1. Thyristor Half-wave Rectifiers
Dr John Fletcher

2. 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.

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

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

5. 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.

6. =193o
φ=70o

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

8. 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

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

10. 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.