Compilation of experiments on ‘Electronics’ for engineering students.
Contents:
- Experiment on Measurement of Currents of the Supplied OP AMP
- Experiment on Closed Loop Gain of an OP AMP
- Experiment on Conversion of Ammeter into a Voltmeter
- Experiment on Characteristics of Semiconductor Diode
- Experiment on Dynamic Characteristic Curves Using p-n Junction Diode
- Experiment on Semiconductor Diode as Rectifier in Power Supply
Experiment #
1. Measurement of Currents of the Supplied OP AMP:
ADVERTISEMENTS:
To measure the input off-set voltage, off-set current and input bias currents of the supplied OP AMP.
Circuit Diagram:
To measure the input off-set voltage Fig. 3.1, to measure the input off-set current Fig. 3.2 and to measure the input bias current Fig. 3.3(a) and (b) are used. Here any one of the IC 741 group of OP AMP is selected for the purpose which is a monolithic silicon integrated circuit with 11 resistors and 19 transistors.
[IC 741 OP AMP consists of a differential input amplifier which effectively drives a gain the level shifting stage with an emitter-follower output. The two transistors of the first stage of an OP AMP draw some base current for a suitable operating condition. OP AMPs are so designed that the input bias current becomes very low and hence the input signal cannot be loaded.]
Working Formula:
Due to the symmetry in the differential amplifier stages of an OP AMP, the two bias currents should be theoretically same. But in practice, they are always unequal causing an off-set current defined as-
Ii0 = IB1 – IB2
ADVERTISEMENTS:
Where IB1 = bias current at inverting input, and
IB2 = bias current at non-inverting input.
This off-set current would always flow resulting in a non-zero output voltage without any input. This can be nulled (or balanced) by applying a low voltage at any one of the internal offset balance terminals of the OP AMP. Such low value of the d.c. input voltage is known as the input off-set voltage (Vi0) which is obtained from the relation-
Since the input bias current is half the sum of the separate bias currents of the two input terminals of the balanced amplifier, we can write it mathematically as-
1. Suitable circuit connections as shown are carefully made. Following the application of the power supply voltage there is normally an initial warm up time drift of Vi0 and IB. This warm up time differs for different OP AMPS. It may vary in the range 2-5 minutes in general but for some special cases it may be even greater than 20 minutes. So measurements are to be made after its initial warm up has taken place.
2. To measure the input off-set voltage the output voltage V01 is noted (Fig. 3.1) for a set of resistances R1 and R2. The observation is repeated for different sets of R1 and Rs. In each case Vi0 can be calculated and hence a mean Vi0 is found out.
ADVERTISEMENTS:
3. To measure the input off-set current a constant value of the source resistance Rs is inserted (say 1 mΩ) to both the inverting and non- inverting inputs. For a set of resistances R1 and R2 the output voltage V02 (Fig. 3.2) is noted. Knowing V02 one may calculate the input off-set current (Ii0) in each case. A mean value of Ii0 is then determined.
4. To measure the bias currents IB1 and IB2 at inverting and non-inverting terminals respectively two separate circuits are to be followed [Fig. 3.3(a) and (b)]. The resistance, Ra in this experimental part, is fixed at a constant value (e.g., 1 mΩ). For inverting input V02 is noted for different sets of R1 and R2. For inverting input V0S is noted for different sets of R1 and R2. For non-inverting input V04 is measured. A mean value of V03 and also of V04 are then calculated.
5. The resistances R1 and R2 for the measurements of (2), (3) and (4) are so chosen that the output voltage does not attain close to ±12 volt which is the supply voltage.
(A) Specifications of OP AMP, used from the manual. (Make a table (mark as table I) showing the type of the OP AMP and its details.)
(B) To find the range and the smallest division of the meters used for measurement. (Make a table (mark as Table II) as tabulated in other experiments)
(C) To measure the input off-set voltage.
Supply voltage = 12 volt (say).
Room temperature = …°C
(D) To measure the input off-set current.
(E) To find the input bias current.
