical close condition, zero voltages are found across the latch and an operating point builds upon the upper end of the DC load line (as has been shown in figure d)

Figure 6.4 – transistor latch with trigger input

Figure 6.4

The second method of closing a latch is called forward bias triggering method. In this method, a sharp pulse or trigger is coupled for providing forward bias on the Q2 base (as elucidated in figure a). As a result of this trigger, Q2 base current increases momentarily, due to which positive feedback kicks off or initiates.

Resultantly, both transistors enter their saturation condition. As a result of saturation, both transistors operate like a typical short circuit and thus latch gets closed (as shown in figure c).

Remember if Q2 breaks down prior to Q1, a break-over can occur in such a situation as well. Although breakover starts with the breakdown of any collector diode, however, it ends when both transistors are in saturation condition. Therefore, the term breakover is used instead of breakdown for the purpose of clarifying this closing latch condition.

Opening a latch

If the latch depicted in figure 6.4 (a) is desired to be opened, one of the methods is to reduce the value of VCC and bring it to zero. By doing so transistors are forced to saturation to cut-off (i.e. they tend to come out from

Its saturation condition and move into the cut-off condition)

This method of latch opening largely depends on reducing the latch current value extensively, due to which transistors extricate from their saturation status. Therefore, this method of latch opening is also called “low current dropout”. Another method is also used for latch opening besides this particular method. It is known as reverse bias trigger method. In this method, a negative trigger or pulse is applied for opening a latch instead of a positive pulse, due to which base current Q2 decreases. As a result of a reduction in the base current of Q2, the base current of Q1 also decreases. Thus, both the transistors reach a cut-off condition due to the consistent decrease in current. Thus, the latch opens.

In short, the following methods are applied for the purpose of opening or closing a latch.

1). For closing a latch- the method of breakover or forward bias triggering method

2). For opening a latch – the method of low current dropout or the reverse bias triggering method

Comparison between a Thyristor and Transistor

In the table, 6.1 comparison between a thyristor and a transistor has been elucidated

Table 6.1

S. No Transistors Thyristors
1. 3-layers, 2 junction devices 4-layer, 2 or more junction devices
2. Fast response  Very fast response
3. High frequency Very high frequency
4. Highly reliable Very highly reliable
5. Small voltage drops Very small voltage drops
6. Long life Very long life
7. Small to medium power ratings Very small to very large power ratings
8. Requires a continuous flow of current to remain in conducting state Require only a small pulse for triggering and thereafter remaining in conducting state
9. Low power consumption Very low power consumption
10. Low control capability High control capability
11. Small turn-on and turn-off time Very small turn-on and turn-off time


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My name is Shahzada Fahad and I am an Electrical Engineer. I have been doing Job in UAE as a site engineer in an Electrical Construction Company. Currently, I am running my own YouTube channel "Electronic Clinic", and managing this Website. My Hobbies are * Watching Movies * Music * Martial Arts * Photography * Travelling * Make Sketches and so on...

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