Basic Power Electronics (22427) Quiz

To protect a DC circuit of 240V with a 25V selenium cell, we need to ensure that the voltage of the selenium cells is higher than the circuit voltage. Since the voltage of each selenium cell is 25V, we can calculate the number of cells required by dividing the circuit voltage (240V) by the voltage of each cell (25V). This gives us 9.6, which rounds up to 10 cells. Therefore, 10 selenium cells are required to protect the DC circuit.

Gate triggering is the most reliable and efficient method to turn ON a Silicon Controlled Rectifier (SCR). By applying a positive voltage pulse to the gate terminal of the SCR, the device can be triggered into conduction. This method allows for precise control and fast switching of the SCR, making it suitable for various applications. Forward voltage triggering relies on applying a forward voltage to the anode-cathode junction, which may not provide the same level of control and efficiency. dv/dt triggering and thermal triggering are not commonly used methods for turning ON an SCR.

The value of the holding current is 10mA. Holding current refers to the minimum current required to keep a device or component in its current state, typically in the ON state for a latching device. In this case, if the latching current is 12mA, the holding current would be lower than that value to ensure the device remains in the latched state. Therefore, the correct answer is 10mA.

The correct answer is "VGS > VGS(TH)". In order for an IGBT (Insulated Gate Bipolar Transistor) to turn on, the gate-source voltage (VGS) must be greater than the threshold voltage (VGS(TH)). This threshold voltage is the minimum voltage required to activate the IGBT and allow current to flow through it. Therefore, when VGS exceeds VGS(TH), the IGBT will turn on and conduct current.

When the pot (potentiometer) in a light dimmer is in the mid position, it means that it is neither turned fully clockwise (high intensity) nor fully counterclockwise (low intensity). This position allows for a medium intensity of light to be emitted.

The correct answer is "All of the above." This means that all of the mentioned methods can be used to turn off SCS (Silicon Controlled Switch). SCS can be turned off by reducing the anode current below IH, by applying a negative gate pulse to GK, or by applying a positive gate pulse to GA. Each of these methods can effectively turn off the SCS device.

The correct answer is "Both 1 & 2". In an IGBT (Insulated Gate Bipolar Transistor), there are three terminals: Drain, Source, and Gate. However, the Drain and Source terminals are equivalent to the Collector and Emitter terminals in a regular bipolar transistor. Therefore, both options 1 (Drain, Source & Gate) and 2 (Collector, Emitter & Gate) are correct.

In a 3 phase half wave controlled rectifier, each thyristor conducts for a period of 120 degrees. This means that during each cycle of the input waveform, the thyristor is turned on for 120 degrees and then turned off for the remaining 240 degrees. This allows for the rectification of the input AC waveform into a pulsating DC waveform.

A diode is called an uncontrolled rectifier because it allows current to flow in only one direction, from the anode to the cathode, without any control or regulation. It acts as a simple switch, conducting when forward biased and blocking when reverse biased. Unlike other devices like SCR, BJT, and PUT, a diode does not have any gate or control terminal to regulate the flow of current. Therefore, it is considered an uncontrolled rectifier.

The reverse characteristics of an SCR (Silicon Controlled Rectifier) are similar to that of a diode. Both the SCR and diode allow current to flow in one direction (forward biased) and block it in the opposite direction (reverse biased). This is because both devices have a p-n junction that behaves like a diode. The TRIAC, DIAC, and UJT do not have the same reverse characteristics as an SCR.

The correct answer is "Both 1 & 2". Latching current is defined as the minimum on-state current required to keep the device (SCR) in the on state after the triggering pulse has been removed. This means that even after the triggering pulse is no longer present, the SCR will remain turned on as long as the on-state current is above the latching current threshold.

In TRIAC, Mode II and Mode III are less sensitive compared to the other modes. Mode II is a quadrant II operation where the gate current is positive and the main terminal current is negative. Mode III is a quadrant III operation where the gate current is negative and the main terminal current is positive. These modes are less sensitive because the TRIAC requires less gate current to trigger the device in these modes compared to Mode I and Mode IV.

