3. Introduction to Electronics – CE/ENVE 320-04
The study of electronics is the branch of physics that deals with the emission and effects of
electrons and with the use of electronic devices.
Have you ever taken a flashlight apart to find out how it works? If not, you most certainly have
taken a flashlight apart to replace the bulb or batteries. The diagram below which shows the
arrangement of parts inside of a basic flashlight:
Structure of an electric flashlight
Why did the designer choose this particular combination of materials? The metal parts of the
flashlight must conduct electric current if the torch is to function, but they must also be able to
stand up to physical forces. The spring holding the cells in place should stay springy, while the
parts of the switch must make good electrical contact and be undamaged by repeated use.
The lamp and reflector make up an optical system, often intended to focus the light into a narrow
beam. The plastic casing is an electrical insulator. Its shape and color are important in making
the torch attractive and easy to handle and use.
A flashlight is a simple product, but a lot of thought is needed to make sure that it will work well.
Can you think of other things which the designer should consider?
A different way of describing the flashlight is by using a circuit diagram in which the
components of the light are represented by symbols:
Circuit diagram of a flashlight
There are two electric cells ('batteries'), a switch and a lamp (the light bulb). The lines in the
diagram represent the metal conductors which connect the system together.
A circuit is a closed conducting path. In the flashlight, closing the switch completes the circuit
and allows current to flow. Flashlights sometimes fail when the metal parts of the switch do no
make proper contact, or when the lamp filament is 'blown'. In either case, the circuit is
incomplete.
Current
An electric current is a flow of charged particles. Inside a copper wire, current is carried by
small negatively-charged particles, called electrons. The electrons drift in random directions
until a current starts to flow. When this happens, electrons start to move in the same direction.
The size of the current depends on the number of electrons passing per second.
Current is represented by the symbol I, and is measured in amperes, or 'amps', A. One ampere is
a flow of 6.24 x 1018 electrons per second past any point in a wire. That's more than six million
million million electrons passing per second. This is a lot of electrons, but electrons are very
small and each carries a very tiny charge.
In electronic circuits, currents are most often measured in milliamps, mA, that is, thousandths of
an amp.
Voltage
In the torch circuit, what causes the current to flow? The answer is that the cells provide a 'push'
which makes the current flow round the circuit. When the cells are new, enough current flows to
light the lamp brightly. On the other hand, if the cells have been used for some time, they may be
'flat' and the lamp glows dimly or not at all.
Each cell provides a push, called its potential difference, or voltage. This is represented by the
symbol V , and is measured in volts, V.
Typically, each cell provides 1.5 V. Two cells connected one after another, in series, provide
3 V, while three cells would provide 4.5 V:
Cells connected in series
Which arrangement would make the lamp glow most brightly? Lamps are designed to work with
a particular voltage, but, other things being equal, the bigger the voltage, the brighter the lamp.
Strictly speaking, a battery consists of two or more cells. These can be connected in series, as is
usual in a torch circuit, but it is also possible to connect the cells in parallel, like this:
Cells connected in parallel
A single cell can provide a little current for a long time, or a big current for a short time.
Connecting the cells in series increases the voltage, but does not affect the useful life of the cells.
On the other hand, if the cells are connected in parallel, the voltage stays at 1.5 V, but the life of
the battery is doubled.
A flashlight bulb which uses 300 mA from C-size alkaline cells should operate for more than 20
hours before the cells are exhausted.
Which way does the current flow?
One terminal of a cell or battery is positive, while the other is negative. It is convenient to think
of current as flowing from positive to negative. This is called conventional current. Current
arrows in circuit diagrams always point in the conventional direction. However, you should be
aware that this is the direction of flow for a positively-charged particle.
In a copper wire, the charge carriers are electrons. Electrons are negatively-charged and
therefore flow from negative to positive. This means that electron flow is opposite in direction to
conventional current.
Current flow in electronic systems often involves charge carriers of both types. For example, in
transistors, current can be carried by electrons and also by holes, which behave as positive
charge carriers.
When the behavior of a circuit is analyzed, what matters is the amount of charge which is being
transferred. The effect of the current can be accurately predicted without knowing about whether
the charge carriers are positively or negatively charged.
A cell provides a steady voltage, so that current flow is always in the same direction. This is
called direct current, or d.c. In contrast, the domestic electric mains provide a constantly
changing voltage which reverses in polarity 60 times every second. This gives rise to alternating
current, or a.c., in which the charge carriers move backwards and forwards in the circuit.
