Basic Voltmeter design.

Experiment No.: 02

Objective:

To design and build a basic voltmeter accurate to within 5% of an acceptable standard.

Apparatus Required:

Sl. No.	Name	Specification	Quantity
1	Variable DC Power Supply	(0-50)V	1 no.
2	Potentiometers	(0-25)kΩ	1 no.
3	Panel meter	(0-100)µA	1 no.
4	Digital Multimeter		1 no.
5	Connecting wires	PVC insulated copper	as per required
6	Potentiometers	(0-100)kΩ	1 no.

Circuit Diagram:

Theory:

To convert a given microammeter into different ranges of voltmeter, we need to connect resistance in series of the microammeter. The amount of resistance, depends upon the internal resistance of the microammeter. So first we will find out the internal resistance of the microammeter and then we will proceed to find the different series resistance value for different ranges.

Procedure:

A. Variable Resistor Method: I

1. Set the 25kΩ potentiometer to maximum and the voltage source to minimum.
2. Slowly increase the voltage to 1V.
3. Slowly decrease the resistance of the potentiometer until the full scale reading is obtained on the meter.
4. Carefully disconnect the potentiometer and measure the value of the potentiometer resistance(using a digital multimeter) that was in the circuit. Record this as R₁.
5. Re connect the circuit and slowly vary potentiometer until the meter reads exactly half scale. Record this as R₂.
6. Compute the internal resistance of the meter movement by the equation R_m1 = R₂-2R₁

B. Potentiometer Method: II

1. Set the 25kΩ potentiometer to maximum and the voltage source to minimum.
2. Slowly increase the voltage to 1V.
3. Slowly decrease the resistance of the potentiometer until a full scale readings is obtained on the meter on the meter under test.
4. Compute the internal resistance R_m2 of the meter under test using the milivolt and micro ammeter readings.

C. Shunt Resistor Method: III

1. Connect the circuit shown and vary the 100 kΩ potentiometer for full scale reading on the meter under test.
2. Close switch S and vary the 100k potentiometer until the meter under test reads exactly one half of scale.
3. Remove the 25 kΩ potentiometer from the circuit and measure the potentiometer shunting the meter under test using a multimeter. Record this as R_m3.

D. Calculate the average internal resistance of the meter under test as R_mavg= (R_m1 + R_m2 + R_m3) /3.

E. Use the average meter resistance from step D as R_m in the calculation for multipliers.

F. Using the test movement design a voltmeter having the 0 to 1V and 0 to 5V.

G. Construct the experimental circuit shown in fig. 2.4 (use fix composition resistor for the multipliers.

H. Apply voltage to each range of the experimental circuit adjust the voltage for full scale deflection on the test-meter and record these readings as E₂ in observation table, also measure the applied voltage on each range with the digital meter and record the reading as E₁ in the observation table.

I. Note down the quantities required for each range.

a. The voltage range.

b. The internal resistance of the microammeter.

c. The required resistance in series for the particular range.

Observation Table: –

Sl. No.	Supply Voltage (E1) in volts.	Design Voltage (E2) in volts.	% Error =[{(E1-E2)/E1}*100]
1
2

Conclusion: –

To be written by student.

For your Reference: –

A voltmeter is an instrument used to measure voltage or electric potential difference between two points in an electric circuit. Here’s the theory behind a basic voltmeter design:

Principle of Operation:

1. Voltage Sensing: A voltmeter works on the principle of voltage sensing. It measures the potential difference between two points in an electrical circuit by connecting across those points.
2. Voltmeter Resistance: Ideally, a voltmeter should have infinite resistance so that it doesn’t draw any current from the circuit under test, thus not affecting the voltage being measured. However, practical voltmeters have finite resistance, but it’s typically very high to minimize loading effects.
3. Voltage Division: To ensure that the voltmeter doesn’t disturb the circuit under test, it’s often connected in parallel with a resistor network that divides the voltage down to a level suitable for the voltmeter to measure.

Viva-Voce Question:

1. Explain the basic design principles of a voltmeter.

Answer:

A voltmeter is an essential instrument in electronics used to measure voltage or electric potential difference between two points in an electrical circuit. Its design involves several key principles:

1. Voltage Sensing Principle: A voltmeter operates on the principle of voltage sensing. It measures the potential difference between two points by connecting across those points without significantly disturbing the circuit.
2. Voltmeter Resistance: Ideally, a voltmeter should have infinite resistance to draw negligible current from the circuit under test, ensuring accurate voltage measurement. While practical voltmeters have finite resistance, it’s typically very high to minimize loading effects on the circuit.
3. Voltage Division: To scale down the voltage being measured, a voltmeter often employs a resistor network connected in parallel with the device. This network divides the voltage to a level suitable for measurement by the voltmeter.
4. Components:

• Analog Panel Meter: The primary display device in a basic voltmeter, consisting of a needle or pointer moving across a calibrated scale to indicate voltage.
• Resistor Network: Precision resistors form a voltage divider network, scaling down the measured voltage for accurate display on the panel meter.
• Switch (Optional): Allows selection of different voltage ranges on the panel meter, each corresponding to a specific resistor configuration.
• Diode (Optional): Protects the voltmeter from damage due to reverse polarity connections.

1. Circuit Operation:

• The voltage to be measured is applied across the input terminals of the voltmeter.
• The resistor network divides down the voltage, and the resulting voltage is applied to the panel meter.
• The panel meter’s pointer moves in proportion to the voltage applied, displaying the voltage on the calibrated scale.

1. Calibration: Before use, a voltmeter requires calibration to ensure accuracy. Calibration involves applying known voltages and adjusting the resistor network or switch settings until the panel meter reads accurately.