Firing Circuits for Thyristor | R Firing Circuit of SCR in Hindi | Power Electronics

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In this video you will learn about firing circuits for SCR in hindi.
What Are The Most Common Thyristor Firing Methods?
Thyristors are controlled by sending the correct signal to the gate connection of the device. It will then continue to let current flow until the gate signal is removed and the voltage through it reaches zero. There are two main methods of firing the thyristors:
Zero Voltage Crossover Firing (burst pulse)
Phase angle firing
What Is Zero Voltage Crossover Firing (burst pulse)?
Zero Crossover Firing sends the gate signals to the thyristors only when the voltage through it is zero. Therefore, the thyristor will turn on and off only at the zero voltage crossover point of the sine wave which occurs every half cycle. The fully on and off periods are based on burst pulses of time which are set by the firing circuit and can be adjusted to suit the user’s load. For example if the total cycle time is set to 2 seconds and a 50% power output from the thyristor controller is required then the output will be fully on for 1 second and fully off for 1 second, this is shown in the diagram below:
Why Use Zero Crossover Firing (burst pulse)?
Zero Crossover Firing (burst pulse) is the preferred method for standard resistive loads, in particular in the HVAC industry. By ensuring that the thyristors are always switched on at the zero voltage point of the mains half cycle, fast rising high voltage wave fronts are avoided which significantly reduces the level of generated electrical noise. This technique is cost effective, as the requirement for substantial filtering is now eliminated. Another benefit of this method is that it can be used on a three phase system with only two of the phases controlled by thyristors; this again saves cost and reduces heat losses produced by the thyristors.
What Is Phase Angle Firing?
Phase angle firing turns the thyristors on at a specific point in each half cycle of the mains frequency. Varying this switch-on point between the initial and final zero voltage points of the sine wave provides a variation from 100% down to 0% of the load voltage (and hence the output power).
Why Use Phase Angle Firing?
Phase angle control allows an extremely tight, accurate and smooth control of the load but will generate a significant amount of electrical noise unless substantial filter networks are employed. The level of electrical interference increases as the switch on point approaches the maximum peak voltage of the sine wave. Power factor is a calculation of available power vs. consumed power, at full output the phase angle method provides a power factor of 1 but this decreases as the output of the thyristors is reduced, i.e. the power factor is 0.5 at 50% output. The phase angle control method is used mainly for inductive loads, such as transformer coupled loads, and also some specific resistive loads which benefit from features that phase angle control can offer such as soft start to limit high inrush current.
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