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. ![]() It is also possible to use an NPN transistor if a positive ground configuration is needed. 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. To ensure that the transistor switch is completely turned OFF, R2 is used in the circuit below. To guarantee our transistor switch is always saturated, it is best to calculate about 30% more current than we will need in actual practice. Getting the transistor in a saturation state is the secret to making a transistor switch work properly. The PNP variety is the cost commonly used transistor switch as shown below. 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.Calculate an approximate value for the base resistor.Choose a transistor which meets the requirements and making a note of its properties.The transistor’s maximum current gain must be at least 5 times the load current divided by the maximum output current from the IC.The transistor’s maximum collector current must be greater than the load current.No matter whether the P section is the emitter or base, the arrow always points in the direction of hole flow. The PNP is distinguished from the NPN transistor by the direction of the arrow on the emitter. The PNP’s emitter is “emitting” electrons and the NPN’s emitter is “emitting” holes. To keep a straight emitter notation, the arrow in a schematic is always the emitter. The layers of semiconductor material used to make the transistor are denoted by the letters. ![]() The two types of standard transistors are PNP and NPN which differs in circuit symbols.
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