V01 = … volt (from Table III)
Rs = 1MΩ (say).
Discussion:
i. IC 741 OP AMP used in this experiment provides output short-circuit protection and internal phase compensation. It also provides latch-free operation i.e., the output voltage can be latched or struck at some value regardless of the value of the input voltage.
ii. Basically, the input bias currents are measured by forcing them to flow in large resistors of the order of 1MΩ which may be bipassed to reject noise by making use of a capacitor (~.01µF).
iii. For every change of the resistive network, off the circuit at first.
iv. Input off-set voltage drifts over a period of time and also with temperature is observed. It is also found to depend on the supply voltage applied.
Special Remarks:
Adjustment of off-set null – the effect of the input off-set voltages are very low and hence can be ignored. But in certain cases it cannot be neglected.
This can then be nulled or balanced by applying the following procedures:
(a) Connections are made as shown in Fig. 3.1. Terminal 1 and 5 of the OP AMP are connected to A and B (Fig. 3.4) of the 5 KΩ potentiometer shown centre is already connected internally to -12V terminal.
Next the following steps are followed:
(b) At first, R1 is made equal to R2 and their values are set at 10 KΩ and the potentiometer knob is adjusted to get zero output voltage.
Next R1 is set at 1KΩ and R2 is set at 10 KΩ and the potentiometer knob is adjusted to get the zero output voltage.
Finally, the value of R1 is set at 100 Ω, while R2 is made equal to 10 KΩ. Next, the knob of the potentiometer is adjusted to obtain zero output voltage.
Caution:
Don’t disturb the knob such adjusted for the second experiment with OP AMP.
Experiment #
2. Closed Loop Gain of an OP AMP:
To find the closed loop gain of an OP AMP and also to use of an OP AMP as an adder and differential amplifier.
Circuit Diagram:
The circuit diagram for studying the gain of an inverting amplifier and non-inverting amplifier is shown in Fig. 3.5(a) and (b) respectively. In both the figures the output is feedback to the inverting input terminal through a resistor Rf. In the inverting amplifier the input voltage Vi is applied to the minus (-) input while in the non-inverting amplifier the input voltage is applied to the plus (+) input.
The circuit diagram of an adder (or summer) is shown in Fig. 3.6. In the figure V2, and V3 are the input voltages connected to the minus input through the resistors R1, R2 and R3 respectively.
The connection of the differential amplifier is shown in Fig. 3.7. Here V1 and V2 are the two input voltages connected through the same resistance R1 to the minus and plus input. The inputs V1 and V2 are respectively connected to the points A’ and B’ whose other ends are earthed. A resistance Rƒ is connected between the output V0 and the point A while another resistance of equal value (i.e., Rƒ) is connected between the point B and the earth.
Working Formula:
Experimental Procedure:
(a) For Measurement of Closed Loop Gain:
1. Connections are made as shown in Fig. 3.5(a). For the inverting amplifier first, the combination of resistances R1 and Rƒ are properly chosen and for the +ve input supply, the input and output voltages are found out.
2. For the same set of R1 and Rƒ and making the supply at the input ‘-ve’, the input and output voltages are noted.
3. Operations (1) and (2) are repeated for different sets of R1 and Rf.
4. Connections are next made as shown in Fig. 3.5(b). For different sets of R1 and Rƒ applying the input +ve supply and the input -ve supply separately the input and output voltages are determined.
5. The gain of the inverting and non-inverting amplifiers are then calculated from the data obtained and compared with the theoretical data.
(b) For Measurements on Adder Circuit:
6. The resistances R1, R2 and R3 are made equal (say 10KΩ) and the value of Rƒ is set at 100KΩ after the suitable circuit connections (Fig. 3.6).
7. The actual values of the input voltages V1, V2 and V3 are recorded and the output voltage calculated.
8. The above observation is repeated for different sets of resistances and for each case the value of the output voltage is noted.
