Measurement

Question 1

Common Electronics Units

Question 2

The Prefixes

Question 3

Describing the Large

Question 4

Describing the Small

Question 5

Binary Prefixes

Question 6

The Multimeter

A meter is a device that measures electrical quantities in your electronics projects. A multimeter, therefore, is a combination of several different types of meters all in one box. At the minimum, a multimeter combines three distinct types of meters (ammeter, voltmeter, and ohmmeter) into a single device.

Question 7

Ammeter measures current

Current is the flow of electric charge through a conductor. Current is measured in units called amperes. It should come as no surprise, then, that a meter that measures amperage is called an ammeter.

Ammeters usually measure current in milliamperes, also called a milliamp, and abbreviated mA. One mA is one-thousandth of an ampere.

Symbol

Question 8

Voltmeter measures voltage

The second fundamental quantity of electricity is voltage, a term that refers to the difference in electric charge between two points. If those two points are connected to a conductor, a current will flow through the conductor. Thus, voltage is the instigator of current. The device that measures voltage is called a voltmeter.

Symbol

Answer 8

It turns out that, all other things being equal, a change in the amount of voltage between two points results in a corresponding change in current. Thus, if you can keep things equal, you can measure voltage by measuring current, and you already know of a device that can measure current: It’s called an ammeter.

Question 9

Ohmmeter measures resistance

As you know, a resistor is a material that resists the flow of current. How much the current is restricted is a function of the amount of resistance in the resistor, which is measured in units called ohms. The symbol for ohms is the Greek letter omega, Ω. A device that measures resistance is called an ohmmeter.

Symbol

Question 10

Other electronic circuit measurements

All multimeters can measure current, voltage, and resistance. Some multimeters can perform other types of measurements as well. For example, some meters can measure the capacitance of capacitors, and some meters can test diodes or transistors. These features are handy, but not essential.

Question 11

Before going in to detail about multimeters, it is important for you to have a clear idea of how meters are connected into circuits. Diagrams below show a circuit before and after connecting an ammeter:

Answer 11

To measure current, the circuit must be broken to allow:

• the ammeter to be connected in series.
• ammeters must have a LOW resistance

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.

Question 12

Diagrams below show a circuit before and after connecting a voltmeter:

Answer 12

To measure potential difference (voltage), the circuit is not changed:

• the voltmeter is connected in parallel
• voltmeters must have a HIGH resistance

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.

Question 13

An ohmmeter does not function with a circuit connected to a power supply. If you want to measure the resistance of a particular component, you must take it out of the circuit altogether and test it separately, as shown in the diagram below:

Answer 13

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

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 misuse.

Question 14

What does an oscilloscope do?

An oscilloscope is easily the most useful instrument available for testing circuits because it allows you to see the signals at different points in the circuit. The best way of investigating an electronic system is to monitor signals at the input and output of each system block, checking that each block is operating as expected and is correctly linked to the next. With a little practice, you will be able to find and correct faults quickly and accurately.

Question 15

The function of an oscilloscope is extremely simple:

it draws a V/t graph, a graph of voltage against time, voltage on the vertical or Y-axis, and time on the horizontal or X-axis.

Many of the controls of the oscilloscope allow you to change the vertical or horizontal scales of the V/t graph, so that you can display a clear picture of the signal you want to investigate.

‘Dual trace’ oscilloscopes display two V/t graphs at the same time, so that simultaneous signals from different parts of an electronic system can be compared.

Answer 15

As you can see, the screen of this oscilloscope has 8 squares or divisions on the vertical axis, and 10 squares or divsions on the horizontal axis. Usually, these squares are 1 cm in each direction:

Question 16

How does an oscilloscope work?

An outline explanation of how an oscilloscope works can be given using the block diagram shown below:

Answer 16

Like a televison screen, the screen of an oscilloscope consists of a cathode ray tube. Although the size and shape are different, the operating principle is the same. Inside the tube is a vacuum. The electron beam emitted by the heated cathode at the rear end of the tube is accelerated and focused by one or more anodes, and strikes the front of the tube, producing a bright spot on the phosphorescent screen.