Resistance
If a thick copper wire was connected from the positive terminal of a battery directly to the
negative terminal, you would get a very large current for a very short time. In a torch, this does
not happen.
Part of the torch circuit limits, or resists, the flow of current. Most of the circuit consists of thick
metal conductors which allow current to flow easily. These parts, including the spring, switch
plates and lamp connections, have a low resistance. The lamp filament, on the other hand, is
made up of very thin wire. It conducts much less easily than the rest of the circuit and has a
higher resistance.
The flow of current through the filament causes it to heat up and glow white hot. In air, the
filament would quickly oxidize. This is prevented by removing all the air inside the glass of the
lamp and replacing it with a non-reactive gas.
The resistance, R , of the filament is measured in ohms, Ω. If the battery voltage is 3 V (2 C-size
cells in series) and the lamp current is 300 mA, 0.3 A, what is the resistance of the filament?
This is calculated from:
where R is resistance, V is the voltage across the lamp, and I is current. (Although it may not
appear logical, the symbol I is always used for current. C is used for capacitance.)
In this case, 10 Ω is the resistance of the lamp filament once it has heated up. Its resistance is less
when cold and there will be a surge of current, more than 300 mA, when the torch is first
switched on.
Resistance values in electronic circuits vary from a few ohms, Ω, to values in kilohms, kΩ,
(thousands of ohms) and megohms, MΩ, (millions of ohms). Electronic components designed to
have particular resistance values are called resistors.
What do resistors do?
Resistors limit current. In a typical application, a resistor is connected in series with an LED:
Enough current flows to make the LED light up, but not so much that the LED is damaged. Later
in this Chapter, you will find out how to calculate a suitable value for this resistor. (LEDs are
described in detail in Chapter 5.)
The 'box' symbol for a fixed resistor is popular in the UK and Europe. A 'zig-zag' symbol is used
in America and Japan:
Resistors are used with transducers to make sensor subsystems. Transducers are electronic
components which convert energy from one form into another, where one of the forms of energy
is electrical. A light dependent resistor, or LDR, is an example of an input transducer.
Changes in the brightness of the light shining onto the surface of the LDR result in changes in its
resistance. As will be explained later, an input transducer is most often connected along with a
resistor to to make a circuit called a potential divider. In this case, the output of the potential
divider will be a voltage signal which reflects changes in illumination.
Microphones and switches are input transducers. Output transducers include loudspeakers,
filament lamps and LEDs. Can you think of other examples of transducers of each type?
In other circuits, resistors are used to direct current flow to particular parts of the circuit, or may
be used to determine the voltage gain of an amplifier. Resistors are used with capacitors (Chapter
4) to introduce time delays.
Most electronic circuits require resistors to make them work properly and it is obviously
important to find out something about the different types of resistor available, and to be able to
, or M , for a particular application.
choose the correct resistor value, in ,
Fixed value resistors
The diagram shows the construction of a carbon film resistor:
During manufacture, a thin film of carbon is deposited onto a small ceramic rod. The resistive
coating is spiraled away in an automatic machine until the resistance between the two ends of the
rod is as close as possible to the correct value. Metal leads and end caps are added, the resistor is
covered with an insulating coating and finally painted with colored bands to indicate the resistor
value.
Carbon film resistors are cheap and easily available, with values within ±10% or ±5% of their
marked, or 'nominal' value. Metal film and metal oxide resistors are made in a similar way, but
can be made more accurately to within ±2% or ±1% of their nominal value. There are some
differences in performance between these resistor types, but none which affect their use in
simple circuits.
Wirewound resistors are made by winding thin wire onto a ceramic rod. They can be made
extremely accurately for use in multimeters, oscilloscopes and other measuring equipment. Some
types of wirewound resistors can pass large currents without overheating and are used in power
supplies and other high current circuits.
Color code
How can the value of a resistor be worked out from the colors of the bands? Each color
represents a number according to the following scheme:
Number
Color
0
black
1
brown
2
red
3
orange
4
yellow
5
green
6
blue
7
violet
8
grey
9
white
The first band on a resistor is interpreted as the FIRST DIGIT of the resistor value. For the
resistor shown below, the first band is yellow, so the first digit is 4:
The second band gives the SECOND DIGIT. This is a violet band, making the second digit 7.
The third band is called the MULTIPLIER and is not interpreted in quite the same way. The
multiplier tells you how many zeros you should write after the digits you already have. A red
band tells you to add 2 zeros. The value of this resistor is therefore 4 7 0 0 ohms, that is,
4 700 , or 4.7
. Work through this example again to confirm that you understand how to
apply the color code given by the first three bands.
