Instructor: Res. Asst. M. Mustafa ATANAKTA : Res. Asst. Özen YELBAŞI

ANADOLU UNIVERSITY

DEPT. OF ELECTRICAL AND ELECTRONICS ENGINEERING

EEM 328ELECTRONICS LABORATORY

Experiment 2: DIODE CHARACTERISTICS

Date : 30.10.2008Group Name: B

16169230356 Osman GÜLERCAN

a) Purpose

The purpose of this experiment is to study the characteristics of the diode and to implement the

rectifier circuits.

b) Background and Theoretical Discussion

—Diodes

A diode is an electrical device allowing current to move through it in one direction with far

greater ease than in the other. The most common type of diode in modern circuit design is the

semiconductor diode, although other diode technologies exist. Semiconductor diodes are

symbolized in schematic diagrams as such:

Diodes are used most commonly in circuits that convert ac voltages and current into dc voltages

and currents (e.g., ac/dc power supply). Diodes are also used in voltage-multiplier circuits,

voltage-shifting circuits, voltage-limiting circuits, and voltage-regulator circuits.

There is an equation describing the exact current through a diode, given the voltage dropped

across the junction, the temperature of the junction, and several physical constants. It is

commonly known as the diode equation: ID =Is (e VD / nVT -1).

A diode’s one-way gate feature does not work all the time. That is, it takes a minimal voltage to

turn it on when it is placed in forward-biased direction. Typically for silicon diodes, an applied

voltage of 0.6 V or greater is needed; otherwise, the diode will not conduct. This feature of

requiring a specific voltage to turn the diode on may seem like a drawback, but in fact, this

feature becomes very useful in terms of acting as a voltage-sensitive switch. Germanium diodes,

unlike silicon diodes, often require a forward-biasing voltage of only 0.2 V or greater for

conduction to occur. Following figure shows how the current and voltage are related for silicon

and germanium diodes.

—Rectifier Circuits

Rectification is the conversion of alternating current (AC) to direct current (DC).

—Half-Wave Rectifier

In this circuit, the diode acts to convert an ac input voltage into a pulsed dc output voltage.

Whenever the voltage attempts to swing negative at the anode, the diode acts to block current

flow, thus causing the output voltage (voltage across the resistor) to go to zero. This circuit is

called a half-wave rectifier, since only half the input waveform is supplied to the output. Note

that there will be a 0,6 V drop across the diode, so the output peak voltage will be 0,6 V less than

the peak voltage of Vin. The output frequency is the same as the input frequency, and the average

dc voltage at the output is 0.318 times zero-to-peak output voltage.

(a) Half-wave rectifier. (b) Equivalent circuit of the half-wave rectifier replaced with

piecewise-linear model.

(c) Input and output waveforms, assuming that rD<<R.

—Full-Wave Bridge Rectifier

This circuit (a) is called a full-wave rectifier, or bridge rectifier. Unlike the half-wave

rectifier, a fullwave rectifier does not merely block negative swings in voltage but also converts

them into positive swings at the output. To understand how the device works, just follow the

current flow through the diode one-way gates. Note that there will be a 1.2-V drop from zero-to

peak input voltage to zero-to-peak output voltage (there are two 0.6-V drops across a pair of

diodes during a half cycle).The output frequency is twice the input frequency, and the average dc

voltage at the output is 0.636 times the zero-to-peak output voltage.

The bridge rectifier circuit operates as follows: During the positive half-cycles of the input

voltage, Vs is positive, and thus current Is conducted through diode D1, resistor R and diode D2.

Meanwhile, diodes D3 and D4 will be reverse biased. Observe that there are two diodes in series

in the conduction path, and thus Va will be lower than Vs by two diode drops.

Next, consider the situation during the negative half-cycles of the mput voltage. The secondary

voltage Vs will be negative, and thus -Vs will be positive, forcing current through D3, R, and D4.

