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Choosing a Suitable PNP or NPN Transistor Switch
The procedure for choosing a suitable PNP transistor is exactly the same as that for an NPN transistor. In choosing a suitable switching transistor:
1. The transistor’s maximum collector current must be greater than the load current
2. The transistor’s maximum current gain must be at least 5 times the load current divided by the maximum output current from the IC
3. Choose a transistor which meets the requirements and making a note of its properties
4. Calculate an approximate value for the base resistor
5. A protection diode is necessary if the load is a motor or relay coil by connecting across the load to protect the transistor from the brief high voltage when the load is switched OFF.
The PNP variety is the cost commonly used transistor switch as shown below. Getting the transistor in a saturation state is the secret to making a transistor switch work properly. To guarantee our transistor switch is always saturated, it is best to calculate about 30% more current than we will need in actual practice. To ensure that the transistor switch is completely turned OFF, R2 is used in the circuit below. The resistor ensures that the base of the transistor does not go slightly negative which cause a very small amount of collector current to flow.
It is also possible to use an NPN transistor if a positive ground configuration is needed. The basic NPN transistor switching circuit is similar to the Common Emitter circuit with the difference of turning the transistor fully OFF or fully ON. The Base input terminal must be made more positive than the emitter in order for the Base current to flow.
A wide variety of applications are using transistor switches. Some are interfacing large current or high voltage devices such as motors, relay or lamps. The actual transistor used as a switch is not critical since any general purpose NPN or PNP transistor can be used virtually. It will only require knowing the minimum HFE and the power dissipation of the transistor. The use of transistor switches is usually a much more reliable and inexpensive alternative to using mechanical relays. It is a good practice to always use a diode when turning on any inductive load. If a power transistor is used to turn on a high current device, it may be necessary to use another lower current transistor switch.