The remaining band is called the TOLERANCE band. This indicates the percentage accuracy of
the resistor value. Most carbon film resistors have a gold-colored tolerance band, indicating that
the actual resistance value is with + or - 5% of the nominal value. Other tolerance colors are:
Tolerance Color
±1%
brown
±2%
red
±5%
gold
±10%
silver
When you want to read off a resistor value, look for the tolerance band, usually gold, and hold
the resistor with the tolerance band at its right hand end. Reading resistor values quickly and
accurately isn't difficult, but it does take practice!
More about color codes
The color code as explained above allows you to interpret the values of any resistor from 100
upwards. How does the code work for values less than 100 ? Here is the code for 12 :
brown, red, black
The multiplier color black represents the number 0 and tells you that no zeros should be added to
the first two digits, representing 1 and 2.
What would be the color code for 47
? The answer is:
yellow, violet, black
Using this method for indicating values between 10
require the same number of bands.
For values bewteen 1
colors:
and 10
and 100
means that all resistor values
, the multiplier color is changed to gold. For example, the
brown, black, gold
indicate a 1
resistor, while the colors:
red, red, gold
refer to a 2.2
resistor.
Metal film resistors, manufactured to 1 or 2% tolerance, often use a code consisting of four
colored bands instead of three. The code works in the same way, with the first three bands
interpreted as digits and the fourth band as the multiplier. For example, a 1
metal film
resistor has the bands:
brown, black, black, brown (+brown or red for tolerance)
while a 56
metal film resistor has the bands:
green, blue, black, red
It is worth pointing out that the multiplier for metal film resistors with values from 1
upwards is brown (rather than red, as in the three color system), while the multiplier for 10
upwards is red (instead of orange).
You are likely to use low value resistors and metal film resistors on some occasions and it is
useful to know how to read their codes. However, most of the resistors you use in building
electronic circuits will be carbon film types with values indicated using the three band color
code. It is this system which you should master first.
E12 and E24 values
If you have any experience of building circuits, you will have noticed that resistors commonly
have values such as 2.2
, 3.3
, or 4.7
and are not available in equally spaced values 2
,3
,4
,5
and so on. Manufacturers don't produce values like these - why not?
The answer is partly to do with the fact that resistors are manufactured to percentage accuracy.
Look at the table below which shows the values of the E12 and E24 series:
E12 series
10%
tolerance
E24 series
5%
tolerance
10
10
11
12
12
13
15
15
16
18
18
20
22
22
24
27
27
30
33
33
36
39
39
43
47
47
51
56
56
62
68
68
75
82
82
91
Resistors are made in multiples of these values, for example, 1.2
, 120
and so on.
, 12
, 120
, 1.2
, 12
Consider 100 and 120 , adjacent values in the E12 range. 10% of 100 is 10 , while 10%
of 120 is 12 . A resistor marked as 100 could have any value from 90 to 110 , while
a resistor marked as 120 might have an actual resistance from 108 to 132 . The ranges of
possible values overlap, but only slightly.
Further up the E12 range, a resistor marked as 680 might have and actual resistance of up to
680+68=748 , while a resistor marked as 820 might have a resistance as low as 820-82=738
. Again, the ranges of possible values just overlap.
The E12 and E24 ranges are designed to cover the entire resistance range with the minimum
overlap between values. This means that, when you replace one resistor with another marked as a
higher value, its actual resistance is almost certain to be larger.
From a practical point of view, all that matters is for you to know that carbon film resistors are
available in multiples of the E12 and E24 values. Very often, having calculated the resistance
value you want for a particular application, you will need to choose the nearest value from the
E12 or E24 range.
Power Rating
When current flows through a resistance, electrical energy is converted into heat. This is obvious
in an electric flashlight where the lamp filament heats up and glows white hot. Although the
result may be less evident or imperceptible, exactly the same process of energy conversion goes
on when current flows through any electronic component.
The power output of a lamp, resistor, or other component, is defined as the rate of change of
electrical energy to heat, light, or some other form of energy. Power is measured in watts, W, or
milliwatts, mW, and can be calculated from:
where P is power.
What is the power output of a resistor when the voltage across it is 6 V, and the current flowing
through it is 100 mA?
0.6 W of heat are generated in this resistor. To prevent overheating, it must be possible for heat
to be lost, or dissipated, to the surroundings at the same rate.
A resistor's ability to lose heat depends to a large extent upon its surface area. A small resistor
with a limited surface area cannot dissipate (lose) heat quickly and is likely to overheat if large
currents are passed. Larger resistors dissipate heat more effectively.