(c) For Measurement on Differential Amplifier:
9. For certain values of V1, V2 and applying suitable values of R1, Rƒ the output voltage V0 is experimentally observed.
10. The above operation is repeated for other sets and for each set the value of V0 is found out.
11. Theoretical value of V0 is calculated in each case and this value is compared with the experimental value of V0.
Experimental Results:
(A) Specification of the OP AMP. (Make a Table mark as Table I)
(B) To find the range and the smallest division of different metres used. (Make a table mark as Table II)
(C) To measure the closed loop gain.
(D) To measure the output voltage of an adder circuit.
(E) To measure the output of the differential amplifier.
Discussion:
i. The excessive gain of an amplifier can always be reduced to our choice by external circuitry.
ii. In a differential amplifier the voltage difference ∆V should be so chosen that the input offset voltage may be neglected in comparison to ∆V. Other-wise off-set null adjustment must be made before taking readings.
iii. In some laboratories the circuit connections of the OP AMP are kept in a closed box where only the binding tags are used by the students for performing the experiment. The internal connections are generally shown by lines on a panel board which, however, varies from one manufacturer to other.
But in some other cases a part of the whole instruments is enclosed in a box while the remaining are kept outside which is to be properly connected following the basic principle of the circuits shown in different figures. The working principle, of course, remains same.
iv. For OP AMP using as an adder the output voltage of the operation amplifier is measured and hence the error from the theoretically predicated value and also the percentage error are calculated. For substraction also from the output voltage measurement, the errors can be calculated.
v. OP AMP can also be used as integrator and differentiator with proper change of passive elements. For using OP AMP as an integrator the feedback resistance Rƒ in Fig. 3.5(a) is to be replaced by a capacitor C, the position of R1 remaining same. For differentiator the position of C and R1 in the case of integrator are to be interchanged.
Experiment # 3. Conversion of Ammeter into a Voltmeter:
To convert a given ammeter into a voltmeter, a given voltmeter into an ammeter and calibrate the instrument and to measure the internal resistance of it in each case.
Apparatus Required:
Adjustable regulated power supply (0-6 V), a rheostat (200 Ω), one microammeter (0-100 µA), resistance boxes (0-5000 Ω).
i. To find the Internal Resistance of the Microammeter construct the Circuit as Shown in Fig. 1.1:
If I be the current observed in the microammeter, then, the potential difference,
(VA – VB) = I (Rin + RS) where Rin is the internal resistance of the meter.
(VA – VB) is measured by a multimeter for different values of I with a given RS and a graph is drawn for (VA – VB) vs I.
The slope of the graph is (RS + Rin). Knowing RS, we can determine Rin.
Alternative method (Half deflection method)
Set up the circuit as shown in Fig. 1.2. For a given shunt resistance RS and R2 = 0, adjust the resistance R1 so that the microammeter shows full scale deflection. Then insert resistances from the box R2 until the deflection in the microammeter becomes half (i.e., 50 µA). The resistance R2 will then be the resistance of the microammeter. Repeat the experiment for various value of RS.
ii. Conversion of Microammeter to Voltmeter:
Redraw the circuit of Fig. 1.1. Find RS so that (RS + Rin) = 10 kΩ. Adjust the rheostat and observe the current I in the microammeter. The potential difference (VA – VB) = I (RS + Rin). Verify the p.d. (VA – VB) using a sensitive multimeter. Range of the meter is (100 µA) x (10 k Ω) = 1V. Repeat the steps for (RS + Rin) = 50 kΩ, 100 kΩ etc., and the 100 µA ammeter is converted to a 5 V, 10 V, and voltmeter, respectively.
iii. Conversion of a Basic Meter to DC Ammeter:
Let us assume that it is required to convert an 1-mA meter to a 10-mA meter. The internal resistance of the supplied meter is obtained by half deflection method. The circuit is made as shown Fig. 1.3 and the resistance of the meter is obtained.
Let the resistance Rin be 50 Ω. We can
now treat the meter as a voltmeter with rating 1mA x 50 Ω = 50 mV. For converting this multivoltmeter to a 10 mA meter, connect a low resistance parallel to the meter as in the circuit of Fig. 1.4.
