Exp 1 Single-Phase Rect1

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Universiti Sains MalaysiaElectrical Engineering DepartmentAdvanced Power Electronic Laboratory EEK471

The Single-Phase Half-wave RectifierOBJECTIVE

To become familiar with the operating principles of the diode.

To demonstrate the operation of a half-wave rectifier, a freewheeling diode and a batterycharging circuit.

INTRODUCTION

Operating principles of the diode

A diode is a two-terminal semiconductor device. The two terminals are called respectively theanode A and cathode K. The diode operates as a high-speed switch which has no movable parts.The symbol for a diode and the characteristic curve of a diode are shown in Figure 1.

Figure 1: Power Diode

In summary, a diode turns ON only when a forward voltage is applied. It remains ON until thecurrent stop flowing. The characteristic curve shows the current flows through the diode versusthe voltage across it. The curve shows that virtually no current flows when the diode is reversedbiased, but that the current increases very rapidly when it is forward biased.

As long as current, I flow between A and K the diode acts as a closed switch. When I stopsflowing, the diode turns off and becomes an open switch.


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Half-wave rectifier

The half-wave rectifier is so called because it delivers a half-cycle of dc output for every full cycleof the applied ac voltage. With a resistive load, the circuit operates as follows (see Figure 2).

Figure 2: Operation of a power diode pure resistive circuit

The conduction angle of a rectifier component is equal to the time that the component conducts

current during each cycle, divided by the period and multiplied by 360. In Figure 2, the

conduction angle is 180.

With an inductive load, the circuit operates as follows (see Figure 3). The conduction angle of thediode has been increased because the inductor must restore the energy which has been suppliedby the source. This principle is fundamental in defining the on state of a diode in inductive circuits.The current in an inductor varies slowly. This is because inductors oppose variations in theircurrent. Also, the phase of I lags that of E.

Figure 3: Operation of a power diode inductive circuit


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Rectifier with free-wheeling diode

When the load is inductive, the negative part of the output voltage waveform reduces the averageoutput voltage. To prevent the output voltage from going negative, a free-wheeling diode can beplaced in the circuit as shown in Figure 4.

Figure 4: Half-wave rectifier with free-wheeling diode

When the output voltage begins to go negative, the free-wheeling diode starts conducting. Thismaintains the output voltage at approximately zero while the energy stored in the inductor isbeing released (see Figure 4).The output voltage waveform is the same as for a purely resistive

load, and the average output voltage is therefore greater than it would be without the free-wheeling diode.

Battery charger circuit

A simple battery charging circuit consists of one diode and a current limiting device (see Figure5). The current limiting device can be either a resistor or an inductor connected in series. Thiscircuit transfers power from an ac source to a dc source (active dc load).

Figure 5: Battery charger with a series resistor


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Figure 6 shows the voltage and current waveforms for a battery charger with a series resistor.Current flows only between angles 1 and 2, when the ac source voltage ES is greater than thebattery voltage EB. This circuit provides no means for electronically controlling the chargingcurrent.

Figure 6: Voltage and current waveforms in the battery charger circuit

EQUIPMENTS

EMS 8821 Enclosure Power Supply

EMS 8840-0A PE Power Supply

EMS 8842-1A Power Diodes

EMS 8412-05 Lab-Volt DC Voltmeter/Ammeter

EMS 8311 Variable Resistance EMS 8325 Smoothing Inductor

EMS 9056-15 and EMS 9056-05 Voltage/Current Isolator

Textronic Oscilloscope

Connection Leads

WARNINGS

The voltages and currents that are used during this lab are larger and rated at 240VACLine-to-Neutralwith current as high as 20 amps (or higher if circuits are improperly connected). Pleasetake the proper precautions and use your head before touching any circuitry. NEVERchange anycircuit connections while the power supply is turned on. Ask the demonstrator to check your

connections before turning on the switches. And follow the rating of voltmeters and ammetersgiven to prevent equipments from damaged.


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Experiment: Part ICharacteristic curve of a power diode

1. Make sure that the main power switch of the Power Supply is set to the O (OFF) position.Set the voltage control knob to 0 %. Make sure that the toggle switches on the ResistiveLoad are all set to the O (open) position.

2. In this part, you will use the Voltage and Current lsolators to display the characteristiccurve of a diode on the oscilloscope. Connect the modules as shown in Figure 1a.

Figure 1a: Diode characteristic circuit

Table 1a: Parameters setting

Line Voltage (VAC) e1 (V) i1 (A) R1()

0 - 240 600 5 229

3. Use the horizontal (X) and vertical (Y) position controls on the oscilloscope to position the

spot in the centre of the screen by selecting DisplayFormatXY mode on theoscilloscope. Then set both channels to dc coupling by pressing CH1 or CH2 and chooseCouplingDC.

