Magnetism and Multimeters

Overview

• Magnetism• Electromagnetism• Introduction to Multimeters

Magnetism

Magnetism is force of attraction or repulsion of certain types of metallic substances.

Magnetism

The Law of Magnetic Force states: Unlike magnetic poles attract and like magnetic poles repel.

Magnets have North and South POLES

MagnetismThe effects of the attraction and repulsion are explained by assuming the existence of invisible lines of force. The invisible lines of force are called flux lines. They go from north to south outside of the magnet. Flux lines are more concentrated at the poles thus a stronger magnetic pull.

The stronger the magnetic field, the

more flux lines.

Magnetism

• Magnetic material are those that magnets attract. Materials such as iron, nickel, cobalt and steel.

• Non Magnetic are those that magnets do not attract. Materials such as aluminum, copper, lead, silver, wood and glass.

• Permeability the ease in which a material can become magnetized.

• Retentivity the ability of a material to retain its magnetism after you remove the magnetizing force.

Magnetic material is affected by the flux lines of a magnet and takes on the characteristics of a magnet. We use certain terminology to describe the differences in magnetic properties.

ElectromagnetismThe source of magnetism in materials is the direction of spinning electrons as they orbit the nuclei of atoms.

A magnetic field is created when most of the electrons are caused to spin in the same direction.

The more aligned, the stronger the magnetic field.

Electromagnetism is a man-made form of magnetism. It is made by applying electricity to magnetic materials.

When electrons flow through a conductor, a circular magnetic field develops around the conductor. This current is responsible for developing the magnetic field. The strength of the magnetic field is directly proportional to how much current is flowing through the conductor.

If the direction of the electron flow in a conductor is known, the direction of the flux lines in the magnetic field can be determined. The relationship between electron flow and the direction of the magnetic field is called the “Conductor Left-Hand Rule”.

Electromagnetism

Conductor Left-Hand Rule: When the left hand is placed so the thumb points in the direction of electron flow, the curled fingers point in the direction of the magnetic field flux lines that circle the conductor.

Electromagnetism

An example of a working electromagnet is one of the huge cranes that moves junk and scrap iron in a wrecking yard. The crane magnet is a big block of iron that is magnetized by an electrical current flowing through the coil. To pick up the scrap iron, the operator turns on the electrical current, which magnetizes the block of iron. The magnetized block of soft iron attracts the scrap iron and picks it up.

To release the scrap iron, the operator turns off the electrical current, which demagnetizes the block of iron.

Electromagnetism

Magnetomotive Force is the magnetizing force set up due to current flowing in a coil. The magnetomotive force is directly proportional to the current in amperes and to the number turns in the coil.

Electromagnetism

The closer to the conductor, the stronger the concentration of Magnetomotive Force.

Electromagnetism

Several factors affect the magnetic field strength or intensity of an electromagnet formed by a coil. Knowing how these factors affect the strength of the field around a coil, a magnetic field of a given strength can be produced.

Magnetic Field Strength is determined by: The number of coil windings.

The more windings the stronger the magnetic field

The distance between windings. The closer together the windings, the stronger the magnetic

field.

The amount of current. The more current, the stronger the magnetic field.

Composition of the core.

Electromagnetism

Electromagnets have many uses in electronic devices. Some of their most important uses are in relays, circuit breakers and meters.

• Circuit breakers electronic devices which open or trip a circuit when an overload condition is sensed.

• Relays are electronic switches that are very useful in many places , such as on a main control panel.

• Most meters used for electrical measurements operate on the principle of magnetism. The action of the basic DC meter is based on the principle of a motor.

Electromagnetism

Relays are switches that use an electromagnet to attract the metal arm of a switch. They give you the ability to close a switch in an electronic circuit without being near the switch.

As current energizes the electromagnet, it attracts the steel plate armature. The armature swings on the swivel toward the electromagnet and connects itself with the stationary contact.

Relays are used for signal control or for power control.

Electromagnetism

ON

Conductor

Conductor

CURRENT SURGE

OFF“tripped”Contacts

Reset

In a Circuit Breaker, when a surge in current creates a magnetic field in the coil, it attracts the magnet, causing the switch to open.

Electromagnetism

ON

Conductor

Conductor

OFF“tripped”Contacts

Reset

Once the reset is pressed on the Circuit Breaker, the switch is closed.

Electromagnetism

The action of the basic DC Meter is based on the principle of a motor. The motor or turning action results from the flow of a small current through a moving coil positioned in a fixed magnetic field.

How far the coil turns against the force of the spring tension depends on how much current is flowing through the electromagnetic coil.

Electromagnetism

A Multimeter is one of a group of test instruments called test equipment. These devices are useful for basic fault finding and field service work. A meter is a device that measures a quantity in a circuit to a very high degree of accuracy.

Multimeters

DigitalAnalog

Multimeters are either analog or digital. Analog meters use a needle or point to display values. Digital meters have a digital

display. Both, analog and digital, vary in size, shape, and power requirements.

Multimeters measure voltage,

current and resistance.