Then changing R, we can find the current by connecting a sensitive meter between the points C and D and verify the reading in the given meter.
Experiment #
4. Characteristics of Semiconductor Diode:
To draw the static characteristic curve of a semiconductor diode, and hence to calculate the static and dynamic impedance (or resistance).
Circuit Diagram:
The circuit diagram for studying the forward and reverse characteristic curves of a semiconductor diode are shown in Figs. (2.1) and (2.2) respectively.
The indices are given below:
mV → a millivoltmeter of suitable range.
mA → a milliammeter of suitable range.
µA → a microammeter of suitable range.
R → a resistance box having resistance from 0 to 5000 ohm.
Rh → a rheostat of suitable power ratings.
pn → the semiconductor diode.
E → a regulated d.c. power supply or a battery.
K → a plug key or a switch.
1. For Diode Characteristics:
When a semiconductor diode is biased in the forward or reverse direction, then holes and electrons flowing in opposite direction cross the junction and constitute a flow of current in the semiconductor.
The current voltage relationship for a semiconductor diode is given by-
In the above equation, I represents the value of the current flowing in the semiconductor when a voltage V is applied across the diode and ln represents the reverse saturation current, k is the Boltzmann constant, T is the absolute temperature of the surroundings at the time of experiment and e is the electronic charge.
When the p side of the semiconductor diode is made positive in the closed circuit, then it is said to be forward biased. Here, the holes move from p to the n region while the electrons flow from n to p region (provide avalanche breakdown does not take place). This give rise to a current flow and it increases with increase of forward bias voltage.
Again when the p side of the junction diode is made negative with respect to the n side, the diode is reverse biased. Here due to the increase in the height of the potential barrier the flow of current decreases owing to the majority carriers. From Eq. (2.1) it is evident that for a large negative value of the applied voltage, I = –lS.
The static impedance Z of a semiconductor diode when it is conducting is the resistance offered under d.c. condition, and is given by-
This dynamic impedance also changes with change of operating point. For non-linear region it is obtained by drawing tangent at the operating point while for the linear region the value is directly obtained by drawing triangle about the operating point. (See Fig. 2.3). It can be also obtained from the reverse characteristic.
Special Note:
Hence, the quantity 1 in the above equation can be neglected in comparison to the first term. Thus, I = IS [exp. (4)]. Therefore, in the forward biased condition a very small voltage gives an appreciable current and the current increases exponentially with the voltage.
Near zero-bias, we can safely assume that V = VT, hence-
The above equation indicates that current linearly increases with the applied voltage, when the biasing-voltage is small enough.
In the reverse bias condition V is negative and so the current flow is very small. For a large value of negative applied voltage, I = –lS, and so the current is independent of the applied voltage. But this is not strictly true, because at the surface of the junction a small leakage current flows. With high negative reverse voltage, avalanche breakdown occurs. With high negative reverse voltage, avalanche breakdown occurs.
The actual leakage current is generally controlled by the conditions existing at the surface boundary of the junction. Whatever may be the manner of forming the junction, it must end at some surface. The surface behaves as a discontinuity in the crystal structure and it can collect foreign elements.
Hence the property of the bulk of the crystal is quite different from those at the surface. The surface of the crystal diode contains adsorbed solids, gases and ordinary water vapour and hence the life time of the surface is governed by the above factors.
Hence, the behaviour of the knee region of the reverse characteristic cannot be completely explained by the above simple theory. The poorer is the condition of the surface, the lesser is the slope of the knee-region.
As the reverse biasing is increased more than that existing at the knee-region a different phenomenon takes place. With increase of the reverse voltage (i.e., the field applied), the velocities of the carriers crossing the depletion layer increase. Here the electrons are knocked off from the atoms of crystal and so electron-hole pairs are created.
With increase of reverse biasing the pairs rapidly separate and hence causes further pair generation through other collisions. At the breakdown voltage a very large current is produced due to a light additional increase of the reverse voltage. The current I in this region is governed by the multiplication factor m, and is given by I = m.IS.
