202/2 – Know the principles of basic electrical circuits

Outcome 2.4 – State how instruments are connected into circuits in order

to measure electrical quantities.

Unit 202 Principles of electrical

science

2.3 Calculate power in a basic

electrical circuits

Series and parallel.

Last session

2.4 State how instruments are

connected into circuits in order to

measure electrical quantities.

Voltmeter, ammeter, wattmeter,

ohmmeter.

This session

Digital Meters

Digital meters take an input signal and converts it to a series of pulses

Digital signals are either on or off

Advantages of digital meters

• Strong

• No moving parts

• Accurate

• Easy to read

Disadvantages of digital meters

• Batteries need replacing regularly

• Not easy to tell if it is the meter or the circuit that is at fault

• Readings may vary

Analogue Meters

Analogue signals are variable

which mean they are changing all

the time.

Moving Iron Meter (repulsion type)

A moving-iron meter is based on the principle that when two

magnetic fields oppose each other they deflect or try to

move in opposite directions

Moving Iron Meters

There are two types of moving iron meters repulsion and attraction which both work in a similar way

This type of meter is less commonly used now

Advantages

• Cheap

• Fairly robust

• Can be used on a.c and d.c.

Disadvantages

• Uneven scale

• Can be affected by heat

• Can be affected by stray magnetic fields

• Only accurate at the lower end of the scale

Moving Coil Meter

A coil is wound on an

aluminium frame. It is

supported via a spindle

resting in jewelled

bearings which enable it

to move freely.

Moving Coil Meter

This meter is much easier to read because the scale is linear, meaning the accuracy is constant

Advantages• Even scale• Accurate• Unaffected by stray magnetic fields

Disadvantages• Fragile• Expensive• Only measures d.c.

Voltmeter

• On site for general purposes we use a

multimeter to test for voltage.

• A specific voltage testing device is called a

voltmeter.

• Voltmeter’s measure the difference in

potential between to point on a circuit.

• This test instrument must be connected in

parallel across the load.

• Voltmeters have very high internal resistance

to ensure accurate readings.

Voltmeters

Voltmeters measure Voltage

Voltmeters have to be connected across the load (In

parallel)

U

Load

U

The voltmeter is connected in parallel with the load.

The same voltage is dropped across the load and the

voltmeter.

Altering the Range of the Voltmeter

The problem with connecting a voltmeter into a circuit is that the

current is supposed to flow through the load, what happens is

that it splits up and some current passes through the voltmeter.

The voltmeter needs to have a resistance that is very high, so

reducing the current to a negligible amount.

To alter the range of the voltmeter we must connect a resistor in

series with the meter, this is called a multiplier.

Voltmeters

Ammeter

• On site for general purposes we use

a multimeter to test for current.

• A specific current testing device is

called a ammeter.

• ammeter’s measure the current that

pass through them (current flow).

• This test instrument must be

connected in series with the circuit.

• Ammeter’s have very low internal

resistance to ensure false readings

aren’t given.

Ammeters

Ammeters measure current

In a circuit it is critically important that the ammeter is placed in the

correct position. (IN SERIES)

To understand why think back to resistors.

In a series circuit the current is constant throughout the circuit.

The ammeter is connected in series with the load and we will assume

that the instrument has a resistance and so a volt drop when the

current flows.

Increasing the Range of the Meter

If we want to increase the range of the instrument.

Which is sometimes necessary as the current can destroy the

sensitive coil in the meter. We have to place a resistor in

parallel with the meter. This resistor has a very low value and is

called a shunt.

We have in effect two resistors in parallel connected to a resistor

(the load) that is in series with them

Ammeters

Wattmeter

• On site for general purposes we use

a multimeter to test for power.

• A specific power testing device is

called a wattmeter.

• wattmeter’s measure the current

and voltage that pass through them

and display (apparent) power in

watts.

Wattmeter

Current coil

(SERIES)

Potential coil

(PARALLEL)

AMMETER

(SERIES)

VOLTMETER

(PARALLEL)

WATTMETER

In DC circuits you can simply measure voltage and

current and using P = I x V and calculate power.

In AC circuits using this method you only calculate

APPARENT POWER. This value will be incorrect unless we

have unity power factor (1).

This is due to factors within a system such as

inductance and capacitance, these factors make he

current lead or lag the voltage. We use P = I x V x cos ø

In circuits with any of these two factors we must use a

wattmeter to measure power.

Wattmeter

Three meter method

Current coil

Potential coil

Meter two

Meter three

Meter one

Neutral

Meter one

Meter two

Wattmeter

Two meter method

Ohmmeter

• Ohmmeter’s measure impedance

(Z) also known as resistance.

• On site for general purposes we use

a multimeter to test for impedance.

• A specific impedance testing device

is called a ohmmeter.

• Impedance is defined as the total

opposition to the flow of alternating

current at a specific frequency.

• Quite often these types of meters

are referred to as LRC metres as the

quantify readings from resistance,

inductance and capacitance.

Ohmmeter

• You must connect the tester

at each end of the resistive

load or circuit.

• Test leads must be zeroed out

before the test or tested

initially and deducted from

the result total.

2.4 State how instruments are

connected into circuits in order to

measure electrical quantities.

Voltmeter, ammeter, wattmeter,

ohmmeter.

Consolidation

Outcome 2 formative assessment

Next session