Scr working pdf

Since J2 is reverse biased the width of the depletion region increases and it acts as an obstacle for conduction, so only a small amount of current will be flowing from J1 to J3. When the voltage applied to the SCR is increased and if it reaches the breakdown voltage of the SCR , the junction J2 gets depleted due to avalanche breakdown. Once the Avalanche breakdown occurs the current will start flowing through the SCR. In this mode of operation, the SCR is forward biased, but, there will not be any current flow.

Forward Conduction Mode. We can make the SCR conduct in two different ways, one we can increase the applied forward bias voltage beyond the breakdown voltage or else we can apply a positive voltage to the gate terminal.

When we increase the Applied forward bias voltage between the anode and cathode the junction J2 will be depleted due to the avalanche breakdown and the SCR will start conducting. We are not able to do this for all the applications and this method of activating the SCR will eventually reduce the lifetime of the SCR. The applied positive voltage will help the SCR to move to the conduction state. During this mode of operation, the SCR will be operating in forward bias and current will be flowing through it.

Reverse Blocking Mode. During this mode of operation the Junctions J1 and J3 will be reverse biased and the junction J2 will be forward biased. Since the junctions J1 and J3 are reverse biased there will not be any current flowing through the SCR. Although there will be a small leakage current flowing due to the drift charge carriers in the forward-biased Junction J2, it is not enough to turn on the SCR. The VI characteristics of the SCR are obtained by operating the SCR in three different regions, namely forward blocking region, forward conduction region and reverse blocking region.

When the SCR is operating in the reverse blocking mode , there will be small leakage current flowing in the reverse direction of the SCR which is mentioned as the reverse leakage current in the graph, the reverse leakage current will be located at the negative quadrants of the graph. Now if you apply positive voltage to anode and negative voltage to cathode the SCR will start operating in the forward blocking mode and a small leakage current will be flowing through the SCR in the positive direction, hence the curve starts rising to a certain level in the positive quadrants of the graph which is mentioned as the forward leakage current.

Once the graph reaches a certain voltage level called the Breakdown voltage or if the gate current Ig is applied to the SCR, the SCR moves to the conduction mode and a high amount of current starts flowing through the SCR. The current flow is represented as the forward conduction in the VI curve. It is not possible to discuss all the Thyristor applications , but basically, they are used to control the current or voltage across a device.

The motor will be having two windings namely field winding and armature winding. By controlling the current given to the armature winding we can control the speed of the DC motor. The armature winding of the motor is connected to an AC supply through a transformer and two SCRs connected in parallel with one another.

During the positive half cycle of the AC supply, the SCR1 is forward biased and starts conducting if the gate pulse is applied, the current to the armature winding will be flowing through the SCR1. By varying the trigger pulse given to the gates of the respective SCRs we will be able to control the input given to the DC motor hence it controls the speed of the motor. There are many types of SCR based on the specification and application.

The SCRs are available in different kinds of packages that can be used for different kinds of applications. The SCRs are available in the following packages. Discrete Plastic: The discrete Plastic package is a commonly known type of SCRs that has three pins attached to a plastic-covered semiconductor material.

They are available up to 25A and V applications , they can be easily mounted on any type of circuit with a large number of other components. In the normal operating conditions of SCR, anode is held at high positive potential w. In a silicon controlled rectifier, load is connected in series with anode.

Under this condition, junction J 2 is reverse biased while junction J 1 and J 3 are forward biased. Hence, the situation in the junctions J 1 and J 3 is just as in a npn transistor with base open. However, if the applied voltage is gradually increased, a stage is reached when the reverse biased junction J 2 breaks down.

The applied voltage at which SCR conducts heavily without gate voltage is called Breakover voltage. The SCR can be made to conduct heavily at smaller applied voltage by applying a small positive potential to the gate as shown in fig.

Now junction J 3 is forward biased and junction J 2 is reverse biased. The electrons from n-type material start moving across junction J 3 towards left whereas holes from p-type towards the right. Consequently, the electrons from junction J 3 are attracted across the junction J 2 and gate current starts flowing. The increased current in turn makes more electrons available at junction J 2. This process continues and in an extremely small time, junction J 2 breaks down and the SCR starts conducting heavily.

Once SCR starts conducting, the gate loses all control. Even if gate voltage is removed, the anode current does not decrease at all. The only way to stop conduction i. The SCR shown in fig. Thus, the equivalent circuit of SCR is composed of pnp transistor and npn transistor connected as shown in fig 4.

It is clear that collector of each transistor is coupled to the base of of the other, thereby making a positive feedback loop. The working of SCR can be easily explained from its equivalent circuit.

Assume the supply voltage V is less than breakover voltage as is usually the case. With gate open i.