The multiplication factor m is given by-
Where Vb = breakdown voltage.
The value of n generally lies between 3 to 6 depending on the nature of the crystal. For silicon and n-type germanium, the value of n may be taken as 3.
Experimental Procedure:
1. The type number of the semiconductor diode is noted.
From the manufacturer’s semiconductor manual and following informations are recorded in the laboratory note book:
i. Semiconductor type (germanium or silicon)
ii. Number of the semiconductor.
iii. Peak inverse voltage (PIV).
iv. Maximum forward voltage drop and power rating.
v. Maximum reverse voltage and power rating.
2. Recording for Forward Characteristics:
i. The circuit diagram as shown in Fig. 2.1 is drawn in the laboratory note book and finally the connections are made (if closed box instrument is not supplied). E is a regulated d.c. supply source. For a regulated power supply. For a regulated power supply source, the input terminals of the power supply is connected to a.c. mains. The circuit should be approved by the instructor before switching on the circuit.
ii. Supply voltage is set to zero volt by regulating the knob (or by adjusting the resistance R in the circuit). The supply voltage is increased in small voltage steps, and in each step the corresponding current is noted. The recordings should be such that the knee region of the forward characteristic can be properly shown on the graph paper.
The maximum voltage applied to the diode for drawing the forward characteristic should be such that the maximum rating of forward current for the diode supplied is not exceeded. For each set of recording observation I and II may be made and from these the mean value may be taken.
Special Note:
For forward characteristic a small forward voltage in millivolt range changes the forward current in milliampere range. Hence the increment of voltage should be such that the current increases by one smallest division of the milliammeter supplied from the preceding value (for knee region).
Current change in the knee region should be about 1 mA at each step. But for the nearly linear region of the forward characteristic, the current change in each step may be taken to about 5 mA from the previous one. Hence the voltage should be increased in such a way that the successive difference between the recorded current is about 5 mA (for the nearly linear region).
iii. A graph (Fig. 2.3) is drawn by plotting the forward voltage (in mV) along the X-axis, and the corresponding current in milliampere along the Y-axis.
iv. From the graph (Fig.2.3) the value of the static resistance and dynamic impedance are calculated at different operating points (using Eqns. 2.2 and 2.3).
v. The circuit is switched off.
Extra Task (To be done if Directed by the Instructor):
A 60 watt bulb (when incandescent) is placed in front of the semiconductor diode so that its distance is about 1.5 cm to 2 cm. from the diode. Proceeding as before draw the forward characteristic curve and compare the curve with the former one. Interpret the result. Also calculate the d.c. resistance of the diode and note the effect of temperature on the forward d.c. resistance of the diode.
3. Recordings for Drawing Reverse Characteristic:
i. The circuit diagram, as shown in Fig. 2.2 is drawn in the laboratory note book and finally the connections are made (if closed box instrument is not supplied). Before switching on the circuit, the connections should be approved by the instructor. At first, the reverse voltage is set to zero volt.
The reverse voltage is increased in very small voltage steps and in each step the corresponding current is noted by the micro-ammeter (or a multimeter supplied). This increment of reverse voltage should be such that the maximum reverse voltage for the diode supplied is not exceeded.
ii. A graph is drawn by plotting the reverse voltage (in mV) along the negative X-axis, and the corresponding current (in uA or in mA) along the negative Y-axis. The nature of the graph is shown in Fig. 2.4.
iii. From the reverse characteristic curve, the d.c. and a.c. resistance of the diode at a given reverse voltage (taken at the region about half the maximum reverse voltage value) are calculated.
Extra Task (To be done if Directed by the Instructor):
A 60 watt bulb (when incandescent) is placed in front of the semiconductor diode at a distance of about 1.5 cm to 2 cm. Care should be taken so that the hot bulb does not touch the diode. The distance should be such that the radiated heat can increase the temperature of the diode. Draw the reverse characteristic curve again as before for the heated diode. The effect of heat on the reverse characteristic, and also on the d.c. and a.c. resistance or impedance are to be noted.
iv. Finally, from the knowledge of IS from the reverse characteristic, and the value of V from the forward characteristic, the theoretical formula given by Eqn. (8-1.1) is compared with the experimental one.
