Using a Multimeter

Introduction

The practical work supporting More About Circuits introduces you to using a multimeter to make measurements from circuits. Once you are able to test circuits, you will understand better how they work and be able to locate and correct faults.

What do meters measure?

A meter is a measuring instrument.

A multimeter combines these functions, and possibly some additional ones as well, into a single instrument.

There is nothing wrong with single function voltmeters and ammeters which are often used in teaching physics. Multimeters are cheaper and more versatile. It is worth the effort of finding out how to use one when you are studying electronics.

Before going into detail about multimeters, it is important for you to have a clear idea of how meters are connected into circuits. Fig.1 below shows a

circuit

A circuit is a closed conducting path.circuit before (A) and after (B) connecting an

ammeter

An ammeter measures current. Ammeters are connected in series in a circuit and must have a low resistance.ammeter:

Figure 1.	Connecting an ammeter.

Think about the changes you would have to make to a practical circuit in order to include the ammeter.

To start with, you need to break the circuit so that the ammeter can be connected in series.

All the current flowing in the circuit must pass through the ammeter.

Meters are not supposed to alter the behaviour of the circuit, or at least not significantly, and it follows that an ammeter must have a very low resistance.

To measure current, the circuit must be broken to allow the ammeter to be connected in series. Ammeters must have a low resistance.
            

Fig.2(C) shows the same circuit after connecting a

voltmeter

A voltmeter measures the voltage, or difference in energy, between two points in a circuit. Voltmeters are connected in parallel and must have a high resistance.voltmeter:

Figure 2.	Connecting a voltmeter.

This time, you do not need to break the circuit.

The voltmeter is connected in parallel between the two points where the measurement is to be made.

Since the voltmeter provides a parallel pathway, it should take as little current as possible. In other words, a voltmeter should have a very high resistance.

To measure potential difference (voltage), the circuit is not changed. The voltmeter is connected in parallel. Voltmeters
               must have a high resistance.
            

Which measurement technique do you think will be the more useful? In fact,

voltage

Potential difference, or voltage V is a measure of the difference in energy between two points in a circuit. Charges gain energy in the battery and lose energy as they flow round the rest of the circuit.voltage measurements are used much more often than

current

Current I is a flow of charged particles, usually electrons.current measurements.

The processing of electronic signals is usually thought of in

voltage

Potential difference, or voltage V is a measure of the difference in energy between two points in a circuit. Charges gain energy in the battery and lose energy as they flow round the rest of the circuit.voltage terms. It is an added advantage that a voltage measurement is easier to make. The original circuit does not need to be changed. Often, the meter probes are connected simply by touching them to the points of interest.

An

ohmmeter

An ohmmeter measures resistance. To find its resistance, the component to be tested must be removed from any circuit and connected to the ohmmeter separately.ohmmeter does not function with a circuit connected to a power supply. If you want to measure the

resistance

Resistance R limits current flow.resistance of a particular component, you must take it out of the circuit altogether and test it separately, as shown in Fig.3 (D):

Figure 3.	Connecting an ohmmeter.

Ohmmeters work by passing a small current through the component and measuring the voltage produced.

If you try this with the component connected into a circuit with a power supply, the most likely result is that the meter will be damaged.

Most multimeters have a fuse to help protect against this kind of mistake.

To measure resistance, the component must be removed from the circuit altogether. Ohmmeters work by passing a current through
               the component being tested.
            

Switched range multimeters

Multimeters are designed and mass produced for electronics engineers. Even the simplest and cheapest types may include features which you are not likely to use.

Digital

In a digital circuit, information is represented by discrete voltage levels. A high voltage is called logic 1, or 1, while a low voltage is called logic 0, or 0. Digital meters give an output in numbers, usually on a liquid crystal display.

The diagram below shows a switched range multimeter:

Figure 4.	Switched range multimeter.

The central knob has lots of positions and you must choose which one is appropriate for the measurement you want to make. If the meter is switched to 20 V

d.c.

In a d.c., or direct current, circuit, current always flows in the same direction.d.c., for example, then 20 V is the maximum voltage which can be measured, This is sometimes called 20 V

f.s.d.

The full scale deflection, or f.s.d., is the maximum reading which a switched range multimeter can display on any particular measurement range.f.s.d., where f.s.d. is short for full scale deflection.

You can change the switch position of the

multimeter

A multimeter can be set up to work as a voltmeter, as an ammeter, or as an ohmmeter. These functions are selected by rotating the central control knob to the appropriate position.multimeter in the diagram. Drag on the control to change the position of the meter switch. Find out which settings make the multimeter work as an ammeter, as a voltmeter, and as an ohmmeter. Where should the multimeter leads be connected? Find out by moving the mouse pointer over the meter sockets.

