To Measure Resistance, Voltage (AC/DC), Current (AC) and Check Continuity of a Given Circuit Using Multimeter

A multimeter is an electronic measuring device used to measure voltage, current, and resistance in electrical circuits. It is also useful for checking continuity, ensuring a circuit is complete.

There are two types of multimeters:

1. Analog Multimeter – Uses a moving needle to show readings.
2. Digital Multimeter (DMM) – Displays readings on a screen.

In this experiment, we will learn how to use a multimeter to measure resistance, AC/DC voltage, AC current, and continuity.

Apparatus Required

• Digital Multimeter
• Connecting Probes (Red & Black Wires)
• Resistors, Wires, and an Electrical Circuit
• AC/DC Power Supply or Battery

Procedure

1. Measuring Resistance (Ω)

Resistance is the opposition to the flow of electric current and is measured in Ohms (Ω).

Steps:

1. Turn the multimeter dial to the Ω (Ohm) symbol.
2. Disconnect power from the circuit.
3. Insert probes:
   • Red probe → V/Ω port
   • Black probe → COM port
4. Touch the probes to both ends of the resistor/component.
5. Read the resistance value on the display.

Observations:

• If the display shows 0 Ω, the component has no resistance.
• If the display shows infinity (OL), the component is broken or open circuit.

2. Measuring Voltage (AC/DC)

Voltage is the electrical potential difference across a component, measured in Volts (V).

(a) Measuring DC Voltage (Battery, Power Supply)

1. Set the dial to DCV (Direct Current Voltage) symbol (V with a straight line and dots).
2. Insert probes:
   • Red probe → V/Ω port
   • Black probe → COM port
3. Touch the red probe to the positive (+) terminal and black probe to the negative (-) terminal.
4. Read the voltage value on the screen.

(b) Measuring AC Voltage (Home Electric Supply)

1. Set the dial to ACV (Alternating Current Voltage) symbol (V with a wavy line ~).
2. Insert probes:
   • Red probe → V/Ω port
   • Black probe → COM port
3. Touch the probes to both terminals of the power source.
4. Read the voltage value displayed.

Observations:

• For DC, polarity matters (positive and negative).
• For AC, polarity doesn’t matter.

3. Measuring Current (AC)

Current is the flow of electric charge in a circuit, measured in Amperes (A).

Steps:

1. Set the dial to ACA (Amperes AC, marked as A~).
2. Insert probes:
   • Red probe → A or mA port (depends on current range).
   • Black probe → COM port
3. Break the circuit and connect the multimeter in series (not parallel like voltage measurement).
4. Turn on the power and note the reading.

Observations:

• If the display shows 0 A, there is no current flow.
• If the display shows a high value, check circuit connections.

Caution: Never measure AC current directly from the wall socket, as it can damage the multimeter and cause injury.

4. Checking Continuity

Continuity is used to check if a circuit is complete and unbroken.

Steps:

1. Set the dial to the continuity mode (🔊 or diode symbol).
2. Insert probes:
   • Red probe → V/Ω port
   • Black probe → COM port
3. Touch both probes to the two points you want to test.
4. If the circuit is complete, the multimeter beeps.
5. If no beep is heard, the circuit is broken.

Observations:

• Beep sound → Circuit is complete.
• No sound → Open circuit or broken wire.

Precautions

• Never measure resistance in a powered circuit.
• Use correct measurement ranges (AC/DC) to prevent damage.
• Never connect the probes directly to a high-voltage AC source.
• Hold probes properly to avoid electric shock.

Question and Answer on AC AND DC Current

1. What is a multimeter?

Answer: A multimeter is an electronic measuring instrument used to measure voltage (V), current (A), and resistance (Ω). It can also be used to check circuit continuity.

2. How do you measure resistance using a multimeter?

Answer:

1. Set the multimeter dial to the Ω (Ohm) mode.
2. Disconnect power from the circuit.
3. Insert probes:
   • Red probe → V/Ω port
   • Black probe → COM port
4. Touch the probes to both ends of the resistor.
5. Read the resistance value on the display.

3. What is the unit of resistance?

Answer:
The unit of resistance is Ohm (Ω).

4. How do you measure DC voltage using a multimeter?

Answer:

1. Set the dial to DCV (V with a straight line and dots).
2. Insert probes:
   • Red probe → V/Ω port
   • Black probe → COM port
3. Touch the red probe to the positive (+) terminal and the black probe to the negative (-) terminal.
4. Read the voltage value on the screen.

5. How do you measure AC voltage using a multimeter?

Answer:

1. Set the dial to ACV (V with a wavy line ~).
2. Insert probes:
   • Red probe → V/Ω port
   • Black probe → COM port
3. Touch the probes to the two terminals of the power source.
4. Read the voltage on the screen.

