Most transistors are constructed from silicon and germanium but other semiconductor materials are also used for construction of transistors. In today’s post, we will discuss how we can use a transistor as a switch and also see its practical working in different circuits. So let’s get started with Transistor as a Switch.
Transistor as a Switch
- For understanding the working of a transistor as switch we use bipolar junction transistor (BJT) and will construct its current versus voltage curve.
- There are 3 regions in which transistor operates the first one is active, second one is saturation and third one is cut-off region.
- In the first region that is active regions, transistor operates as an amplifier.
- But in other 2 regions that are saturation region in which transistor is in on state and cut-off region in which transistor is off, work as a switch.
- Now we discuss these three regions one by one with detailed.
Transistor Operation Region
- In the given figure the current and voltage characteristics curve is shown.
- In this curve you can see that the portion at the below of curve has pink colour is denoted as Cut-off portion and blue colour portion is known as saturation portion of the transistor.
Let’s discuss these two regions of transistors with detailed.
Transistor Cut-off Region
- In this operation region of a transistor the value of current at base is zero (IB=0) so the value of current at collector will also 0.
- The value of voltage across the collecter and emitter terminals (VCE) is higher that cause to make larger depletion layer in the transistor and zero current flows through the component.
- So the transistor is completely off meant it is an open cirucit.
- In this portion, the transistor will have such biasing that the amount of current at base terminal is maximum that causes to flow extreme current through the collector.
- The value of the voltage at collector and emitter terminals will be zero so there will be no depletion layer and a large amount of current will passes through the transistor and it behaves like a closed switch.
- In a simple way, we can define saturation region as it will occur when the current flows through the collector are extreme and the voltage across base terminals is 0.7 volts it is for NPN transistor.
- In the case of PNP, the emitter should be connected with a positive terminal of battery.
Working of Transistor as Switch
- For a practical understanding of transistor as a switch we discuss a circuit that is shown in a given figure.
- In this circuit, NPN transistor is used as a switch its collector and emitter is points are working as terminals of switch.
- A circuit that consists of the bulb as load is connected with the collector and emitter terminals of transistor.
- The base and emitter of transistor working as a controller that decides open and closed condition of switch.
- For closed the switch the battery is connected between base and emitter terminals.
- This source provides large amount of base current due to that collector current flows in the collector and emitter circuitry.
- The value of collector current will be larger if the resistance between collector and emitter is almost zero.
- In the above figure, you can see that emitter is at ground potential so we can also suppose that collector is also zero potential. So, in this case, resultant circuit can be constructed as.
- You can see that terminal of switch that are collector and emitter are closed and bulb is illuminating as a large collector current flowing through it.
- To open the terminals of switch we remove the current passing through the base.
- As Ic=βIb so due to zero value of base current the collector current also zero and it behave like an open switch.
Applications of Transistor as a Switch
- The structure of a transistor is such that collector current will not flow until there is no source of current at base.
- Due to this feature, it mostly used in different electronics circuitry as a switch.
- So we discuss such circuits that used transistor as a switch, for an explanation of such circuits we use NPN transistor.
- In given below circuit transistor is used as switch for on and off bulb. In this circuitry there is LDR, bulb and voltage divider circuitry is formed.
- This circuitry is operated in light and in dark it does not work.
- When photons of light collide with light-dependent resistance it starts to operate and current flows through base than collector that glows the bulb.
- In given below the transistor is used in heat operated switch the main element of this circuitry is a thermistor.
- A thermistor is a type of resistance that operates with the variation of temperature.
- There is an increment in its resistance with the decrement in temperature and with the increment of temperature resistance decreases.
- So in this circuitry when temperature increases the resistance of thermistor decreases so base current starts to flow that causes the movement of current through the circuit.
- Then at output alarm starts to operate after receiving a signal from the transistor.