Chapter 02Direct Current Meters

Warm - ups

Objectives• At the end of this chapter, the

students should be able to:•Describe about the types of suspension

used in the d’Arsonval meter movement.•Explain in detail the principles of

operation of the pmmc or d’Arsonval meter movement.

Objectives•Explain the purpose of shunts across a meter and multipliers in series with a meter.

•Analyze a circuit in terms of Voltmeter Loading Effect and Ammeter Insertion Errors.

Objectives•Describe the construction and operation of a basic Ohmmeter.

•Perform calculations to obtain specific meter range.

•Apply the concepts related to error to the circuits calculation.

Outlines•Introduction•pmmc = d’Arsonval meter movement

•Ayrton Shunt•d’Arsonval used in DC Voltmeter•d’Arsonval used in DC Ammeter

Outlines•Voltmeter Loading Effects•Ammeter Insertion Effects•Ohmmeter•Multiple-range Ohmmeter

Outlines•Multi-meter•Example of applications•Assignment 1

What is a meter?• A meter is any device built to accurately

detect and display an electrical quantity in a form readable by a human being.

• Usually this "readable form" is visual: motion of a pointer on a scale, a series of lights arranged to form a "bargraph," or some sort of display composed of numerical figures.

What is a meter?• Most modern meters are "digital" in

design, meaning that their readable display is in the form of numerical digits.

• Older designs of meters are mechanical in nature, using some kind of pointer device to show quantity of measurement.

What is a meter?• The display mechanism of a meter is

often referred to as a movement, borrowing from its mechanical nature to move a pointer along a scale so that a measured value may be read.

• Mechanical meter movement designs are very understandable.

What is a meter?• Most mechanical movements are based

on the principle of electromagnetism: that electric current through a conductor produces a magnetic field perpendicular to the axis of electron flow.

• The greater the electric current, the stronger the magnetic field produced.

What is a meter?• If the magnetic field formed by the conductor is

allowed to interact with another magnetic field, a physical force will be generated between the two sources of fields.

• If one of these sources is free to move with respect to the other, it will do so as current is conducted through the wire, the motion (usually against the resistance of a spring) being proportional to strength of current.

What is a meter?• Practical electromagnetic meter movements

can be made now where a pivoting wire coil is suspended in a strong magnetic field, shielded from the majority of outside influences.

• Such an instrument design is generally known as a permanent-magnet, moving coil, or PMMC movement .

What is a meter?

pmmc=d’Arsonval• In the picture above, the meter movement

"needle" is shown pointing somewhere around 35 percent of full-scale, zero being full to the left of the arc and full-scale being completely to the right of the arc.

• An increase in measured current will drive the needle to point further to the right and a decrease will cause the needle to drop back down toward its resting point on the left.

pmmc=d’Arsonval• The arc on the meter display is labeled with numbers

to indicate the value of the quantity being measured, whatever that quantity is.

• In other words, if it takes 50 microamps of current to drive the needle fully to the right (making this a "50 µA full-scale movement"), the scale would have 0 µA written at the very left end and 50 µA at the very right, 25 µA being marked in the middle of the scale.

• In all likelihood, the scale would be divided into much smaller graduating marks, probably every 5 or 1 µA, to allow whoever is viewing the movement to infer a more precise reading from the needle's position.

pmmc=d’Arsonval• The basic principle of this device is the

interaction of magnetic fields from a permanent magnet and the field around a conductor (a simple electromagnet).

• A permanent-magnet moving-coil (PMMC) movement is based upon a fixed permanent magnet and a coil of wire which is able to move, as in next figures.

pmmc=d’Arsonval• The basic principle of this device is the

interaction of magnetic fields from a permanent magnet and the field around a conductor (a simple electromagnet).

• A permanent-magnet moving-coil (PMMC) movement is based upon a fixed permanent magnet and a coil of wire which is able to move, as in next figures.

•When the switch is closed, the coil will have a magnetic field which will react to the magnetic field of the permanent magnet. The bottom portion of the coil in Figure 2(a) will be the north pole of this electromagnet. •Since opposite poles attract, the coil will move to the position shown in Figure 2(b).

pmmc=d’Arsonval

pmmc=d’ArsonvalTo use pmmc as a meter, 2 problems must be solved. •First, a way must be found to return the coil to its original position when there is no current through the coil.•Second, a method is needed to indicate the amount of coil movement.

pmmc=d’ArsonvalThe first problem is solved by the:• use of hairsprings attached to each end of the coil. •These hairsprings can also be used to make the electrical connections to the coil. • With the hairsprings, the coil will return to its initial position when there is no current. •The springs will also tend to resist the movement of the coil when there is current through the coil.

pmmc=d’Arsonval• As the current through the coil increases, the magnetic field generated around the coil increases.• The stronger the magnetic field around the coil, the farther the coil will move. This is a good basis for a meter. • But, how will you know how far the coil moves? • If a pointer is attached to the coil and extended out to a scale, the pointer will move as the coil moves, and the scale can be marked to indicate the amount of current through the coil.

pmmc=d’Arsonval• 2 other features are used to increase the accuracy& efficiency of this meter.• First, an iron core is placed inside the coil to concentrate the magnetic fields.• Second, curved pole pieces are attached to the magnet to ensure that the turning force on the coil increases steadily as the current increases.• The meter movement as it appears when fully assembled is shown in this figure.

pmmc=d’Arsonval• The d’Arsonval meter movement is very widely

used.• Current from a measured circuit passes tru the

windings of the moving coils causes it to behave as an electromagnetic.

• The poles of EMT interact with the poles of PM, causing the coils to rotate.

• The pointer deflects up scale whenever current flows in proper direction in the coil.

pmmc=d’Arsonval• For this reason, all DC meter movements

show polarity markings.• d’Arsonval meter movement is a current

responding device.• Regardless of the units (volt,ohm,etc) for

which the scale is calibrated, the moving coil responds to the amount of current thru its windings.

summary• The basic principle and operation of pmmc or d’Arsonval

meter movement.• The two (2) features used to increase the accuracy&

efficiency of this PMMC meters are: • First, an iron core is placed inside the coil to

concentrate the magnetic fields.• Second, curved pole pieces are attached to the magnet

to ensure that the turning force on the coil increases steadily as the current increases.

• Regardless of the units (volt,ohm,etc) for which the scale is calibrated, the moving coil responds to the amount of current thru its windings.

conclusion

•The students should be able to describe in detail about the basic principles of operation of the pmmc or d’Arsonval meter movement.

evaluationLabel the figure appropriately