In order to turn off a GTO (Gate Turn-Off) device with a voltage rating of 4000V and a current rating of 3000A, a gate current of -750A is required. The negative sign indicates that the gate current needs to be negative in order to turn off the device.

A Power MOSFET is a type of transistor that is used for switching and amplifying electronic signals. It is specifically designed to handle high power and voltage levels. One of its key characteristics is that it can be controlled by varying the voltage applied to its gate terminal. This means that it can effectively control the flow of current in a circuit by adjusting the voltage across it. Therefore, a Power MOSFET is considered a voltage control device.

In a three-phase half wave controlled rectifier with a resistive load, the primary side of the transformer is connected in a delta configuration. This means that the three primary windings of the transformer are connected in a triangular shape, forming a closed loop. The delta connection allows for a balanced and efficient distribution of the three-phase AC input voltage to the rectifier circuit. This configuration is commonly used in power systems to convert AC power to DC power with minimal losses.

The intrinsic stand off ratio refers to the ratio of the collector current to the emitter current in a bipolar junction transistor (BJT) when the base-emitter junction is not forward biased. This ratio is typically between 0.5 and 0.8, indicating that the collector current is about half to four-fifths of the emitter current. This range is important for understanding the amplification characteristics and biasing requirements of a BJT.

In a 2 quadrant converter, the polarity of the output voltage can be either positive or negative, depending on the input and load conditions. However, the output current of a full converter has only one polarity. This means that the current always flows in the same direction, regardless of the input and load conditions. Therefore, the correct answer is Full converter.

A complementary SCR, or Silicon Controlled Rectifier, is a device that consists of two SCRs connected in parallel. It is designed to provide bidirectional current flow and is commonly used in applications such as motor control and power inverters. The PUT, or Programmable Unijunction Transistor, is not a complementary SCR. It is a three-layer, four-terminal device that functions as a voltage-controlled trigger for SCRs and other thyristors. Therefore, the correct answer is PUT.

If α approaches unity, it means that α is very close to 1. In the given equation, if α is close to 1, the numerator (α2Ig + IcEo) will dominate over the denominator (1 - α). As a result, the anode current will increase.

The correct answer is "switch with an arrow" because a chopper is an electronic device used to convert a fixed DC voltage to a variable DC voltage. The switch with an arrow symbol represents a semiconductor switch, such as a transistor or a MOSFET, which is commonly used in chopper circuits to control the flow of current and vary the output voltage. The arrow indicates the direction of current flow when the switch is closed, allowing the voltage to be chopped or switched on and off rapidly to achieve the desired output.

The arrow in the MOSFET symbol indicates the direction of electron flow. In a MOSFET, electrons are the charge carriers, and the arrow represents the movement of these electrons. The arrow points from the source to the drain, indicating the direction in which the electrons flow through the channel when a voltage is applied to the gate terminal. Therefore, the correct answer is "Direction of electron flow."

The correct answer is "It convert AC to pulsating DC." The freewheeling diode is not responsible for converting AC to pulsating DC. Its main function is to prevent the reversal of load voltage by providing a path for the current when the main rectifier is turned off. This allows the thyristors to regain their blocking state and protects the circuit from voltage spikes.

Crowbar protection is the best option for Thyristor overcurrent protection. A crowbar protection circuit is designed to quickly short-circuit the Thyristor in case of overcurrent, preventing damage to the device. It operates by sensing the overcurrent condition and triggering a short circuit across the Thyristor terminals, effectively diverting the excessive current away from the Thyristor. This fast and effective response helps to protect the Thyristor from damage and ensures the safe operation of the circuit. Selenium diode protection, snubber circuit protection, and metal oxide varistor are not specifically designed for overcurrent protection in Thyristors, making crowbar protection the better option.

The correct answer is 4K Ohm to 10k Ohm. This is because the interbase resistance (RBB) in a UJT (Unijunction Transistor) exists within this range.

The correct answer is "It is mimimum forward anode current required to keep SCR on if the current fall below the holding level,then SCR turn off". This means that the holding current is the minimum amount of current required to keep the SCR (Silicon Controlled Rectifier) in the "on" state. If the current falls below the holding level, the SCR will turn off.