Thyristor Forward Conduction and Blocking Modes

Thyristor is a semiconductor device that can switch high voltages and currents. It has three terminals, anode, cathode, and gate. When the anode is connected to a positive voltage and the cathode to a negative voltage, the thyristor is in the forward blocking mode or off state. When a positive pulse is applied to the gate terminal, the thyristor switches into the forward conduction mode or on state. The thyristor remains in the on state even after the gate pulse is removed until the anode current drops below a certain level.

Methods to Switch Thyristor Modes

There are different methods to switch thyristor modes, including:

1. dv/dt triggering method - This method uses a high rate of voltage rise across the anode-cathode terminals to trigger the thyristor into the forward conduction mode. This method is not reliable and can cause false triggering.

2. Applying a negative gate signal - This method can switch the thyristor from the on state to the off state by applying a negative pulse to the gate terminal. This method is not commonly used because it requires a high negative voltage.

3. Applying a positive gate signal - This method is the most commonly used method to switch the thyristor from the off state to the on state. A positive pulse is applied to the gate terminal to trigger the thyristor into the forward conduction mode.

4. Applying a reverse voltage across anode-cathode terminals - This method can switch the thyristor from the on state to the off state by applying a reverse voltage across the anode-cathode terminals. This method is used in AC circuits to control the power by turning off the thyristor at the end of each half-cycle.

Conclusion

The thyristor can be switched from the forward blocking mode to the forward conduction mode by applying a reverse voltage across anode-cathode terminals. This method is commonly used in AC circuits to control the power by turning off the thyristor at the end of each half-cycle.