KING ABDULAZIZ UNIVERSITY

THE COLLEGE OF ENGINEERING, THE KINGDOM

OF SAUDI ARABIA

EE303

ELECTRICAL MEASRMENT & INSTRMENT

SPRING 2013

EXPERIMENT # 7

Design a Half-wave, Rectifier Type, AC Voltmeter

GROUP # 4

Team Member Group ID Section

Fahad Mohammad Al-Jdanni 1008538 DA

Faisal Alawi Baroom 1007396 DA

Mazen Almuqati 1007539 DA

Abdulaziz Hammouda 1007055 DA

Lab. Instructor: Mohammad Mottahir

Experiment Date: 9. April. 2013

Lab. Time: Sunday 11:00 – 1:00

Experiment No.7

Introduction In this experiment we will going to design a half-wave rectifier type, ac voltmeter.

This experiment will be related to dc meter because ac voltmeters are similar to dc

meters, except that ac is rectified before it is applied to the dc meter circuit.

Objective The objective of this experiment is to design a half-wave, rectifier type, ac voltmeter.

In addition, the properties encountered due to the non ideal properties of real diodes

will be explored.

Theory If a diode D1 is added to the dc voltmeter, as shown in Fig.1, we have an ac voltmeter

using half wave rectifier circuit capable of measuring ac voltages. The sensitivity of

the dc voltmeter is given by

Sdc=1/Ifsd =1/1 mA=1 kΩ

Figure 1. Design a Half-wave, Rectifier Type, AC Voltmeter

A multiple of 10 times this value mean a 10 V dc input would cause exactly full scale

deflection when connected with proper polarity. Assume D1 to be an ideal diode with

negligible forward bias resistance. If this dc input is replaced by a 10 V rms sine wave

input. The voltages appearing at the output is due the +ve half cycle due to rectifying

action. The peak 10 V rms sine wave is

Ep = 10 V rms x 1.414= 14.14

The dc will respond to the average value of the ac input, therefore

Eav = Ep x 0.636 = 14.14 x 0.636 = 8.99 V

Since the diode conducts only during the positive half cycle, the average value over

the entire cycle is one half the average value of 8.99 V, i.e. about 4.5 V. Therefore,

the pointer will deflect for a full scale if 10 V dc is applied and 4.5 V when a 10 V

rms sinusoidal signal is applied. This means that an ac voltmeter is not as sensitive as

a dc voltmeter.

As Edc = 0.45 x Erms

∴ The value of the multiplier resistor can be calculated as

Rs = – Rm = - Rm

Circuit diagram

Circuit diagrams are shown in figure 2 and 3

.

Figure 2: for activity 1 and activity 2

Figure 3: for activity 3

Lab Safety

Before moving ahead, we have to take a look at the following instructions that we

should follow it during the lab time.

General safety instructions

1. Move carefully in the lab.

2. No eating, no drinking and no smoking in the lab.

3. Use appropriate and available safety precautions and tools.

4. Never use chairs or boxes to reach to high places.

5. Concentrate to your experiment and equipment.

6. Don't keep any liquid close or on-top of any electrical device.

7. While doing experiment , be sure of the followings :

Connect circuit wiring carefully, let lab engineer check it.

Keep away any wire or equipment not used.

If you need to do any change in your circuit, switch power off, do necessary

modifications, double check –it, then switch power on.

Component and hand should be dry while doing the experiment.

If you got unexpected results ask assistance of responsible.

Never touches or play with denuded wires or cables.

After finishing the experiment, switch-OFF the equipment then botches put back

all components and wires in their places.

Safety rules of electrical laboratories: 1. Proper the grounding for lab .Power-Supply and equipment.

2. Fire extinguisher fixed in appropriate places.

3. Emergency exit signs for any emergency case.

4. Coincidence of power cables /plugs and current loads – regular check of cables

and wires insulation.

5. Cables should be in insulated trunks.

6. Warning tags for high voltage or radiating equipment.

7. First aid kit in appropriate place and well equipped.

Equipments 1mA meter movement.

DC Power supply.

Two variable resistor boxes.

Wires for connection.

Function Generator.

1N4001 diode (general- purpose rectifier diode).

Oscilloscope.

Steps

1. Connect the positive terminal of the power supply to the variable resistor box

(RB1).

2. Connect the negative terminal from the variable resistor box (RB1) to the positive

of the 1mA meter movement. Set the resistor box to be 10KΩ.

3. Connect the circuit by connecting the negative terminal of the 1mA meter

movement to the negative terminal of the power supply

4. Adjust the power supply to 10 volts. Note that the meter movement will not give

1mA exactly which means there is internal resistor of meter movement.

5. Decrease the variable resistor box (RB1) until the meter movement reach of full

scale deflection (1mA).

6. Connect another variable resistor box (RB2) parallel to the 1mA meter movement.

7. Increase the variable resistor box (RB2) until the pointer of 1mA meter movement

become in the half of the full scale deflection (0.5mA). Note the value of the

second variable resistor box (RB2) represents the internal resistor value (Rm).