For circuits with power supplies of up to 20 V, which includes all the circuits you are likely to build, the 20 V d.c. voltage range is the most useful. However, you may sometimes want to measure smaller voltages, and in this case, the 2 V or 200 mV ranges are used. The different ranges available are indicated by

on the meter.

What does d.c. mean?

It stands for direct current.

In any circuit which operates from a steady voltage source, such as a battery, current flow is always in the same direction.

Every constructional project described in Absorb Electronics works in this way.

Some power supplies are referred to as a.c.

This means alternating current.

In an electric lamp connected to the domestic mains electricity, current flows first one way, then the other. That is, the current reverses, or alternates, in direction. With UK mains, the current reverses 50 times per second. In the USA, the current reverses 60 times per second.

For safety reasons, you must never connect a multimeter to the mains supply.

You are not at all likely to use the

a.c.

In an a.c., or alternating current, circuit, current flows first in one direction, then in the other.a.c. ranges, indicated by

on your multimeter.

In prototype board diagrams in Absorb Electronics, a switched range meter is represented by a simplified symbol, as shown in Fig.5:

Figure 5.	Switched range multimeter symbol.

Different versions of the symbol are used to represent the use of the multimeter as a voltmeter, ammeter, or ohmmeter.

Autoranging multimeters

An alternative style of multimeter is the autoranging multimeter:

Figure 6.	Autoranging multimeter.

This is a more sophisticated instrument capable of measuring capacitance and frequency in addition to the current, voltage, and resistance. Nevertheless, the central knob has fewer positions and all you need to do is to switch it to the quantity you want to measure. Once switched to V, the meter automatically adjusts its range to give a meaningful reading, and the display includes the unit of measurement, V or mV. This type of meter is more expensive, but obviously much easier to use.

Drag the control on the meter knob to find out which functions are available.

Where are the two meter probes connected? To make a voltage reading, the black lead is always connected into the socket marked COM, short for 'common'. The red lead is connected into the socket labelled VΩF. This particular meter has an additional socket labelled mACx. When you want to use the meter as an ammeter, you need to transfer the red lead to this socket. The 10A socket is very rarely used.

In prototype board diagrams in Absorb Electronics, an autoranging meter is represented by a simplified symbol, as shown in Fig.7:

Figure 7.	Switched range multimeter symbol.

Different versions of the symbol are used to represent the use of the multimeter as a voltmeter, ammeter, or ohmmeter.

Analogue multimeters

An

analogue

In an analogue circuit, information is represented by continuous changes in voltage. An audio signals is an example of an analogue signal. analogue meter moves a needle along a scale.

Switched range

The central control knob of a switched range multimeter has different positions for each measurement range.Switched range

analogue

In an analogue circuit, information is represented by continuous changes in voltage. An audio signals is an example of an analogue signal. analogue multimeters are very cheap but are difficult for beginners to read accurately, especially on resistance scales. The meter movement is delicate and dropping the meter is likely to damage it.

Figure 8.	Analogue multimeter.

Each type of meter has its advantages. Used as a voltmeter, a digital meter is usually better because its resistance is much higher – 1 MΩ or 10 MΩ, compared to 200 kΩ for an analogue multimeter on a similar range. On the other hand, it is easier to follow a slowly changing voltage by watching the needle on an analogue display.

Used as an ammeter, an analogue multimeter has a very low resistance and is very sensitive, with scales down to 50 μA. More expensive digital multimeters can equal or better this performance.

Most modern multimeters are digital and traditional analogue types are destined to become obsolete.

Making voltage measurements

Build the circuit shown below using

prototype board

Prototype board is used for building temporary circuits. Connections are made by pushing components and wire links into the holes in the prototype board.prototype board and four 10 kΩ resistors:

Figure 9.	Resistors in series.

You can see what the circuit looks like with a switched range or an autorange meter by clicking the button.

Expand this section to be reminded about how to make link wires.

The joining wires which you use to connect up your circuit must be made from insulated single core wire, described in catalogues as '1/0.6 mm single core equipment wire'. To make the wire links, you need to remove about 5 mm of insulation from each end. This is done using wire strippers:

Figure 10.	Miniature wire stripper.

Adjust the position of the screw, tightening the locking nut on the other side, so that the jaws of the stripper do not close completely. The jaws grip and cut into the outer insulation without penetrating into the wire core. Automatic strippers, shown below, are quick and easy to use and work well provided you are careful with them:

Figure 11.	Automatic wire stripper.

A pair of cutting blades in the handle lets you cut the wires to length. Place the end of the wire in the jaws against the adjustable stop and close the handles to remove the insulation. Cut links 50 mm long and keep them for reuse.