N.B.:
During recording of data for forward and reverse characteristic and the other constants involved therein, the temperature during the performance of the experiment must be noted. This is because, the nature of the characteristics are highly temperature dependent.
Experimental Result:
N.B.:
The data shown in different tables should never be taken as the standard. The value differ from instrument to instrument. These are shown only for illustration.
(A) Typical Parameters of the Diode:
(B) Range and Graduation of the Meters Used:
(C) Data for Forward Bias:
(D) Data for Reverse Bias:
(E) Calculation of Static Impedance (Z) and Dynamic Impedance (ZD) (Or Dynamic Resistance) From the Forward Characteristic:
Note:
For calculation of Z (the static resistance), a point P is considered on the forward characteristic (Fig. 2.3). Let the operating current be I0 mA. The corresponding voltage and current are noted at this operating current (I0). Hence the value of Z is calculated with the aid of Eqn. (2.2). The value of static resistance is determined at various operating currents.
For calculation of dynamic impedance (ZD) in the non-linear region, say at a point Q on the curve (Fig. 2.3), a tangent is drawn. From this, the triangle abc is constructed. Hence ZD = ab/bc, can be calculated at this operating current (I0)1. For linear region, drawing of a tangent is not necessary. Here a triangle ABC is drawn about the operating point (I0)2 and the dynamic impedance (or resistance) is calculated simply by using the relation-
ZD = AB/BC.
In a similar way, the value of the static and dynamic impedances are calculated from the reverse characteristic curve (Fig. 2.4) at different operating points.
(F) Calculation of Static and Dynamic Impedances from Forward and Reverse Characteristic:
(G) Comparison of the Calculated Value of I with the Experimental Value:
Remark:
From the calculation of I at room-temperature it is clear that the calculated and experimental values of I are nearly equal. A slight departure is due to the fact that the value of I cannot be very correctly obtained.
i. The specifications of the diodes should be properly noted. During recordings the forward and reverse power ratings should never be exceeded.
ii. The initial errors in different meters used should be eliminated, if possible. Otherwise, the corresponding corrections must be made.
iii. During recordings, the parallax error should be eliminated. Each reading should be taken twice, and the mean value should be taken for correct procedure.
Experiment # 5
. Dynamic Characteristic Curves Using p-n Junction Diode:
To draw the dynamic characteristic curves using p-n junction diode.
Apparatus Required:
(1) Regulated power supply (E) from zero to 25 volt range, (2) a voltmeter (V) and a milliammeter (A) of suitable range, (3) p-n junction diode, (4) a resistance box, (5) connecting wires, (6) key (K) etc.
Theory:
When a resistance box (R) is inserted between the milliammeter and the source, then for a fixed value of load resistance (R) if we vary the plate voltage; the plate current will change. For each plate voltage, the corresponding plate current for that load resistance can be noted. Such curve is known as the dynamic characteristic curve. For different load resistances, dynamic characteristic curves are obtained.
Experimental Procedure:
1. Circuit diagram is drawn in laboratory note book and connections are made as shown in Fig. 2.5. The input terminals of the regulated power supply are connected to the a.c. main. Finally the circuit is approved by the instructor.
2. Range of different meters used are noted.
3. Different important characteristics (namely maximum reverse power rating, maximum forward power rating etc.,) are noted from the semiconductor manual.
4. At first the load resistance R is kept to a fixed value, say 100Ω, for the diode supplied. By varying the regulated power supply (E), the voltage is gradually increased in steps of 0.2 volt (or to other values depending on the smallest division of the voltmeter V), and in each case the corresponding plate current in milliampere is noted. This procedure is done upto the maximumpere is noted. This procedure is done upto the maximum forward voltage across the diode. In this case the supply voltage may be varied from zero volt to 22 volt.