8. Calculate the sensitivity by using the law: S = .

9. Calculate the input resistance by using: Rin = S×VFS(rms). (VFS(rms) without diode

voltage drop)

10. Calculate the series resistance by using: Rs = Rin-Rm.

11. Use the function generator in place of power supply.

12. Adjust the variable resistor box (RB1) to value of Rs.

13. Disconnect the variable resistor box (RB2).

14. Connect the diode series with variable resistor box (RB1) (the anode of the diode

with the RB1 and cathode with 1mA meter movement).

15. Adjust the function generator with the peak voltages that given in the results table.

16. Measure the relative deflection of 1mA meter movement (that becomes AC

voltmeter) at each voltage.

17. Calculate the percentage of error by using: % error

= .

18. Fill the table of activity 1

19. Calculate Vpeak effective by using: Vpeak effective = Vpeak -0.7.

20. Calculate the input resistance by using: Rin = S×VFS(rms). (VFS(rms) with diode

voltage drop)

21. Calculate the series resistance by using: Rs = Rin-Rm.

22. Adjust the variable resistor box (RB1) to value of Rs.

23. Adjust the function generator with the peak voltages that given in the results table.

24. Measure the relative deflection of 1mA meter movement (that becomes AC

voltmeter) at each voltage.

25. Calculate the percentage of error by using: % error

= .

26. Fill the table of activity 2

27. Connect the variable resistor box (RB2) parallel to the 1mA meter movement.

28. Adjust the variable resistor box (RB2) to value of Rm.

29. Adjust the function generator with the peak voltages that given in the results table.

30. Measure the relative deflection of 1mA meter movement (that becomes AC

voltmeter) and multiply it by 2 at each voltage.

31. Calculate the percentage of error by using: % error

= .

32. Fill the table of activity 3.

33. Use the oscilloscope to adjust the peak voltages of function generator in each

activity.

Results & Calculation with Rsh = Rm with diode voltage drop without diode voltage

drop

% error

(calculated)

2*(%deflection)

(measured)

% error

(calculated)

%deflection

(measured)

% error

(calculated)

%deflection

(measured)

%deflection

(expected)

Vpeak

(set)

Vrms

(calculated)

1.0% 2×(49.5 %) 7.0% 93 % 17.5% 82.5 % 100% 10 = 7.071

5.4% 2×(42.57 %) 10.0% 81 % 22.2% 70 % 90% 9 = 6.364

5.0% 2×(38 %) 12.5% 70 % 20.0% 64 % 80% 8 = 5.657

8.6% 2×(32 %) 14.3% 60 % 22.9% 54 % 70% 7 = 4.950

10.0% 2×(27 %) 17.5% 49.5 % 26.7% 44 % 60% 6 = 4.243

16.0% 2×(21%) 21.0% 39.5 % 25.0% 37.5 % 50% 5 = 3.536

20.0% 2×(16 %) 27.5% 29 % 31.3% 27.5 % 40% 4 = 2.828

23.3% 2×(11.5 %) 35.0% 19.5 % 33.3% 20 % 30% 3 = 2.121

35.0% 2×(6.5 %) 45.0% 11 % 32.5% 13.5 % 20% 2 = 1.414

42.0% 2×(2.9 %) 50.0% 5 % 55.0% 4.5 % 10% 1 = 0.707

The ammeter resistance was measured by:

Rm = (166) Ω

Rin = 3181

RS = Rin - Rm

= 3181 - (166) = 3015 Ω

And with diode voltage drop

Rin = 2961

RS = 3961 – 166 = 3795

error = | | ×100

Example in how the calculation was done when VPeak= 8V as shown (without diode):

Deflection % = (0.8) ×100 = 80%

By observing the meter, the percentage of the deflection measured as 64%

error = ((80-64) / 80) ×100 = 20%

Comments It is clear that the percentage of error of the reading is high when the reading taken

without using the diode. For example first read has error of 17.5% and this number is

high.

By connecting the diode to the meter, the percentage of error of the same reading

which taken in activity 1 become smaller and here the role of the diode in decreasing

the percentile of error. For example when we used the diode, for the first read, the

error decreased from 17.5% to 7%

To make the error more smaller, it is recommended to apply a shunt resistor (in

parallel with internal resistance of the meter Rm ) which has same value of Rm

(notice that the diode still connected). As mentioned above, this will decrease the

percentage of error of the same reading which taken in activity 1 and 2 and this

considered as optimum case. For example when we applied the shunt resistor with the

diode, for the first read, the error decreased from 7% to 1%

Notice that when we apply a shunt resistor (in parallel with internal resistance of the

meter Rm ), we actually increased the range of the meter to the double, and this is

another benefit of this case.

1

Conclusion

After finishing this experiment, and after we have learned that the internal resistance of any

measurement device effects in the readings from the previous experiments, we used in this

experiment the internal resistance and variable resistances box and diode in order to help us

to build an ac voltmeter device. Although in this experiment there are some errors in the

readings, it still extremely helpful in order to let students know about manipulating in the

devices functions. Finally, we hope that this report gained the acceptance.