Look at the diagram below which shows resistors of different sizes:
The standard size of carbon film resistor used in most circuits has a power rating of 0.5 W. This
means that a resistor of this size can lose heat at a maximum rate of 0.5 W. In the example above,
the calculated rate of heat loss was 0.6 W, so that a resistor with a higher power rating, 1 W or 2
W, would be needed. Some resistors are designed to pass very large currents and are cased in
aluminum with fins to increase surface area and promote heat loss.
Input and signal processing subsystems in electronic circuits rarely involve large currents, but
power rating should be considered when circuits drive output transducers, such as lamps, LEDs,
and loudspeakers.
OHM'S LAW
Ohm's Law deals with the relationship between voltage and current in an ideal conductor. This
relationship states that:
The potential difference (voltage) across an ideal conductor is proportional to the current
through it.
The constant of proportionality is called the "resistance", R.
Ohm's Law is given by:
V=IR
where V is the potential difference between two points which include a resistance R. I is the
current flowing through the resistance. For biological work, it is often preferable to use the
conductance, g = 1/R; In this form Ohm's Law is:
I=gV
Material that obeys Ohm's Law is called an Ohmic conductor or a linear conductor because
the potential difference across it varies linearly with the current.
Ohm's Law can be used to solve simple circuits. A complete circuit is one which is a closed loop.
It contains at least one source of voltage (thus providing an increase of potential energy), and at
least one potential drop i.e., a place where potential energy decreases. The sum of the voltages
around a complete circuit is zero.
An increase of potential energy in a circuit causes a charge to move from a lower to a higher
potential (ie. voltage). Note the difference between potential energy and potential.
Because of the electrostatic force, which tries to move a positive charge from a higher to a lower
potential, there must be another 'force' to move charge from a lower potential to a higher inside
the battery. This so-called force is called the electromotive force, or emf. The SI unit for the
emf is a volt (and thus this is not really a force, despite its name). We will use a script E, the
symbol
, to represent the emf.
A decrease of potential energy can occur by various means. For example, heat lost in a circuit
due to some electrical resistance could be one source of energy drop.
Because energy is conserved, the potential difference across an emf must be equal to the
potential difference across the rest of the circuit. That is, Ohm's Law will be satisfied:
=IR
RESISTORS IN SERIES
Resistors can be connected in series; that is, the current
flows through them one after another. The circuit in Figure 1
shows three resistors connected in series, and the direction of
current is indicated by the arrow.
Figure 1 Resistors connected in
series.
Note that since there is only one path for the current to travel, the current through each of the
resistors is the same.
[1]
Also, the voltage drops across the resistors must add up to the total voltage supplied by the
battery:
[2]
Since V = I R, then
[3]
But Ohm's Law must also be satisfied for the complete circuit:
[4]
Setting equations [3] and [4] equal, we get:
[5]
We know what the current through each resistor (from equation [1]) is just I.
[6]
So the currents cancel on both sides, and we arrive at an expression for equivalent resistance for
resistors connected in series.
[7]
In general, the equivalent resistance of resistors connected in series is the sum of their
resistances. That is,
[8]
This can also be written in terms of conductances, since conductance is just the reciprocal of
resistance:
[9]
RESISTORS IN PARALLEL
Resistors can be connected such that they branch out from a single point (known as a node), and
join up again somewhere else in the circuit. This is known as a parallel connection. Each of the
three resistors in Figure 1 is another path for current to travel between points A and B.
Figure 1 Example of a circuit
containing three resistors connected
in parallel
Figure 2 Circuit containing resistors
in parallel, equivalent to Figure 1
Note that the node does not have to physically be a single point; as long as the current has
several alternate paths to follow, then that part of the circuit is considered to be parallel. Figures
1 and 2 are identical circuits, but with different appearances.
At A the potential must be the same for each resistor. Similarly, at B the potential must also be
the same for each resistor. So, between points A and B, the potential difference is the same. That
is, each of the three resistors in the parallel circuit must have the same voltage.
[1]
Also, the current splits as it travels from A to B. So, the sum of the currents through the three
branches is the same as the current at A and at B (where the currents from the branch reunite).
[2]
By Ohm's Law, equation [2] is equivalent to:
[3]
By equation [1], we see that all the voltages are equal. So the V's cancel out, and we are left with
[4]
This result can be generalized to any number of resistors connected in parallel.
[5]
Since resistance is the reciprocal of conductance, equation [5] can be expressed in terms of
conductances.
