Skip to content

Silicon Controlled Rectifier(SCR): power semiconductor electronic equipment

Silicon Controlled Rectifier(SCR) power semiconductor electronic equipment

Silicon Controlled Rectifier(SCR) is defined as such a device that allows the unidirectional flow of current to flow and opposes the other direction. These type of devices are known as unidirectional devices.It is a power control device based on a thyristor, and its core is an intelligent digital control circuit. Thyristor rectifier plays a great role in AC voltage regulation, controllable rectification and non-contact power static switch.

What is SCR in PCB?

Silicon Controlled Rectifier is the technical name. In power electronics, the SCR is a member of the thyristor family of components.  As a result, the term “Thyristor” is frequently used to describe it. SCRs are unidirectional electronics that, like diodes, allow electricity to flow in one direction while obstructing it in the other. A group of electrical engineers invented the SCR in 1957. An SCR is a four-layer, three-terminal, three-junction semiconductor power electronic device. The SCR’s inherent capacity to turn ON or OFF is controlled by the gate input terminal and biassing circumstances. These results cause the SCR’s ON times to shift, which alters the average power delivered to the load. It can manage thousands of voltages and currents.

What is SCR and its types in PCB

What’s the type of SCR in PCB?

Press Pack:
For the separation of anode and cathode, the press pack SCRs are used to enclose the electrodes in a ceramic. Additionally, they provide support for applications requiring 200A or more and 1200V or more.

Stud Base:
This unit’s screwed base makes it simple to attach and it offers little thermal resistance. The stud base device also supports full voltage and currents between 5 and 150A.

Discrete Plastic:
In this type, three pins are affixed to a plastic covering for the semiconductors. Also available from 25A to 1000V, these integrate on a PCB.

Flat Base:
Insulation is provided with a flat base to protect it from the heatsink. This can manage current measuring from 10 to 400A and has similar properties to the stud base.

Plastic Module:
This provides the same benefits as the discrete plastic but comprises of many devices. It has a mounted heatsink attachment and can handle currents of up to 100A.

What is an SCR also called?

1.SCR diode
3.4-layer diode
4.4-layer device

What is a SCR symbol?

The following diagram represents the parts of a silicon controlled rectifier. The direction of the conventional current is represented through the arrow.
Anode (A)
Cathode (K)
Gate (G)

What is the construction of SCR?

The Anode, a positive electrode, is one of the three terminals of the SCR. It will be on the P-layer, while the Cathode, a negative electrode, will be on the N-layer. The terminal of SCR is controlled through the Gate. SCR is a semiconductor with four layers for the creation of NPNP or PNPN structure, as a result, finally three junctions are created i.e. J1, J2, and J3. Following diagram shows the Construction of SCR.
What is the construction of SCRIn comparison to the outside P and N layers, which will house the two electrodes, the center P and N layers, to which the gate terminal will be connected, will be weakly doped. The three types employed in the building are planar type, mesa type, and SCRs.

What’s the usage of silicon controlled rectifier?

Silicon controlled rectifiers are used when the power delivered must be controlled, such as when it is going to electric motors, relay controls, or induction heating elements. You can utilize thyristors as controlled rectifiers. It is functional as a static switch. It is utilized in fan speed regulators, lamp dimmers, and battery chargers. The speed of both DC and AC motors can be controlled with the use of SCR.

What are the 3 states of SCR?

There are 3 states of SCR or Thyristor:
1)Forward Blocking Mode (Off State)
2)Forward Conducting Mode (On State)
3)Reverse Blocking Mode (Off State)

What is the working principle of SCR

What is the working principle of SCR?

Following is the explanation to understand the working of thyristor:

Forward Blocking Mode

In this mode of operation, the cathode receives a negative voltage while the anode receives a positive voltage. The gate remains open, as there is no pulse received by it. After the voltage is supplied, junctions J1 and J3 will be forward-biased, while junction J2 will be reverse-biased. Reverse biassing J2 causes the depletion region to widen and create a barrier to conduction, which causes a small amount of current to flow from J1 to J3 as a result.

When the voltage provided to the SCR is raised or approaches the breakdown voltage of the SCR, avalanche breakdown occurs, causing junction J2 to become depleted. Current will begin to flow through the SCR after the Avalanche breakup takes place. The SCR is forward biased in this mode of operation even though there won’t be any current flow.

Forward Conduction Mode

In this mode, the SCR will be conducted in the ON state. To cause the SCR to conduct, we have two options: either we increase the applied forward bias voltage over the either we apply a positive voltage to the gate terminal or use the breakdown voltage. In this mode, the SCR will be conducted in the ON state. To cause the SCR to conduct, we have two options: either we increase the applied forward bias voltage over the either we apply a positive voltage to the gate terminal or use the breakdown voltage.

As a result of the avalanche breakdown, the junction J2 will become depleted as with the increase in the applied forward bias voltage between the anode and cathode, the SCR will begin to conduct.

Reverse Blocking Mode

No gates will remain open circuit if a pulse is supplied to them since the positive voltage is applied to the Cathode (-) and the negative voltage is given to the Anode (+) in this mode. While junction J2 will work in forward bias, junctions J1 and J3 will operate in reverse bias. Junctions J1 and J3’s reverse bias prevents electricity from passing through the SCR. A small amount of leakage current will flow as a result of the drift charge carriers in the forward-biased Junction J2, but not enough to trigger the SCR.

What are the characteristics of silicon controlled rectifier?

A “Thyristor” has different following characteristics.

StructureFour alternating semiconductor layers make up the PNPN structure
TerminalsSCR consists of 3 three terminals: Anode, Cathode and Gate.
PN JunctionsSCR consists of 3 PN junctions.
OperationSCR starts conduction in forward bias only when the positive gate pulse is provided.
Forward BlockingThe SCR can block the flow of current in forward bias only if there is no provision of the gate signal.
RectificationCan perform controlled rectification where the power to load can be controlled.
Voltage DropThe voltage drop across a conducting SCR is higher than diode around 1.5 volts.
Power LossesThe SCR has higher power losses.
Voltage RatingsThe SCR can handle very high voltages.
Power HandlingThe SCR is specially designed to handle very high-power applications.
ApplicationsMostly used for controlled rectification to manage the power fed to the load.

What are the advantages and disadvantages of SCR?

What are the advantages and disadvantages of SCR


1) It can handle large supply of voltage, current and power.
2) The Triggering circuit is simple. The SCR is easy to turn ON.
3) Protection of the SCR with the help of fuse.
4) It is able to control AC power.
5) Low cost.


1) SCR is a unidirectional device, hence it can only regulate power on the positive side of the AC supply, which means that it can only be used to control DC power.
2) A negative gate current is not permitted.
3) Must be switched on for each cycle in an AC circuit.
4) High frequencies cannot be used with SCR. It operates at a maximum frequency of 400 Hz.It is not easy to turn off the conducting SCR. We have to use special circuits called commutation circuits to turn off a conducting SCR.
5) It can conduct only in one direction. So it can control power only during one half cycle of AC.

What is difference between SCR and diode?



I hope this article can help you learn more about SCR and electronic components, and you can also browse other articles on our website for more information. In addition, if you are looking for a PCB assembly manufacturer, then IBE with 17 years of experience in PCB manufacturing, design and assembly will be a good choice.



Product Categories

Most Popular

Contact Us

IBE News

Related Posts


INDUSTRIES WE SERVE Click on each industry to learn more about how we make custom printed circuit board solutions for each market.Our industries served include

Read More »