5. Now load resistance R is changed and kept to another fixed value, say 200Ω. Similar to the procedure no. (4), the voltage is gradually increased by varying the power supply (E) and in each case the corresponding plate current is noted. Again the load resistance is kept fixed to 300Ω (say) and similar procedure is done.
6. Now a graph is plotted with input voltage (V0) along X-axis and corresponding current (I0) along Y-axis. The graph will be nearly straight line passing through the origin. Actually the graph is slightly bent near the origin. The nature of the graph is shown in Fig. 2.6.
N.B.:
The data shown in different tables should never be taken as the standard. The values differ from instrument to instrument. These are shown only for illustration-
(A) Value of Graduation and the Smallest Divisions of Different Meters Used:
(B) Data for Dynamic Characteristic Curves:
N.B.:
Record different values of Ia and Va for other values of load resistances (say 400 ohms, 500 ohm etc.). Draw the Ia – Va curve as shown in Fig. 2.6.
Experiment # 6
. Semiconductor Diode as Rectifier in Power Supply:
To study the semiconductor diode as rectifier in power supply.
Circuit Diagram:
To use the diode as half wave rectifier the circuit as shown in Fig. 2.7(a) is to be made. In the figure, the input is an a.c. voltage applied across the series connected diode and the load RL. To study the full wave rectifier characteristics, circuit of Fig. 2.7(b) is used. In both the figures, C is a capacitor, mA is the milliammeter and RL is the load resistance. The output voltage Vd.c is obtained across RL.
A half-wave rectifier (consisting of one diode) converts the applied alternating voltage to a pulsating voltage using half cycle of the applied voltage. In a half-wave rectifier during the positive half cycle the p-side of the semiconductor is positive with respect to the n-side of the diode and the current flows in the circuit. But during the negative half cycle, the p-side of the diode is negative with respect to the n-side, and hence no current flows (provided the avalanche breakdown does not occur).
On the other hand, a full-wave rectifier (consisting of two diodes) converts the applied alternating voltage to a pulsating voltage using full-cycle of the applied voltage. That is the conduction takes place by the one rectifier element during one half-cycle and by the other element during the other half-cycle of the alternating voltage.
Voltage Regulation:
Voltage regulation for any rectifier is a measure of the ability of the rectifier to maintain the output voltage to a specified range though the load (or the supply current) varies.
Percentage of voltage regulation ρ is defined as-
Where Vno-load = voltage across the diode when load is infinity (i.e. lL = 0) and Vrated-Ioad = load voltage for a particular current flowing through the circuit.
1. The connections for the study of half-wave and full-wave rectifications are made as shown in Fig. 2.7(a) and (b) respectively. The a.c. is supplied by using a step-down transformer (30 – 0 – 30 volt say).
2. By varying RL (in the resistance box or in a potentiometer) the load current ldc without the capacitance filter is noted. For each value of the load resistance RL two observations for load current are found out-from which a mean ldc is determined.
3. The same procedure as mentioned in procedure no. (2), is to be followed with a capacitance filter. If possible instead of using only a capacitor filter or L-filter, (L – C) or a π-filter (C – L – C) may be suitably used, as shown in Fig. 2.8.
4. The above operations are to be followed both for a half-wave and a full-wave rectifier circuits.
5. Different values of d.c. output voltages are then calculated from the values of load resistance and d.c. load current.
6. A graph is then drawn by plotting load current along X-axis and the corresponding load voltage along the Y-axis with and without the filter circuit for the half-wave rectifier. Another similar graph is drawn on a separate graph paper for the full wave rectifier.
Experimental Results (For a Half Wave Rectifier):
Make similar tables as Table I and II.
(C) Data for Drawing Half-Wave and Full-Wave Rectification Characteristics:
(D) To Draw the Rectification Characteristics:
Current through the load (Idc) in mA is plotted along the X-axis and the output voltage (Edc) in volt is plotted along the Y-axis. The-upper curve represents the variation with C.L.C. filter. The lower curve represents the variation with L.C. filter.