[6]
RESISTORS IN COMBINATION CIRCUITS
Here, we will combine series circuits and
parallel circuits. These are known as
combination circuits. No new equations
will be learned here.
We can imagine a branch in a parallel
circuit, but which contains two resistors in
series. For example, between points A and
B in Figure 1.
In this situation, we could calculate the
equivalent resistance of branch AB using
our rules for series circuits. So,
Figure 1 Combination Circuit 1
Now, we can replace the two resistors with a single, equivalent resistor with no effective change
to the circuit.
As can be seen in Figure 2, the circuit is now a
parallel circuit, with resistors RAB and R3 in
parallel. This circuit can be solved using the same
rules as any other
Figure 2 Circuit 1 simplified to give a
parallel circuit
Kirchhoff's Current Law
This fundamental law results from the conservation
of charge. It applies to a junction or node in a circuit - a point in the circuit where charge has several
possible paths to travel.
In Figure 1, we see that IA is the only current flowing
into the node. However, there are three paths for
current to leave the node, and these current are
represented by IB, IC, and ID.
Once charge has entered into the node, it has no place
to go except to leave (this is known as conservation
of charge). The total charge flowing into a node must
be the same as the the total charge flowing out of the
node. So,
IB + IC + ID = IA
Bringing everything to the left side of the above
equation, we get
Figure 1 Possible node (or junction) in a
circuit
(IB + IC + ID) - IA = 0
Then, the sum of all the currents is zero. This can be generalized as follows
Note the convention we have chosen here: current flowing into the node are taken to be negative,
and currents flowing out of the node are positive. It should not really matter which you choose to
be the positive or negative current, as long as you stay consistent. However, it may be a good
idea to find out the convention used in your class.
Kirchhoff's Voltage Law
Kirchhoff's Voltage Law (or Kirchhoff's Loop Rule) is a
result of the electrostatic field being conservative. It states
that the total voltage around a closed loop must be zero. If
this were not the case, then when we travel around a
closed loop, the voltages would be indefinite. So
In Figure 1 the total voltage around loop 1 should sum to
zero, as does the total voltage in loop2. Furthermore, the
loop which consists of the outer part of the circuit (the
path ABCD) should also sum to zero.
Figure 1 Around a closed loop, the
total voltage should be zero
We can adopt the convention that potential gains (i.e. going from lower to higher potential, such
as with an emf source) is taken to be positive. Potential losses (such as across a resistor) will
then be negative. However, as long as you are consistent in doing your problems, you should be
able to choose whichever convention you like. It is a good idea to adopt the convention used in
your class.
Here are a number of simulated experiments based on Kirchoff's Laws:
http://webphysics.ph.msstate.edu/javamirror/ipmj/java/kirch5/
What is a voltage divider?
You are going to find out but don't be in too much of a hurry. Work through the Chapter and
allow the explanation to develop.
The diagram below shows a light dependent resistor, or LDR, together with its circuit symbol:
The light-sensitive part of the LDR is a wavy track of cadmium sulphide. Light energy triggers
the release of extra charge carriers in this material, so that its resistance falls as the level of
illumination increases.
A light sensor uses an LDR as part of a voltage divider.
The essential circuit of a voltage divider, also called a potential divider, is:
As you can see, two resistors are connected in series. with Vin , which is often the power supply
voltage, connected above Rtop . The output voltage Vout is the voltage across Rbottom and is given
by:
It may help you to remember that Rbottom appears on the top line of the formula because Vout is
measured across Rbottom .
What happens if one of the resistors in the voltage divider is replaced by an LDR? In the circuit
below, Rtop is a 10
resistor, and an LDR is used as Rbottom :
Suppose the LDR has a resistance of 500
(these values are reasonable).
, 0.5
, in bright light, and 200
in the shade
When the LDR is in the light, Vout will be:
In the shade, Vout will be:
In other words, this circuit gives a LOW voltage when the LDR is in the light, and a HIGH
voltage when the LDR is in the shade. The voltage divider circuit gives an output voltage which
changes with illumination.
A sensor subsystem which functions like this could be thought of as a 'dark sensor' and could be
used to control lighting circuits which are switched on automatically in the evening.
Perhaps this does not seem terribly exciting, but almost every sensor circuit you can think of uses
a voltage divider. There's just no other way to make sensor subsystems work.
Here is the voltage divider built with the LDR in place of Rtop :
What effect does this have on Vout ?
The action of the circuit is reversed. that is, Vout becomes HIGH when the LDR is in the light,
and LOW when the LDR is in the shade. Substitute the appropriate values in the voltage divider
formula to convince yourself that this is true.
