Thyristor Half-wave Rectifiers Dr John Fletcher John.Fletcher@eee.strath.ac.uk

Half-wave Thyristor Rectifier A controlled rectifier. The flywheel diode is optional but alters the transfer function of the circuit. The thyristor is triggered at ωt=α. Let us consider the case of circuit with flywheel diode.

With flywheel diode Mean Load Voltage Mean Load Current As α is increased, Vav and Iav tend to zero.

Without flywheel diode Mean Load Voltage Mean Load Current β is a function of α and the load impedance angle φ.

Using Chart We need to know If it is a rectifier α=0o. φ If it is a rectifier α=0o. Examples: α=60o and φ=70o from graph β=253o.

=193o φ=70o

Calculating Peak Diode Current Peak diode current occurs when the thyristor commutates off and the freewheel diode conducts the load current.

Active Loads A load that has a source of emf (or current) In the circuit shown the EMF has the effect of forcing the current to zero earlier than with a passive load. Current starts at α and ceases at β

Active Loads The average load voltage And the average current which is true for all rectifiers.

Example 1 EX.1 A half wave, controlled rectifier supplies a load consisting of R=10 and L = 50mH from a 50Hz, 240V supply For a)=0 and b)=90 calculate (1) the mean load current and (2) the mean load voltage. If a flywheel diode is connected across the load calculate (1) and (2) for  = 90, and the peak flywheel diode current.