Controlled rectifiers:
A controlled rectifier is an electronic device that converts alternating current (AC) to direct current (DC). It is widely used in various applications such as power supplies, motor drives, and battery chargers. The controlled rectifier consists of semiconductor devices like thyristors or diodes that control the flow of current in the circuit.

Function of a freewheeling diode:
A freewheeling diode, also known as a flyback diode or a snubber diode, is connected in parallel with an inductive load in a controlled rectifier circuit. Its main function is to provide a path for the inductive load current during the non-conduction period of the main thyristors or diodes.

Inductive load:
An inductive load is a type of load that stores energy in an electromagnetic field. It is characterized by the presence of inductance, which opposes changes in current flow. Examples of inductive loads include motors, solenoids, transformers, and relays.

Need for a freewheeling diode:
In a controlled rectifier circuit, when an inductive load is connected, it causes a problem known as the back EMF (electromotive force). When the main thyristors or diodes turn off, the inductive load tries to maintain the flow of current due to its inherent property of self-induction. This results in a rapid decrease in the current, which induces a reverse voltage across the inductive load.

If there is no freewheeling diode present, this reverse voltage can cause several issues:

1. Voltage spikes: The sudden decrease in current induces a high reverse voltage across the inductive load, leading to voltage spikes. These spikes can exceed the breakdown voltage of the semiconductor devices, causing their failure.

2. High voltage stress: The reverse voltage can stress the insulation of the circuit, leading to breakdown and potential damage to the components.

3. Electromagnetic interference (EMI): The voltage spikes can generate electromagnetic interference, affecting the performance of other electronic devices in the vicinity.

Role of the freewheeling diode:
The freewheeling diode is connected in parallel with the inductive load. During the non-conduction period of the main thyristors or diodes, the freewheeling diode provides a low-resistance path for the inductive load current. It allows the current to circulate through the inductor, dissipating the stored energy in the form of heat.

By providing a safe path for the inductive load current, the freewheeling diode prevents the reverse voltage spikes, protects the semiconductor devices, reduces EMI, and ensures the smooth operation of the controlled rectifier circuit.

Conclusion:
In a controlled rectifier circuit, a freewheeling diode is necessary when the load is inductive. It serves to protect the circuit from voltage spikes, prevent damage to the semiconductor devices, and reduce electromagnetic interference.

Controlled Rectifier:

A controlled rectifier is an electronic device that converts alternating current (AC) into direct current (DC) using power semiconductor devices such as thyristors or diodes. It is widely used in various applications including power supplies, motor drives, and voltage regulators.

Freewheeling Diode:

A freewheeling diode, also known as a flyback diode or a snubber diode, is a diode connected in parallel with an inductive load in a circuit. Its purpose is to provide a path for the inductive current when the controlling switch or thyristor is turned off.

Need for Freewheeling Diode:

In a controlled rectifier circuit, the load can be either inductive, resistive, or capacitive. However, the need for a freewheeling diode arises primarily when the load is inductive.

Explanation:

When an inductive load is connected to a controlled rectifier circuit, it stores energy in its magnetic field when the thyristor or switch is conducting. When the thyristor is turned off, the magnetic field collapses, causing a sudden change in the current.

Without a freewheeling diode, this sudden change in current can result in a high voltage spike across the inductive load. This voltage spike is known as a back EMF (electromotive force) and can damage the semiconductor devices or cause electromagnetic interference in the circuit.

Working of Freewheeling Diode:

The freewheeling diode provides an alternate path for the inductive current when the controlling thyristor or switch is turned off. When the thyristor is conducting, the diode is reverse-biased and does not conduct any current. However, when the thyristor is turned off, the diode becomes forward-biased, allowing the inductive current to flow through it.

The freewheeling diode effectively dissipates the energy stored in the inductive load, preventing the voltage spike and protecting the semiconductor devices. It also ensures a smooth and continuous current flow through the load.

Conclusion:

In a controlled rectifier circuit, a freewheeling diode is necessary when the load is inductive. It provides a path for the inductive current when the controlling thyristor or switch is turned off, preventing voltage spikes and protecting the circuit components.