Temperature sensors
A temperature-sensitive resistor is called a thermistor. There are several different types:
The resistance of most common types of thermistor decreases as the temperature rises. They are
called negative temperature coefficient, or ntc, thermistors. Note the -t° next to the circuit
symbol. A typical ntc thermistor is made using semiconductor metal oxide materials.
(Semiconductors have resistance properties midway between those of conductors and insulators.)
As the temperature rises, more charge carriers become available and the resistance falls.
Although less often used, it is possible to manufacture positive temperature coefficient, or ptc,
thermistors. These are made of different materials and show an increase in resistance with
temperature.
How could you make a sensor circuit for use in a fire alarm? You want a circuit which will
deliver a HIGH voltage when hot conditions are detected. You need a voltage divider with the
ntc thermistor in the Rtop position:
How could you make a sensor circuit to detect temperatures less than 4°C to warn motorists that
there may be ice on the road? You want a circuit which will give a HIGH voltage in cold
conditions. You need a voltage divider with the thermistor in place of Rbottom :
This last application raises an important question: How do you know what value of Vout you are
going to get at 4°C?
To answer this question, you need to estimate the resistance of the thermistor at 4°C.
Lots of different types of thermistor are manufactured and each has its own characteristic pattern
of resistance change with temperature. The diagram below shows the thermistor characteristic
curve for one particular thermistor:
On the y-axis, resistance is plotted on a logarithmic scale. This is a way of compressing the
graph so that it is easier to see how the resistance changes. Between 100 and 1000 , each
horizontal division corresponds to 100 . On the other hand, between 1000 and 10000 ,
each division corresponds to 1000 . Above 10000 , each division respresents 10000 .
As you can see, this thermistor has a resistance which varies from around 70
at 0°C to about
1
at 100°C. Suppliers catalogues usually give the resistance at 25°C, which was 20
in
this case. Usually, catalogues also specify a 'Beta' or 'B-value'. When these two numbers are
specified, it is possible to calculate an approximate value for the resistance of the thermistor at
any particular temperature from the equation:
Where:
RT is the resistance at temperature T in Kelvin (= °C +273)
RT0 is the resistance at a reference temperature T0 in Kelvin. When the reference temperature is
25°C, T0 = 25+273.
e is the natural logarithm base, raised to the power
B is the B-value specified for this thermistor.
in this equation.
You don't need to think about applying this equation at the moment, but it is useful to know that
the information provided in catalogues is sufficient to allow you to predict thermistor
performance. Using a spreadsheet such as Excel, it is possible to to generate characteristic curves
for any thermistor by calculating resistance values for a range of temperatures.
With RT0 = 20
and B =4200, resistance changes from 0 to 10°C are as follows:
From the graph, the resistance at 4°C can be estimated as just a little less than 60
calculation using the equation, the exact value is 58.2
.
. By
What this means is that selecting a value for Rtop close to 58.2
will make the voltage divider
for the ice alert most sensitive at 4°C. The nearest E12/E24 value is 56
. This matters because
large changes in Vout make it easier to design the other subsystems in the ice alert, so that
temperatures below 4°C will be reliably detected.
Sensor devices vary considerably in resistance and you can apply this rule to make sure that the
voltage dividers you build will always be as sensitive as possible at the critical point.
Thermistors turn up in more places than you might imagine. They are extensively used in cars,
for example in:
•
•
•
•
•
•
electronic fuel injection, in which air-inlet, air/fuel mixture and cooling water
temperatures are monitored to help determine the fuel concentration for optimum
injection.
air conditioning and seat temperature controls.
warning indicators such as oil and fluid temperatures, oil level and turbo-charger switch
off.
fan motor control, based on cooling water temperature
frost sensors, for outside temperature measurement
acoustic systems
Thermistors are used to measure surface and deepwater sea temperatures in helping to monitor El
Niño ocean currents. Less obviously, thermistors are used to measure air flow, for instance in
monitoring breathing in premature babies.
Wheatstone bridge
Sir Charles Wheatstone was a talented and versatile scientist. He invented the concertina,
experimented with stereoscopic photography and invented the stereoscope and played an
important part in the early development of telegraph communications. He didn't claim to have
invented the circuit named after him, but was among the first to exploit the circuit effectively in
making resistance measurements.