Caution: Never measure AC voltage from a wall socket without proper safety precautions.

6. Why do we measure voltage in parallel?

Answer:
Voltage is measured across a component, so the multimeter is connected in parallel to check the potential difference between two points.

7. How do you measure AC current using a multimeter?

Answer:

1. Set the dial to ACA (A~).
2. Insert probes:
   • Red probe → A or mA port
   • Black probe → COM port
3. Break the circuit and connect the multimeter in series with the circuit.
4. Turn on the power and read the current value.

Caution: Never measure AC current directly from a power socket.

8. Why is current measured in series and not in parallel?

Answer:
Current flows through a circuit, so to measure it, the multimeter must be placed in series to allow the current to pass through it.

9. How do you check continuity using a multimeter?

Answer:

1. Set the dial to continuity mode (🔊 or diode symbol →|—).
2. Insert probes:
   • Red probe → V/Ω port
   • Black probe → COM port
3. Touch both probes to the two points you want to test.
4. If the circuit is complete, the multimeter will beep.
5. If no beep is heard, the circuit is open or broken.

10. What is the symbol for continuity on a multimeter?

Answer:
The continuity mode is represented by a diode symbol (→|—) or a sound wave symbol (🔊).

11. What happens if you try to measure resistance in a powered circuit?

Answer:
The reading will be incorrect and may damage the multimeter. Always turn OFF the power before measuring resistance.

12. How do you safely use a multimeter?

Answer:

• Always set the correct mode before measuring.
• Do not measure resistance in a powered circuit.
• Do not measure high AC voltage or current without proper safety precautions.
• Hold probes properly to avoid electric shock.

13. What happens if you measure AC voltage in DC mode?

Answer:
The multimeter may display 0V or show an incorrect reading.

14. Why should we use the correct range on a multimeter?

Answer:
Using the wrong range can give incorrect readings or even damage the multimeter.

15. What is the difference between AC and DC voltage?

Answer:

• AC Voltage (Alternating Current) changes direction periodically (e.g., home electricity supply).
• DC Voltage (Direct Current) flows in only one direction (e.g., battery voltage).

16. What should you do if the multimeter does not show any reading while checking continuity?

Answer:

• Ensure the probes are properly connected.
• Check if the circuit is open or broken.
• Make sure the multimeter is working correctly.

17. What does ‘OL’ or ‘1’ on the screen mean while measuring resistance?

Answer:

• ‘OL’ or ‘1’ means ‘Over Limit’, indicating an open circuit or infinite resistance.

18. What are the main functions of a multimeter?

Answer:
A multimeter can measure:

1. Voltage (AC/DC)
2. Current (AC/DC)
3. Resistance (Ω)
4. Continuity
5. Diode testing

19. Can a multimeter be used to check batteries?

Answer:
Yes, set the multimeter to DC voltage mode and measure the voltage of the battery terminals.

20. What should you check before using a multimeter?

Answer:

• The battery of the multimeter should be working.
• The probes should be properly connected.
• The correct measurement mode should be selected.

Viva Questions and Answers

These are some important viva questions that might be asked during your practical exam.

1. What is a multimeter?

Ans: A multimeter is an instrument used to measure voltage, current, and resistance in electrical circuits.

2. How do you measure resistance with a multimeter?

Ans: Set the dial to Ω (Ohm), connect probes across the resistor, and read the display.

3. What happens if you measure AC voltage in DC mode?

Ans: The multimeter may show 0V or give an incorrect reading.

4. What is the symbol of continuity on a multimeter?

Ans: It is usually represented by a diode symbol (→|—) or a sound wave symbol (🔊).

5. Why should we measure current in series but voltage in parallel?

Ans:

• Current is measured in series because it flows through the circuit.
• Voltage is measured in parallel as it is the potential difference between two points.

6. What is the unit of resistance?

Ans: The unit of resistance is Ohm (Ω).

7. What is the difference between AC and DC voltage?

Ans:

• AC Voltage (Alternating Current) changes direction periodically.
• DC Voltage (Direct Current) flows in one direction only.

8. What happens if the circuit is open while checking continuity?

Ans: If the circuit is open, the multimeter will not beep, indicating no continuity.

9. What safety measures should be followed while using a multimeter?

Ans:

• Never connect probes directly to a high-voltage AC source.
• Always select the correct range before measurement.
• Do not touch the metal tips while measuring live circuits.

Conclusion

Using a digital multimeter, we can easily measure resistance, voltage (AC/DC), current (AC), and check continuity in a circuit. It is an essential tool for electronics and electrical work, helping in troubleshooting and circuit testing.

Mastering these basic measurements will help in both academic studies and real-world applications in electrical and electronics engineering.