Bipolar Junction Transistor As a Switch

Switching:

The Bipolar junction transistors can also be used for switching in a circuit. It means that we can use BJT as for current controlling switch. The current in the base will decide whether the current will flow from emitter to collector or not, that’s how it can be used as switch. BJT in circuit used for switching the high voltage or high power circuit with the very low power control variable. We will try to figure it out with the help of example.

Example of BJT as Switch:

As shown in figure below simple lamp circuit with a battery. As switch is open then lamp is off as in fig (1), while when switch is closed, the circuit is closed and lamp is glowing in fig (2).

Fig (1)

Fig (2)

Now when we used bipolar junction transistor in place of switch in above circuit and connect a 1 K resistor with base to provide some current for the base. NPN type of Bipolar junction transistor is being used in this circuit. PNP could also be used, it is just difference of polarity in connections. Keep in mind that the direction of flow of current should be opposite to the direction of arrow on emitter of BJT. The direction of current flow is shown by red arrows in fig (3).

Fig (3)

The BJT do switching in between two state regions; cut off region and saturation region. When there will be no resistor connected with base or if there is open circuit at base of BJT, then the flow of current from emitter to collector will not occur and transistor would be in its “cut off” state. And thus lamp will not glow.

While when there will be resistance connected with the base, some current will flow through base (where we have studied before base current is controlling current). When there will be current through base then current will pass through collector and lamp will glow up and transistor would be in "saturated" state.

So we have seen that we can do switching of high level with the very small amount of current in base. That’s a big advantage of having a BJTs.

Applications:

Due to this switching property of BJT, it has many application in industry and smart production.

1. In a thermocouple circuit, we can use BJT as switch while taking heat as a source for base current. So that with heat on switch on.

2. In automatic door, with the presence of a person near the door, will initiate the base and switch will open.

Like these it has many application in this era.

Electric Potential: definition, explanation, example.

Definition:

"Electric potential is the amount of word done in moving a point test charge from its original position(reference position) to some specific point or position, while moving the charge in direction opposite to direction of field in which charge is present."

Explanation:-

The definition above seem to be difficult to understand the electric potential but I will explain it briefly.
Consider their is a positive point charge A, which have its own electric field around it. And there is another positive test charge B which has no electric field.

 When we will move the test charge B towards the electric field of point charge A then test charge B will move away from the point charge A. This is natural that charge always move from high potential to low potential.

low potential is the area where the charge feels less force while high potential is the area where the force is high.

So to move charge, against its natural movements. we have to provide electric potential or force or to do work. To move test charge B towards the charge A we have to apply some potential which is electric potential, to move the charge from low potential to high potential. So charge B will gain potential energy while moving towards high potential from low potential.

Electric potential is the basic studies of charges behavior with each other. So if you feel there is some type of confusion feel free to comment. The post will be update!