CHAPTER 4 Half Wave Rectifiers AC DC Conversion

CHAPTER 4: Half Wave Rectifiers AC – DC Conversion

AC to DC CONVERSION (RECTIFIER) Single-phase, half wave rectifier a) Uncontrolled : R load, R-L load b) Controlled : R load, R-L load c) Free wheeling diode 2. Single-phase, full wave rectifier a) Uncontrolled: R load, R-L load b) Controlled : R load, R-L load c) Continuous & discontinuous current mode 1.

RECTIFIERS : INTRODUCTION � A rectifier converts AC to rectifier AC DC signal. � DEFINITION: Converting AC (from mains or other AC source) to DC power by using power diodes or by controlling the firing angles of thyristors/ controllable switches. � The purpose of a rectifier may be to produce an output that is purely dc @ to produce a voltage @ current waveform that has a specified dc component. � This reason limits the half wave rectifier in low applications only.

Uncontrolled Half - Wave Rectifier Circuit with R-LOAD

Uncontrolled Half-Wave Rectifier with Resistive Load + vd is D 1 • During the positive half-cycle of input voltage, D 1 conducts and the input voltage appears across the load. • During the negative half – cycle of the input voltage, D 1 is in a blocking condition and the output voltage is zero • The current produces a voltage across the load, which has the same shape as the positive half-cycle the input voltage. • Positive half cycle ⇒ diode is on • Negative half-cycle ⇒ diode is off

R R

Uncontrolled Half-Wave Rectifier with Resistive Load …cont. The average value of the output (load) voltage, Vdc or Vo or Vavg is determined by finding the area under the curve over a full cycle. (4. 0) (4. 1)

Uncontrolled Half-Wave Rectifier with Resistive Load …cont. The dc component of the current for the purely resistive load is (4. 2) The root-mean-square (rms) value of the output voltage, Vrms (4. 3) The rms value of the output current, Irms Average power absorbed by the resistor can be computed from

Uncontrolled Half - Wave Rectifier Circuit with RL-LOAD

Uncontrolled Half-Wave Rectifier with Resistive- INDUCTIVE Load

Uncontrolled Half-Wave Rectifier with Resistive- INDUCTIVE Load cont. .

Uncontrolled Half-Wave Rectifier with Resistive- INDUCTIVE Load cont. .

Uncontrolled Half-Wave Rectifier with Resistive- INDUCTIVE Load cont. .

Uncontrolled Half-Wave Rectifier with Resistive- INDUCTIVE Load cont. .

The Half-wave Controlled Rectifier � Normal rectifiers are considered as uncontrolled rectifiers. � Once the source and load parameters are established, the dc level of the output and power transferred to the load are fixed quantities. � A way to control the output is to use SCR instead of diode. � Two condition must be met before SCR can conduct: 1. The SCR must be forward biased (VSCR>0) 2. Current must be applied to the gate of SCR

� The simplest controlled rectifier uses a single device, such as a thyristor, to produce variable voltage D. C. from fixed voltage A. C. mains. The circuit arrangement is shown below

Voltage waveforms for two delay angles are shown below: � The thyristor is turned on in the positive half-cycle, some time after supply voltage zero, by the application of a gate pulse with delay angle α. � In the negative halfcycle, the thyristor is reverse biased and cannot switch on. � The larger the delay angle, the smaller is the average load voltage.

HALF - WAVE CONTROLLED RECTIFIER CIRCUIT WITH R-LOAD

Average voltage: � RMS current: � Average power absorbed by resister:

Example 1: Design a circuit to produce an average voltage of 40 V across 100Ω load resistor from a 120 Vrms 60 Hz ac source. Determine the power absorbed by the resistor. Briefly describe what happen if the circuit is add by inductor to produce the same average output.

Half - Wave Controlled Rectifier Circuit with RL-LOAD

q When ωt=π, the source becomes zero, but at this instant, the current through the circuit is not zero and there is some energy stored in the inductor. q When Vs becomes negative, the q Let Vs(t) be Vm sin (wt). At ωt=0, the current through the circuit is zero. q As wt becomes > 0, Vs becomes positive. q If a diode instead of an SCR has been used, the diode would start conduction at ωt = 0. q With an SCR, the conduction does not start till the SCR is triggered. Let the SCR be triggered when ωt=α. Then α is called the firing angle and the SCR continues to conduct. current through the circuit would not become zero suddenly because of the inductor. The inductor acts as a source and keeps the SCR forward -biased till the energy stored in the inductor becomes zero. q Let the current through the circuit become zero at ωt= and the value of β > π. q For β< ωt < 2π, the current through the circuit is zero

� Average output voltage: � RMS current: � Average power absorbed by the load:

Example 2: � A half wave rectifier has a source of 120 V RMS at 60 Hz. R=20 ohm, L=0. 04 H, and the delay angle is 45 degrees. Determine: (a) the expression for i(ωt), (b) average current, (c) the power absorbed by the load.

Half - Wave RECTIFIER WITH FREEWHELLING DIODE

� The average voltage and current can be increased by making β=1800. � This can be done by adding a freewheeling diode Dm. � The effect of this diode is to prevent a negative voltage appearing across the load, and as a result, the magnetic stored energy is increased � At ωt = π, the current from D 1 is transferred to Dm and this process is called commutation of diodes. � Depending on the load time constant, the load current may be discontinuous. � Load current is discontinuous with a resistive load and continuous with very high inductive load. � The continuity of the load current depends on its time constant τ = ωL/R.