SINGLE PHASE HALF WAVE CONTROLLED RECTIFIER
These rectifiers are widely used in many electronics based converters, only one SCR is used in half wave single phase rectifier and it is placed in between the AC source and the load the controlled rectifier, performance is depended on the type of the output circuit
They would provide an intermediate unregulated DC voltage and it can be further processed to get a regulated DC or AC output
CLASSIFICATION
According to the load requirements the single-phase rectifiers are classified into
- R LOAD
- RL LOAD
- RL LOAD with Freewheeling diode
R LOAD
In this type resistance is used as load, in the positive half cycle anode is connected to the positive terminal of the supply and cathode to the negative and the thyristor is forward biased. When the thyristor is fired full supply voltage is applied to the load and thus the load is directly connected to the AC supply by varying the firing angle the output voltage can be controlled, the angle at which the thyristor conducts is called the conduction angle
During the negative half cycle, the negative terminal of the supply is connected to the anode and the cathode to the positive and the thyristor is reverse biased
RL LOAD
In this type of inductance is used as the load and the load is highly inductive during the positive half cycle anode is connected to the positive and cathode is connected to the negative of the supply and the thyristor is forward biased the load current will increase when the thyristor is fired and from this instant the supply voltage appears across the load
Due to the inductive load the increase in current is gradual during the time t01 to t1 , during the time T1 the supply voltage reverses but the thyristor continues to conduct because current through the inductance can’t be reduced to zero
In the negative half cycle anode is connected to the negative terminal and cathode to the positive and the thyristor is reverse biased, a current will flow till the energy stored in the inductance is dissipated in the load resistor
Current will flow to the instant t11 due to the energy stored in the inductor at the instant t11 the load current is zero and thyristor turns off due to the negative supply voltage
The above cycle repeats at the instant t02 when the pulse is applied and thus the conducting period of SCR increase due to the inductive load
RL LOAD using Freewheeling diode
A freewheeling diode is connected in parallel with the load in this half wave rectifier and this diode is known as commutating diode because its function is to transfer load current away from the rectifier when the load voltage goes to reverse state
- The diode prevents the load voltage reversal except for the small diode voltage drop
- The load current will transfer away from the rectifier by the diode and thus the thyristor regain its blocking state
Due to the diode thyristor won’t be able to conduct beyond 180 degree, power flow from the input will take place only when the thyristor is conducting with the diode the freewheeling action takes place and no power will be returned to the source and thus the freewheeling diode improves the input power factor