So, what is a Wheatstone bridge? This is the circuit:
It is obvious that the circuit consists of two voltage dividers. Suppose RX is an unknown resistor
value. RC is adjusted until Vout from the second voltage divider is equal to Vout from the voltage
divider containing RX . When the Vout values are equal, the bridge is said to be balanced. The
balance point can be detected by connecting either a voltmeter or an ammeter across the output
terminals. Both sorts of meter give a zero reading when balance is achieved.
In a balanced circuit, the ratio RX / RA is equal to the ratio RB / RC . Rearranging:
In other words, if the values of RA , RB and RC are known, it is easy to calculate RX . In
Wheatstone bridge instruments, RA and RB were fixed and RC was adjusted on a sliding scale in
such a way that the value of RX could be read off directly.
Today, Wheatstone bridge circuits are not usually used to measure resistance values, but they are
used in designing sensor circuits.
A variometer is an instrument used in gliders to detect changes in air pressure due to sudden
changes in altitude. The variometer alerts the glider pilot to updrafts or thermals. By circling
within a thermal the pilot can gain height and keep flying for longer.
One type of variometer uses thermistors to monitor pressure changes:
Altitude changes cause pressure changes which produce air flow. A heating element in the flow
passage heats air which arrives at different temperatures at a thermistor sensor upstream and
downstream of the heating element depending on the rate of air flow.
The thermistor sensors are part of a Wheatstone bridge:
When the instrument is first set up, the preset resistor is adjusted for zero output. The advantage
of the Wheatstone bridge is that only temperature differences between the two sensors will put
the bridge out of balance. Cold or warm weather conditions affect both sensors equally. Air flow
into or out of the reference chamber has the opposite effect on the two sensors: one will be
heated by the airstream, while the other is cooled. As a result, the output changes by more than it
would if there was just a single sensor device.
By the way, Wheatstone bridge circuits are supposed to be difficult to understand. The circuit is
usually drawn as a diamond:
It's less obvious that you should be thinking about two voltage dividers, but once you know, the
action of the circuit is easy to follow.
Sound sensors
Another name for a sound sensor is a microphone. The diagram shows a cermet microphone:
'Cermet' stands for 'ceramic' and 'metal'. A mixture of these materials is used in making the
sound-sensitive part of the microphone. To make them work properly, cermet microphones need
a voltage, usually around 1.5 V across them. A suitable circuit for use with a 9 V supply is:
and the 1
resistors make a voltage divider which provides 1.6 V across the
The 4.7
microphone. Sound waves generate small changes in voltage, usually in the range 10-20 mV. To
isolate these small signals from the steady 1.6 V, a capacitor is used. Capacitors are described in
Chapter 5 and an investigation of the microphone circuit is included in the practical which
accompanies Chapter 4.
Signals from switches
When a switch is used to provide an input to a circuit, pressing the switch usually generates a
voltage signal. It is the voltage signal which triggers the circuit into action. What do you need to
get the switch to generate a voltage signal? . . . You need a voltage divider. The circuit can be
built in either of two ways:
The pull down resistor in the first circuit forces Vout to become LOW except when the push
button switch is operated. This circuit delivers a HIGH voltage when the switch is pressed. A
is often used.
resistor value of 10
In the second circuit, the pull up resistor forces Vout to become HIGH except when the switch is
operated. Pressing the switch connects Vout directly to 0 V. In other words, this circuit delivers a
LOW voltage when the switch is pressed.
In circuits which process logic signals, a LOW voltage is called 'logic 0' or just '0', while a HIGH
voltage is called 'logic1' or '1'. These voltage divider circuits are perfect for providing input
signals for logic systems.
What kinds of switches could you use. One variety of push button switch is called a miniature
tactile switch. These are small switches which work well with prototype board:
As you can see, the switch has four pins which are linked in pairs by internal metal strips.
Pressing the button bridges the contacts and closes the switch. The extra pins are useful in
designing printed circuit boards for keyboard input and also stop the switch from being moved
about or bent once soldered into position.
There are lots of other switches which you might want to use in a voltage divider configuration.
These include magnetically-operated reed switches, tilt switches and pressure pads, all with
burglar alarm applications.
What do meters measure?
A meter is a measuring instrument. An ammeter measures current, a voltmeter measures the
potential difference (voltage) between two points, and an ohmmeter measures resistance. A
multimeter combines these functions, and possibly some additional ones as well, into a single
instrument.
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 A and B below show a circuit before and after
connecting an ammeter:
A
B
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 behavior of the circuit, or at least not significantly, and it follows that an
ammeter must have a very LOW resistance.
Diagram C shows the same circuit after connecting a voltmeter:
A
C
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.
Which measurement technique do you think will be the more useful? In fact, voltage
measurements are used much more often than current measurements.
