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What is a silicon-controlled rectifier (SCR)

Don't understand how an SCR works? I'll explain it to you in a few minutes.

Today, I’m going to talk to you about the thyristor, commonly known as the silicon-controlled rectifier (SCR).

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As we all know, resistors are ubiquitous and versatile. We can use resistors to open and close tiny currents (the principle behind computer memory) and convert small currents into larger ones (the operation of amplifiers). However, when dealing with larger currents, they are not very effective, and they have a drawback – once you remove the current, they completely turn off. This means that they are not suitable for applications where you want the circuit to trigger an alarm device that remains open indefinitely.

For such situations, the thyristor, or SCR, comes into play.

1. What is a thyristor (SCR)?

A thyristor is a unidirectional semiconductor device made of silicon. Basically, the thyristor (SCR) is a three-terminal, four-layer semiconductor device made up of alternating P-type and N-type materials. The thyristor has three PN junctions, J1, J2, and J3. The diagram below shows a thyristor with PNPN layers. The thyristor has terminals: anode (A), cathode (K), and gate (G). The gate terminal (G) is connected to the P-layer near the cathode (K) terminal.

Thyristor structure diagram
Thyristor structure diagram
circuit symbol for a thyristor
circuit symbol for a thyristor

2. Thyristors are like two transistors

Thyristors operate like two transistors connected together, as shown in the diagram below. The output of one forms the input of the other, with the gate serving as a “starting motor” to activate them.

A single silicon-controlled rectifier (SCR) is a combination of a PNP transistor (Q1) and an NPN transistor (Q2). Here, the emitter of Q1 serves as the anode terminal of the SCR, while the emitter of Q2 is its cathode. Additionally, the base of Q1 is connected to the collector of Q2, and the collector of Q1 is connected to the base of Q2. The gate terminal of the thyristor is also connected to the base of Q2.

Thyristor structure diagram
Thyristor structure diagram

3. Three States of the Thyristor

So, how does a thyristor work? We can place it in three possible states, in which it is either completely off or completely on. This essentially means that it is a binary digital device.

A. Forward Blocking Mode

Usually, when no current is applied to the gate terminal, the thyristor is in the off state. No current can flow from the anode to the cathode.

You can think of the thyristor as two diodes connected together, with both the upper and lower diodes forward-biased. However, this means that the central junction is reverse-biased, preventing current from flowing from the top to the bottom. This state is known as forward blocking. Although it resembles forward biasing in a conventional diode, no current flows in this case.

Here, by connecting the anode terminal (A) to the positive terminal of the battery and the cathode terminal (K) to the negative terminal of the battery, a positive bias is applied to the thyristor, as shown in the diagram. In this case, junctions J1 and J3 are forward-biased, while junction J2 is reverse-biased.

In this state, no current can pass through the thyristor, except for a small leakage current, as shown by the blue curve in the characteristic curve below.

Thyristor characteristic curve
Thyristor characteristic curve
Thyristor circuit diagram
Thyristor circuit diagram

B. Reverse Blocking Mode of the Thyristor

Now, let’s assume we reverse the anode/cathode connection. In this case, you will likely observe that both the upper and lower diodes are reverse-biased, and thus, no current flows through the thyristor. This is known as reverse blocking (similar to reverse bias in a simple diode).

In this mode, by connecting the anode terminal (A) of the thyristor to the negative terminal of the battery and the cathode terminal (K) to the positive terminal of the battery, the thyristor is reverse-biased. This results in the reverse biasing of junctions J1 and J3, which, in turn, prevents current from flowing through the thyristor, even though junction J2 remains forward-biased. In this state, the thyristor behaves like a typical diode.

Under these reverse-biased conditions, only reverse saturation current flows through the device, similar to the case of a reverse-biased diode, as represented by the blue line in the characteristic curve. The thyristor also exhibits reverse breakdown beyond the reverse voltage safety limit, just like a diode.

Thyristor reverse blocking mode
Thyristor reverse blocking mode

C. Forward Conduction Mode of the Thyristor

The third state is an interesting one. In this state, we need the anode to be the positive terminal and the cathode to be the negative terminal. Then, when current is applied to the gate, it opens the lower transistor, followed by the upper transistor, and then back to the lower transistor, and so on. Each transistor activates the other.

You can think of it as an internal positive feedback, where two transistors continuously feed current to each other until they are both fully activated. At this point, current can flow from the anode to the cathode. This state is called forward conduction, and it’s how the thyristor “latches” (remains permanently on).

