Test: Transistor As a Switch - Electrical Engineering (EE) MCQ

 This is an inverter, in which the transistor in the circuit is switched between cut off and saturation. The load, for example, can be a motor or a light emitting diode or any other electrical device.

Depending on the type of load, a collector current is induced that would turn on the motor or LED. The transistor in the circuit is switched between cut off and saturation. The load, for example, can be a motor or a light emitting diode or any other electrical device.

Output devices like LED’s only require a few milliamps at logic level DC voltages and can therefore be driven directly by the output of a logic gate. However, high power devices such as motors or lamps require more power than that supplied by an ordinary logic gate so transistor switches are used.

From the cut off characteristics, the base emitter voltage (V_BE) in a cut off region is less than 0.7V. The cut off region can be considered as ‘off mode’. Here, V_BE > 0.7 and I_C=0. For a PNP transistor, the emitter potential must be negative with respect to the base.

Here, the transistor will be biased so that maximum amount of base current is applied, resulting in maximum collector current resulting in minimum emitter voltage drop which results in depletion layer as small as possible and maximum current flows through the transistor.

From the saturation mode characteristics, the transistor acts as a single pole single throw solid state switch. A zero collector current flows. With a positive signal applied to the base of transistor it turns on like a closed switch.

Sometimes DC current gain of a bipolar transistor is too low to directly switch the load current or voltage, so multiple switching transistors is used. The load is connected to ground and the transistor switches the power to it.

Sometimes DC current gain of a bipolar transistor is too low to directly switch the load current or voltage, so multiple switching transistors is used. The load is connected to supply and the transistor switches the power to it.

• Concept:-A transistor can be used in a myriad of applications due to its capability to act as an amplifier or a switch. Here are some key uses:
• Amplifier: Transistors can amplify weak electrical signals, making them stronger. This is crucial in devices like radios, televisions, and other audio equipment where small input signals need to be amplified to drive speakers or other output devices.
• Switch: In digital circuits, transistors can function as switches, turning current on or off. This property is fundamental in the operation of computers and digital electronics, where transistors control the flow of electricity based on binary logic.
• Oscillator: Transistors can be used in circuits designed to generate oscillating signals (AC signals), which are important in radio transmission, clocks, and computer processors.

Additional Information

• Voltage Regulation: They can be used in circuits designed to stabilize voltage, ensuring that electronic devices receive constant voltage even when the supply voltage fluctuates.
• Signal Modulation: Transistors can modulate signals, which is essential in communication devices for encoding information onto a carrier wave for transmission.
• Sensing and Detection: Transistors can be utilized in sensing circuits to detect and respond to light, temperature, and other environmental factors, often found in sensors.
• Memory Storage: In modern electronics, transistors are integral components in memory devices, including RAM (Random Access Memory), flash memory, and SSDs (Solid State Drives), for storing data by representing binary information.
• Logic Gates: By configuring transistors in different ways, they can perform basic logical operations like AND, OR, NOT, NAND, NOR, XOR, and XNOR, which are the building blocks of digital electronics and computing.
• Analog to Digital Conversion: Transistors are used in circuits that convert analog signals to digital form, making the signals compatible with digital systems.
• Power Control: In power electronics, transistors manage and switch high-power devices efficiently, including inverters, power supplies, and motor controllers.
• This versatility makes transistors foundational to modern electronics, enabling a broad spectrum of technologies from the smallest handheld devices to large-scale industrial applications.

Low voltage switching devices:

• Normally, electrical switching devices rated up to 1kV are termed as low voltage switching devices or low voltage switchgear.
• The term LV Switchgear includes low voltage circuit breakers, switches, isolators, HRC fuses, MCB(miniature circuit breaker), and Molded case circuit breakers (MCCB), etc.
• The main contacts of the LV switching devices usually carry the current at low voltage under normal operation.
• During the faulty condition, the current flows through the main contact is very high. And the contact should carry this current for some time. In that case there will be I²R losses in the main contacts.
• Hence, thermal consideration comes into the picture, because due to I²R loss heat will be developed, if the heat developed is more than the thermal capacity of the main contacts then properties of the main contacts may change, which may lead to permanent damage of the LV switching device.
• To reduce this thermal effect we can use low contact resistance so that the losses can be reduced.
• The thermal capacity of the main contacts should be high.
• And one more thing low value of resistance means the area of cross-section of the contacts is more, which helps the device to increase the thermal capacity by the radiation process.