Meanwhile, diodes D1 and D2 will be reverse biased. The important point to note, though, is that

during both half-cycles, current flows through R in the same direction (from right to left), and

thus Va will always be positive.

—Basic AC-to-DC Power Supply

By using a transformer and a full-wave bridge rectifier, a simple ac-to-dc power supply can

be constructed. The transformer acts to step down the voltage, and the bridge rectifier acts to

convert the ac input into a pulsed dc output. A filter capacitor is then used to delay the discharge

time and hence“smooth”out the pulses. The capacitor must be large enough to store a sufficient

amount of energy to provide a steady supply of current to the load.

If the capacitor is not large enough or is not being charged fast enough, the voltage will drop as

the load demands more current. A general rule for choosing C is to use the following relation:

Rload.C >> 1/f where f is the rectified signal’s frequency (100 Hz).

The ripple voltage (deviation from dc) is approximated by Vrippl e= Iload/fC

c) Procedure

Firstly, in order to plot diode’s I-V characteristics curve, Vs voltage was increased step by step.

Then, the diode voltage VD and diode current ID were measured for each values of VS. By using

these values, the I-V forward characteristics curve was plotted. In addition, saturation current Is

and emission coeffient η were calculated by given formulas. Also, the piecewise linear model of

the diode was drawed to compare with ideal diode model of the circuit. In the second step, the

half-wave rectifier circuit was implemented. Connected the A.C. voltage source to the circuit’s

input, the half wave rectefier was analysed with comparing input/output waveforms. Also, the

diode voltage and resistor current were measured. Thirdly, the full-wave rectifier circuit was

built. A.C. voltage source was connected its input, then how the output waveforms were occured

was studied. Also a filter circuit was constructed with connecting a capasitor its output.

d) Results

1-a)

Vs ID VD 0,2 0,0013 0,2940,4 0,036 0,4330,6 0,175 0,5021 0,54 0,5563 2,5 0,6325 4,5 0,6597 6,58 0,6779 8,66 0,6912 11,7 0,704

0

2

4

6

8

10

12

14

0,294 0,433 0,502 0,556 0,632 0,659 0,677 0,69 0,704

Vd(V)

Id(mA)

Seri 1

1-b)

η = (VD1 - VD2) / VTln(ID1/ ID2) = (0,690-0,740) / 26.10-3ln(8,66/11,7) = 1,79

ID=IS(eVd/ηVt-1)

8,66= IS(e0,690/1,79.26.10^3-1)

IS=3,15μA.

2) Vin=2sin(2000t)V ; Vout=1,5V DC

3)a) Vin=17sin(2000t) ; Vout=17V DC

b) Vin=17sin(2000t) ; Vout=17V DC

16169230356

Osman GÜLERCAN

Conclusion:

In this experiment we studied the diode characteristics and compared the relationship between

diode current and diode voltage. Also, we implemented the rectifier circuits with filtering output

signal. In the first part of experiment, we connected a simple diode, resistor and A.C. voltage

source serially. In order to obtain I-V curve, we increased the input voltage of the circuit, then

measured the diode current ID and the diode voltage VD for all the values of VS. In addition

plotting I-V curve, we also calculated the saturation current IS and emission coeffient η . We

observed that the diode voltage VD was almost constant when the forward-biasing voltage was

forced to exceed. In the second part of experiment, we implemented a half-wave rectifier circuit

which are used to convert A.C. signal to D.C. We proved that the negative pulses are absorbed

and the positive pulses are passed by diodes when the forward-biasing voltage is exceeded.

Finally, we constructed a full-wave bridge rectifier which provides us to convert negative pulses

to positive pulses additinally. In this circuit, two diodes work at the same alternance, so the

voltage drop on the output is doubled(0,6x2=1,2V). In addition, connecting a capasitor at the

output, we filtered the D.C. signal and analysed how the smooth is changing for distinct

capasitor values. To sum up, we learned that how to measure and how to calculate the diode’s

characteristics properties, and compared different kinds of rectification methods.