Using the multimeter as a voltmeter, measure the power supply voltage, as indicated in Fig.9.
Adjust the power supply to 6 V if required.
Now move the crocodile clip to position A and make a new measurement.
Similarly, measure the voltages at points B and C.
Complete the table to show your voltage reading at each of these points:

Location	Measured voltage / V
Point A
Point B
Point C

The four resistors are connected in

series

Components are connected in series when they are joined end to end in a circuit, so that the same current flows through each.series, making a chain known as a voltage divider, or potential divider. The total power supply voltage is shared between the four resistors and, allowing for variations in the

resistor

A resistor is an electronic component with a particular resistance values. Resistors limit current.resistor value, each resistor receives an equal share. (You will find out a lot more about voltage dividers in the unit, Voltage Dividers.)

Based on this idea, what voltage reading is expected at each of the points?

At point A:
At point B:
At point C:

Your voltage readings should approximate to these values, provided the power supply is close to 6 V and the resistor values are equal.

Modify the circuit, replacing one or more of the 10 kΩ resistors with 1 kΩ or 100 kΩ values. Are the results as you expect?

The diagram below shows a light sensor circuit built in a similar way:

Figure 12.	Light sensor circuit.

The circuit uses a light-dependent resistor or LDR. The resistance of the

LDR

A light-dependent resistor, or LDR, has a high resistance in the dark, and a low resistance in the light.LDR changes with the level of illumination. In the dark, the resistance is high, up to 1 MΩ or more. When light shines on the LDR, the light energy increases the number of charge carriers available to transfer current, and the resistance falls. In bright light, the resistance can be as little as 100 Ω.

Making resistance measurements

To get the multimeter to function as an ohmmeter, you will need to select a resistance range. With a switched range meter, the 200 kΩ position is usually suitable.

Remove the LDR from the circuit and measure its resistance, as follows:

Figure 13.	Measuring resistance of LDR.

You will see the resistance measurement change as the light level changes. Covering the LDR with your hand increases the resistance of the LDR.

If the meter reads

this means that the resistance is more than the maximum which can be measured on this range and you may need to switch to a new position, 2000 kΩ, to take a reading.

You can check the value of any fixed value resistor in the same way, and confirm that you have worked out the colour code correctly. A colour code converter program is available from the 'Tools' menu to help you. (The resistor colour code system is explained in the unit, Resistors.)

Making current measurements

Fig.14 below shows a prototype board set up for the measurement of current:

Figure 14.	Measuring current in a prototype board circuit.

Note that the current must flow through the ammeter in order to reach the circuit.

It is quite easy to 'blow' the

fuse

A fuse is included in a circuit as a safety device that isolates the circuit if excess current flows. One type of fuse consists of a fine wire which heats up and melts, breaking the circuit if too much current flows.fuse inside the multimeter while working with current ranges. if you have wired everything up correctly and set the meter range correctly, but the circuit fails to function, this is the most likely reason. The fuse can be replaced by opening up the meter. Sometimes the fuse is accessible from the

battery

A battery consists of two or more cells. The cells may be connected in series or in parallel.battery compartment, but often you will need to remove the small screws holding the back of the meter in place:

Figure 15.	Fuse inside a multimeter.

If necessary, replace the fuse with another of the same rating, 200 mA in this case. Don't lose or forget about the screws!

Take a reading of the current in Fig.14 with the link wire to 0 V in position A. Write down the current value you observe. Take new readings after moving the link to positions B, C, and D.

Enter your current readings in mA in each of the boxes.

location	measured current / mA
link wire in position A
link wire in position B
link wire in position C

As the resistance is reduced, the current increases. Calculate the current expected in each case using the formula:

What are the expected current readings?

Current with link wire in position A:
Current with link wire in position B:
Current with link wire in position C:
Current with link wire in position D:

Small variations, up to ±5%, can be attributed to the manufacturing accuracy, or

tolerance

Tolerance describes manufacturing accuracy. If a component is manufactured to a tolerance of ±5%, this means that its actual value is guaranteed to be within 5% of its marked, or nominal value.tolerance, of the resistors.

By working through this practical, you have found out quite a lot about multimeters and how to use them. Most often, you will use a multimeter to measure voltage. Look back to this section to confirm that you understand how to do this.

Summary

The step-through animation below summarizes the prototype boards constructed in this unit.

Figure 16.	Using a multimeter.

Parts list:

Quantity	Items
	switched range multimeter
	red and black 4 mm leads
	crocodile clips
	prototype board (breadboard)
	+6 V d.c. power supply
4	100 Ω 0.5 W carbon film resistors
4	10 kΩ 0.5 W carbon film resistors
4	100 kΩ 0.5 W carbon film resistors
	additional 0.5 W carbon film resistors in assorted values
1	light-dependent resistor, ORP12 or similar
	link wires
	wire stripper
	pliers
	side cutters