Working Principle of Voltmeter

Any basic meter mechanism has a voltage Vm across its terminals when a full-scale current Im, flows through the meter. It follows that basic meter mechanism can be calibrated in micro-volts, milli-volts, or volts, depending on the voltage needed to produce a full-scale deflection. The symbol for the voltmeter is a circle with the enclosed letter V, as indicated in Figure.



An AC voltmeter does not have any polarity signs on it, but the instrument is still connected in parallel to the component for which the voltage is desired. As a voltage measurement does not involve breaking the circuit, the voltmeter needs to be connected only when a reading is desired.

A voltmeter with a higher voltage range, V, is constructed by connecting a resistance, Rsc in series with a meter mechanism, having a full-scale voltage capability of Vm as shown in the following Figure


The series resistance is called a multiplier; its value is determined from the voltage equation.


Multiple scale voltmeters are constructed either by using switching arrangements or multiple terminals.


In above figure, each of the multiplier resistors is separately calculated and are then selected by a non-shorting switch.


The multiplier resistors for above figure are calculated by first evaluating the resistance needed for the lowest range. The additional resistance needed for each higher range is then determined.

Voltage Sensitivity

It is defined as the reciprocal of the current necessary for full-scale deflection.


Where Im is the full-scale current. A smaller meter current results on a larger voltage sensitivity. The actual voltmeter resistance equals the sensitivity times the full-scale voltage. The voltmeter resistance remains constant even through the voltage reading may not be a full-scale reading.

Example

A multi-range voltmeter with 50 and 250 V ranges use a 50 μA meter mechanism.

(a) What is the sensitivity?

(b) What resistance does the voltmeter present n each range?

Solution

(a)
Sensitivity=1Im=150∗10−6=20kΩ/V

(b)
R=sensitivity ∗ range

50 V range:

R=(20∗103)(50)=1∗106=1 MΩ

250 V range:

R=(20∗103)(250)=5∗106=5 MΩ

This is very informative and will surely help to the electrical students like me

What is the essence of understanding this principle?

Does the DC and AC voltmeter has a same principle?

Why we need to study the principle of volt meter?

What is the essence of understanding this principle?

General purpose analog voltmeters may have an accuracy of a few percent of full scale, and are used with voltages from a fraction of a volt to several thousand volts. Digital meters can be made with high accuracy, typically better than 1%. Specially calibrated test instruments have higher accuracies, with laboratory instruments capable of measuring to accuracies of a few parts per million. Meters using amplifiers can measure tiny voltages of microvolts or less.

Does DC and AC has a same principle?

The electric current flows in a constant direction, distinguishing it from alternating current (AC). A term formerly used for this type of current was galvanic current. The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage.

Why we need to study the principle of Voltmeter?

A multimeter or a multitester, also known as a VOM (volt-ohm-milliammeter), is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter can measure voltage, current, and resistance. Analog multimeters use a microammeter with a moving pointer to display readings. Digital multimeters (DMM, DVOM) have a numeric display, and may also show a graphical bar representing the measured value. Digital multimeters are now far more common due to their cost and precision, but analog multimeters are still preferable in some cases, for example when monitoring a rapidly varying value.

A multimeter can be a hand-held device useful for basic fault finding and field service work, or a bench instrument which can measure to a very high degree of accuracy. They can be used to troubleshoot electrical problems in a wide array of industrial and household devices such as electronic equipment, motor controls, domestic appliances, power supplies, and wiring systems.

What is the difference between AC and DC?

AC and DC are different types of voltage or current used for the conduction and transmission of electrical energy. ... According to Karl K. Berggren, professor of electrical engineering at MIT, the fundamental difference between AC and DC is the direction of flow. DC is constant and moves in one direction.

What is the difference between the AC and DC electric motors?

D.C wound field motors are constructed with brushes and a commutator, which add to the maintenance, limit the speed and usually reduce the life expectancy of brushed D.C. motors. A.C. induction motors do not use brushes; they are very rugged and have long life expectancies. The final basic difference is speed control.

Can DC motor run on AC supply?

When A.C. supply is given to a D.C. motor:
1)In case of Series connection of armature winding and field winding , it may run.
2)But, In case of parallel connection , it won't rotate at all and will start humming and will create vibrations, as a torque produced by positive and negative cycle will cancel out each other.

What is AC and DC voltage?

What is the importance of this principle according to the topic?

Is this study, will help us in our every day life of living?

What will happen if voltmeter is connected in series?

Does current flow through a voltmeter?

What is the range of voltmeter?

How do you connect a voltmeter?

johnarnold wrote:How do you connect a voltmeter?

Connect a voltmeter to what?

johnarnold wrote:Does current flow through a voltmeter?

Since voltmeters are always connected in parallel with the component or components under test, any current through the voltmeter will contribute to the overall current in the tested circuit, potentially affecting the voltage being measured.

MasterJake wrote:Is this study, will help us in our every day life of living?

Yes, this study will help you learn more and will guide you to the proper usage of the voltmeter.

denden wrote:This is very informative and will surely help to the electrical  students like me

Thanks den!