Multimeters

The heart of an analog multimeter is a D'Arsonval (or Weston) meter movement.

This device uses DC current passing through a coil and magnet assembly to swing a pivot-mounted needle. The amount of current flowing through the coil determines how far the needle will swing. The selector dial on the front of the meter determines the type of filtering the test value will go through, whether it is fixed resistances, rectifiers or an internal voltage source. The needle is moved according to the magnetic field developed inside the meter.

Multimeters

Display section shows the value of the measurement taken.

A typical multimeter has five control sections.

Input Jacks are uses to connect test leads to the multimeter.

Range Selector Switch is used to select the size of the measurement that

will be taken.

Function Switch chooses which electrical

value to measure.

Multimeters

Zero Adjustment sets the analog meter’s needle to zero before

making a measurement

- Com + V

DCV

Multimeters measure voltage, current and resistance. Voltage is the difference in potential between two parts of a circuit. The

meter must be set-up correctly before you measure voltage. A meter that is incorrectly set-up can cause personal injury or

equipment damage.

To measure voltage ensure:1. Display is zeroed.2. Set function switch to

voltage.3. Select the range.4. Ensure test leads are

properly placed to make a voltage measurement.

Multimeters

The Reading is 1.0 Volts DC

Reading Voltage Measurements:Analog multimeters use different scales (30, 12, or 6)

based on the Function/Range switch.

Multimeters

The Reading is 100 Volts DC

Reading Voltage Measurements:The Range switch also indicates the multiplier of the

Display scale reading.

Multimeters

The Reading is 20 Volts DC

Reading Voltage measurements:Compare the Range Switch and the Display scale reading

to get the correct measurement.

Multimeters

The Reading is 400 Volts AC

Reading Voltage Measurements:AC Voltage can also be read using the same scales on the

Display. The 12 scale includes readings for 1000.

Multimeters

- Com + ADCA

Multimeters measure voltage, current and resistance. The meter must be placed "IN" the circuit to measure current. Remember current is the flow of electrons. The electrons must flow through

the meter. In order for current to flow through the meter, the circuit must be broken (opened) and the meter inserted.

Multimeters

Some circuits have switches to open the circuit to measure current. If the switches are not available, the trick is to find the

easiest place to break into the circuit. One way is to lift the end of a component and then insert the meter between two components.

Another possibility is to lift the negative side of the battery.

Multimeters

- Com + ADCA

The Reading is 10m Amps DC

Reading Current Measurements:Current can be read using the same scale on the Display,

based on the Range switch.

Multimeters

- Com + Ω

OHMS

Multimeters measure voltage, current and resistance. Resistance is the opposition to current. Remember, never

measure resistance in a circuit with power applied.

To measure Resistance ensure:1. Display is zeroed2. Set function switch to Ohms.3. Select the range.4. Ensure test leads are properly

placed to make a voltage measurement. The red lead must be connected to the Ω input jack and the black lead must be connected to the common input jack.

Multimeters

Note: The meter supplies the power, if the equipment is powered on, it will give false reading and may damage the equipment.

- Com + Ω

OHMS

When measuring resistance, if there are two or more resistors in series, the leads are placed across one resistor, the meter will

read that resistor. If the leads are placed across two resistors, the meter will read the total resistance of both resistors.

In a parallel circuit with two branches or more and a resistor in

each branch, place the leads across either resistor and the meter

will read the total resistance. To measure the resistance of one resistor in parallel, it must be

isolated from the rest of the circuit because each branch is connected

electrically.

Multimeters

The Reading is 60k Ohms

Reading Resistance Measurements:Resistance can be read using the “OHMS” scale on the

Display, based on the Range switch.

Multimeters

A continuity check is the test or check you make to find out if continuity exists between two points in a circuit. A continuity check

is the first check you make when troubleshooting an electrical circuit. By making a continuity check, it can be determined

whether or not the system wiring is in order. If there is a broken wire or other conductor, a continuity check will pinpoint the

problem for you.

Multimeters

- Com + Ω

OHMS

- Com + Ω

OHMS

To make a continuity check, place the test leads across the two points in the circuit that need verification. If no break exists, there

is continuity. A multimeter with no built-in continuity checker responds by giving you a low resistance reading. If the reading is high, excess resistance exists in the circuit. In this case, there

would be a need for further checks.

Multimeters

Magnetism is an invisible force that can do mechanical work.

The Law of Magnetic Force states: Unlike magnetic poles attract and like magnetic poles repel.

The factors affecting the strength of an electromagnet coil are: The number of coil windings. The distance between windings. The amount of current. Composition of the core.

Magnetmotive force is the magnetizing force set up when current flows from an electromagnetic coil.

Summary

A Multimeter is an electronic test instrument, which measures and displays quantities of voltage, current, and resistance.

The meter must be set-up correctly before you measure voltage. A meter that is incorrectly set-up can cause personal injury or equipment damage.

The meter must be placed "IN" the circuit to measure current.

Never measure resistance in a circuit with power applied.

A continuity check is the test or check you make to find out if continuity exists between two points in a circuit.

Summary