To make the thyristor conduct, you can use the following two methods:

  1. By increasing the positive voltage applied to the anode terminal (A) beyond the breakdown voltage (V_B).
  2. By applying a positive voltage to the gate terminal (G), as shown in the diagram.
Thyristor circuit diagram
Thyristor circuit diagram

However, by providing a small positive voltage at the gate terminal, it’s also possible to turn on the SCR at much lower voltage levels. Understanding the reasons behind this is better illustrated by considering the equivalent circuit of a silicon-controlled rectifier (SCR) as shown in the diagram.

Here, it can be observed that when a positive voltage is applied to the gate terminal, transistor Q2 conducts, and its collector current flows into the base of transistor Q1. This causes Q1 to conduct, which in turn leads to its collector current flowing into the base of Q2.

Thyristor circuit diagram
Thyristor circuit diagram

This leads to the saturation of either transistor at a very fast rate, and even removing the bias applied to the gate terminal cannot stop the action, provided the current through the thyristor is greater than the latching current.

In this context, latching current is defined as the minimum current required to keep the thyristor in the conducting state even after the gate pulse is removed.

In this state, the thyristor is referred to as “latched,” and it cannot be controlled simply by removing the current flowing to the gate. At this point, gate current becomes irrelevant, and you must interrupt the main current flow from the anode to the cathode, typically by turning off the power to the entire circuit. To turn off a latched thyristor, various techniques like natural commutation, forced commutation, reverse bias shutdown, and gate turn-off are employed.

4. How the Thyristor Latches

Once a thyristor is latched, you cannot simply turn it off by removing the current flowing to the gate. In this situation, gate current becomes insignificant. To turn off the thyristor, you must interrupt the main current flow from the anode to the cathode, typically by turning off the power to the entire circuit. You can refer to the animated graphic below created by someone else to help understand this process.

This brief animation provides a simple summary of how a thyristor latches. You may notice that the thyristor is redrawn to resemble two transistors (PNP at the top and NPN at the bottom) connected together, with the anode, cathode, and gate forming the three external connections. Each transistor serves as the input for the other. So, how does it work?

  • With no current applied to the gate, the thyristor is off, and there’s no current flowing between the anode and cathode.
  • When current is applied to the gate, it effectively enters the base (input) of the lower (NPN) transistor, turning it on.
  • Once the lower transistor is on, current can flow through it, activating the base (input) of the upper (PNP) transistor and turning it on as well.
  • Once both transistors are fully conducting (“saturated”), current can flow through the entire thyristor from the anode to the cathode.
  • Because the two transistors keep each other conducting, the thyristor remains in the conducting state, or “latched,” even if the gate current is removed.

In a slightly simplified form, this is the key principle behind how a thyristor works.

5. Working Principle of the Thyristor

In a thyristor, a silicon chip is doped with four alternating layers of P and N-type material, resembling two transistors connected back-to-back, as shown in the diagram.

Thyristor working principle diagram
Thyristor working principle diagram

Here, P (cathode) and N (anode) are connected in series, giving us three terminal pins: anode, gate, and cathode.

When we forward-bias the anode and cathode, meaning the anode and cathode are connected to the positive and negative terminals of a battery, the first PN junction and the last PN junction (j1 and j3) become forward-biased due to the breakdown of the depletion layers. Junction j2 remains reverse-biased because no current is supplied to the gate.

When we apply current to the gate, the j2 junction layer starts to break down, and current begins to flow in the circuit. When a sufficient positive signal current or pulse is applied to the gate terminal, it triggers the thyristor to enter the conducting state.

The thyristor can only be fully turned on or off, which means it cannot operate in the in-between state like a transistor. This makes the thyristor unsuitable for use as an analog amplifier but useful as a switching device.

6. SCR Application

The thyristor has various variants, such as the Reverse Conducting Thyristor (RCT), Gate Turn-Off Thyristor (GTO), Gate-Assisted Turn-Off Thyristor (GATT), Asymmetric Thyristors, Static Induction Thyristor (SITH), MOS-Controlled Thyristor (MCT), and Light-Activated SCR (LASCR), among others. Typically, thyristors have high switching speeds and can handle high currents. This makes silicon-controlled rectifiers (SCRs) suitable for a wide range of applications, including:

  • Power switching circuits (AC and DC)
  • Zero voltage switching circuits
  • Overvoltage protection circuits
  • Controlled rectifiers
  • Inverters
  • AC power control (including lighting, motors, etc.)
  • Pulse circuits
  • Battery charging voltage regulators
  • Latching relays
  • Computer logic circuits
  • Remote switch units
  • Phase angle trigger controllers
  • Timing circuits
  • IC trigger circuits
  • Welding control and temperature control systems

This summarizes the various applications of thyristors in different electronic and electrical control systems. If you have any questions, please feel free to ask in the comments.

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