The electron beam is bent, or deflected, by voltages applied to two sets of plates fixed in the tube. The horizontal deflection plates, or X-plates produce side to side movement. As you can see, they are linked to a system block called the time base. This produces a sawtooth waveform. During the rising phase of the sawtooth, the spot is driven at a uniform rate from left to right across the front of the screen. During the falling phase, the electron beam returns rapidly from right ot left, but the spot is ‘blanked out’ so that nothing appears on the screen.

In this way, the time base generates the X-axis of the V/t graph.

The slope of the rising phase varies with the frequency of the sawtooth and can be adjusted, using the TIME/DIV control, to change the scale of the X-axis. Dividing the oscilloscope screen into squares allows the horizontal scale to be expressed in seconds, milliseconds or microseconds per division (s/DIV, ms/DIV, µs/DIV). Alternatively, if the squares are 1 cm apart, the scale may be given as s/cm, ms/cm or µs/cm.

The signal to be displayed is connected to the input. The AC/DC switch is usually kept in the DC position (switch closed) so that there is a direct connection to the Y-amplifier. In the AC position (switch open) a capacitor is placed in the signal path. As will be explained in Chapter 5, the capacitor blocks DC signals but allows AC signals to pass.

The Y-amplifier is linked in turn to a pair of Y-plates so that it provides the Y-axis of the the V/t graph. The overall gain of the Y-amplifier can be adjusted, using the VOLTS/DIV control, so that the resulting display is neither too small or too large, but fits the screen and can be seen clearly. The vertical scale is usually given in V/DIV or mV/DIV.

The trigger circuit is used to delay the time base waveform so that the same section of the input signal is displayed on the screen each time the spot moves across. The effect of this is to give a stable picture on the oscilloscope screen, making it easier to measure and interpret the signal.

Changing the scales of the X-axis and Y-axis allows many different signals to be displayed. Sometimes, it is also useful to be able to change the positions of the axes. This is possible using the X-POS and Y-POS controls. For example, with no signal applied, the normal trace is a straight line across the centre of the screen. Adjusting Y-POS allows the zero level on the Y-axis to be changed, moving the whole trace up or down on the screen to give an effective display of signals like pulse waveforms which do not alternate between positive and negative values.

Question 17

Oscilloscope Controls

TIME / DIV: Allows the horizontal scale of the V/t graph to be changed.

Question 18

Oscilloscope Controls

VOLTS / DIV: Adjust the vertical scale of the V/t graph. The vertical scales for CH I and CH II can be adjusted independently.

Question 19

Oscilloscope Controls

Intensity and Focus: Adjusting the INTENSITY control changes the brightness of the oscilloscope display. The FOCUS should be set to produce a bright clear trace.

Question 20

Oscilloscope Controls

CH I and CH II inputs: Signals are connected to the BNC input sockets using BNC plugs.

The smaller socket next to the BNC input socket provides an additional 0 V, GROUND or EARTH connection.

Question 21

Oscilloscope Controls

DC/AC/GND slide switches: In the DC position, the signal input is connected directly to the Y-amplifier of the corresponding channel, CH I or CH II. In the AC position, a capacitor is connected into the signal pathway so that DC voltages are blocked and only changing AC signals are displayed.

In the GND position, the input of the Y-amplfier is connected to 0 V. This allows you to check the position of 0 V on the oscilloscope screen.

The DC position of these switches is correct for most signals.

Question 22

Oscilloscope Controls

X-POS: Allows the whole V/t graph to be moved from side to side on the oscilloscope screen.

This is useful when you want to use the grid in front of the screen to make measurements, for example, to measure the period of a waveform.

Question 23

Oscilloscope Controls

Y-POS: These controls allow the corresponding trace to be moved up or down, changing the position representing 0 V on the oscilloscope screen.

To investigate an alternating signal, you adjust Y-POS so that the 0 V level is close to the centre of the screen. For a pulse waveform, it is more useful to have 0 V close to the bottom of the screen.

Question 24

Oscilloscope Controls

X-MAG: In the IN position, the horizontal scale of the V/t graph is increased by 10 times. For example, if TIME/DIV is set for 1 ms per division and X-MAG is pushed IN, the scale is changed to 0.1 ms per division.