Discussion:
i. While changing the resistance in the box care should be taken so that the current greater than the amount prescribed for the diode may not pass through it. Because in that case the diode may be damaged.
ii. Since the values of the output voltage in the rectification characteristic are found to be irregular in lower current region so a mean curve is to be drawn. One must be careful enough to note the diode current in each case.
Extra Work:
To study the regulation characteristics of a full wave rectifier using either a L-C filter or a π-filter (C-L-C). Fig. 2.8 may be employed. In the figure RB is the bleeder resistance and RL is the load resistance. When the switch S is made off L and C2 come in the circuit and it behaves as an L-C filter; but when the switch S is made on then C1, L and C2 all come in the circuit and so it behaves as a π-filter.
The voltage regulation is calculated by using the relation given below and is expressed in percentage.
We have,
Procedure:
1. The switch S is kept open (Fig. 2.8) so that the circuit behave as a full-wave rectifier with L- C filter. Under this condition the load current, IL is varied in regular steps and in each case the output d.c. voltage is noted.
2. Knowing the value of the bleeder resistor RB the current through the bleeder IRB is found out. The total current which is the sum of IL and lRB is thus determined.
3. The switch S is now kept on (Fig. 2.8) so that the circuit behaves as a full-wave rectifier with π-filter. The above two operations are then repeated.
Experimental Results:
(A) Full wave rectifier using L filter (when the switch S off).
(B) Full-wave rectifier using π-Filter (when the switch S is on)
(C) A graph is drawn by plotting load current in milliampere along the X-axis and output d.c. voltage in volt along the Y-axis for the L-C and π -filter supplied. The nature of the graphs are shown in Fig. 2.9.
(D) Calculation of voltage regulation.
i. From load current and load voltage (Shunt regulation)
ii. From total plate current and load voltage (line regulation)
Special Note:
Percentage of voltage regulation changes with change in the operating current as the value of Erated-load varies, while Eno-load remains the same. It must be remembered that the no-load voltage is the voltage across the diode rectifier when the load is infinity, i.e., load is cutoff from the rectifier circuit. Evidently, the no-load voltage has a fixed value for a particular diode rectifier. The designer has to adjust the no-load voltage to such a level that it does not exceed the power ratings of the diode as mentioned in the semiconductor manual.
In order to study the variation of percentage regulation of voltage with operating current, the value of Erated-load is noted for a particular operating current as shown in Fig. 2.9.
The value of percentage regulation of voltage is then calculated. The operating point is taken to another value, and again the value of percentage regulation is calculated.
Finally a graph is drawn with operating load current along X-axis and the calculated value of the percentage voltage regulation along Y-axis. The nature of the graph is different for different type of filter used. For a poor regulator, the nature of the graph is fully unsymmetrical, while for a good regulator, the graph is more or less symmetrical. For a pure resistance as the load, the regulation is very poor.
(E) Data for percentage regulation characteristics.
(F) Recordings for ld.c and Vd.c for a half wave rectifier (To be done if directed by the instructor).
Note:
The value of Rf is calculated from the forward characteristics of the diode supplied. In the case an a.c signal of proper magnitude is applied to the input of the rectifier (instead of the d.c source). The value of Vm is measured by an a.c voltmeter. The value of RL: is known. Hence Im, ld.c and Vd.c can be calculated from equations (2.5), (2.7). Finally ld.c and Vd.c are measured by a d.c milliammeter and a d.c voltmeter. The theoretical and experimental values are compared.
(G) Recordings of Id c and Vd for a full wave rectifier (To be done if directed by the instructor). Make similar table as above.
Discussion:
i. For no load current in the circuit the output d.c. voltage is almost equal to the peak value of the input voltage. The latter is measured by an a.c. voltmeter by the voltage drop between the anode and the centre tap of the transformer multiplied by √2.
ii. A fraction of total plate current is passed through the bleeder resistor RB connected parallel to the load RL. This effective load thus consists of a parallel combination of RL and RB and their equivalent resistance Reff is to be used when asked to calculate the ripple factor of CLC circuit. The ripple factor for the two filters can be obtained by using the relations given below.