4. Set the main power switch to 1 (ON) and set the voltage control knob to 90(%). Observethe curve displayed on the oscilloscope screen. The horizontal axis represents theinstantaneous value of the voltage across the diode and the vertical axis theinstantaneous value of the current through the diode. Use Figure 1b to reproduce thecurve displayed on the oscilloscope.

Figure 1b: The voltage-current characteristic of a diode


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5. Return the voltage control knob to 0 then set the main power switch to O (OFF). Can youconclude from the appearance of the curve that current flows in one direction only? Canyou also conclude that the diode operates as a switch? Explain.

Experiment: Part IISingle phase half-wave rectifier circuit

1. Make sure that the main power switch of the Power Supply is set to the O (OFF) position.Set the voltage control knob to 0 %. Make sure that the toggle switches on the Resistiveand Inductive Load are all set to the O (open) position.

2. In this part, you will set up a half-wave rectifier circuit. Set up the circuit of Figure 2ausing the resistive load Z1(a).

Figure 2a: Half-wave rectifier circuit

Table 2a: Parameters setting

LineVoltage (V)

I1 dc (A) i1 (A) E1 dc (V) e1, e2 (V) Z1(a) Z1(b)

0 - 240 1.5 5 400 600 R=240 R=240

L=0.8 H

3. Then set the main power switch of the Power Supply is set to 1 (ON) position. Set thevoltage control knob to 90%. Record the output voltage, current, and power of the rectifiercircuit in the first row of Table 2b. Enter the conduction angle of the diode.

Table 2b: Results

Load Z1 E1 dc (V) I1 dc (A) PO = E1 X I1(watt)

ConductionAngle ()

(a) Resistive

(b) Inductive

4. Sketch the voltage and current waveforms displayed on the oscilloscope in Figure 2b. Asine wave supply 4-N is drawn in this figure as a reference and can be viewed as e 1. Youmay find it helpful to change the time base of the oscilloscope. For example, you couldadjust the time base so that one complete cycle of the waveform occupies 6 horizontaldivisions. Each horizontal division would then represent 60.


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Figure 2b: Output waveforms

Record the ripple frequency (frequency of the rectified waveform).Ripple frequency = _______Hz

5. Set the voltage control knob to the 0 % position then set the main power switch to the Oposition.

6. Change the load in the circuit to the inductive load Z1(b) as in Figure 2c. Repeat theprocedure steps necessary to complete Table 2b and Figure 2b. Explain the effect of aninductive load on the voltage and current waveforms and on the conduction angle.

Figure 2c: Z1(b) connection.

7. Set the voltage control knob to the 0 % position then set the main power switch to the Oposition.


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Experiment: Part IIIRectifier with free-wheeling diode

1. Make sure that the main power switch of the Power Supply is set to the O (OFF) position.Set the voltage control knob to 0 %. Make sure that the toggle switches on the Resistiveand Inductive Load are all set to the O (open) position.

2. In this part, you will add a free-wheeling diode to the circuit as shown in Figure 3a. Set upthe circuit using the inductive load Z1(b). Use the similar setting for parameters as inTable 2a. Set the main power switch to 1 (ON), and set the voltage control knob to 90(%).Fill in Table 3a.

Figure 3a: Rectifier circuit with free-wheeling diode

Table 3a: Results

Load Z1 E1 dc (V) I1 dc (A) PO = E1 X I1 (watt)

(b) Inductive

3. Sketch the voltage and current waveforms displayed on the oscilloscope as in Figure 2b.

4. Set the voltage control knob to 0 % then set the main power switch to O (OFF).

5. Connect the Current lsolator as shown in Figure 3b(a) and 3b(b) to observe the waveformof the current through the rectifying diode then through the free-wheeling diode. Beforechanging any connections always set the main power and voltage control knob of thePower Supply to 0. What effect does the free-wheeling diode have on the operation of thecircuit and on the parameters measured?

(a) Through rectifying diode (b) Through free-wheeling diode

Figure 3b: Observing current waveforms


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6. Sketch the current waveforms of ID1 and ID2.

7. Set the voltage control knob to 0 % then set the main power switch to O (OFF).

REVIEW QUESTIONS

1. Indicate how a diode conducts or does not conduct, when a voltage E is applied to itsterminals.

2. What effect has a series inductor on the output of a half-wave rectifier circuit?

3. Explain what happens in a battery charger as the battery charges up.

Exp 1 Single-Phase Rect1

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