The processing of electronic signals is usually thought of in 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 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 diagram D:
A
D
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.
Digital 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 meters give an
output in numbers, usually on a liquid crystal display.
The diagram below shows a switched range multimeter:
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 DC, for example, then 20 V is the
maximum voltage which can be measured, This is sometimes called 20 V fsd, where fsd is short
for full scale deflection.
For circuits with power supplies of up to 20 V, which includes all the circuits you are likely to
on the meter.
build, the 20 V DC voltage range is the most useful. DC ranges are indicated by
Sometimes, you will want to measure smaller voltages, and in this case, the 2 V or 200 mV ranges
are used.
What does DC mean? DC means 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 descirbed in Design Electronics works in this way.
AC 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.
You are not at all likely to use the AC ranges, indicated by
, on your multimeter.
An alternative style of multimeter is the autoranging multimeter:
Autoranging multimeter
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.
Where are the two meter probes connected? The black lead is always connected into the socket
marked COM, short for COMMON. The red lead is connected into the socket labeled V mA.
The 10A socket is very rarely used.
Analogue multimeters
An analogue meter moves a needle along a scale. Switched range 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!
Each type of meter has its advantages. Used as a voltmeter, a digital meter is usually better
because its resistance is much higher, 1 M or 10 M , compared to 200
for a 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.
Batteries are devices that translate chemical energy into electricity. But that simple definition
greatly understates the pervasive role of batteries in our life. Batteries are an efficient way to
make electricity portable. In addition, batteries provide power to replace electricity from the
utility electrical grid for a variety of critical functions. As the world becomes increasingly
addicted to electricity and mobility batteries play an ever greater role in all aspects of our life.
Batteries come in a variety of shapes and sizes. Some are small enough to fit on a computer
circuit board while others are large enough to power a submarine. There are batteries that are
used once and thrown away and there are batteries that are recharged and reused thousands of
times. Some batteries are used several times every day while others may sit for ten or twenty
years before they are ever used. Obviously for such a diversity of uses, a variety of battery types
are necessary. But all of them work from the same basic principles.
The first battery was demonstrated nearly 200 years ago and batteries have been extensively
researched since then. Even so, there is much yet to be learned about the details of battery
chemistry. New battery types and significant improvements in the performance of existing
batteries have spurred the increased use of batteries throughout society.
Batteries can be frustrating. We have all had experiences where we went to use a
battery-powered flashlight or appliance and it hasn't worked. Or we have gone to
start the car and nothing happened. Much of the time it isn't the battery's fault.
We left the flashlight on or forgot to recharge the appliance or left the headlights
on. But the battery gets the blame anyway. However, batteries do have their
quirks. As active chemical systems, they are sensitive to how they are used, the
environment around them, and, even, how they have been treated in the past. An
improved understanding of how batteries work can pay substantial dividends in
better utilizing battery strengths and avoiding battery weaknesses.
A battery consists of one or more electrochemical cells. Although the terms battery and cell are
often used interchangeably cells are the building blocks of which batteries are constructed.
Batteries consist of one or more cells that are electrically connected.
Cells
A cell normally consists of the four principal components shown in Figure 1. These are:
•
•
•
•
a positive electrode that receives electrons from the external circuit when the cell
is discharged,
a negative electrode that donates electrons to the external circuit as the cell
discharges,
electrolyte which provides a mechanism for charge to flow between positive and
negative electrodes, and
a separator which electrically isolates the positive and negative electrodes.
In some designs, physical distance between the electrodes provides the electrical isolation and
the separator is not needed.
In addition to the critical elements listed above, cells intended for commercial batteries normally
require a variety of packaging and current collection apparatus to be complete.
How a Cell Works
When a battery or cell is inserted into a circuit, it completes a loop which allows charge to flow
uniformly around the circuit. In the external part of the circuit, the charge flow is electrons
resulting in electrical current. Within the cell, the charge flows in the form of ions that are
transported from one electrode to the other. As mentioned above, the positive electrode receives
electrons from the external circuit on discharge. These electrons then react with the active
materials of the positive electrode in "reduction" reactions that continue the flow of charge
through the electrolyte to the negative electrode. At the negative electrode, "oxidation" reactions
between the active materials of the negative electrode and the charge flowing through the
electrolyte results in surplus electrons that can be donated to the external circuit.
It is important to remember that the system is closed. For every electron generated in an
oxidation reaction at the negative electrode, there is an electron consumed in a reduction reaction
at the positive.
As the process continues, the active materials become depleted and the reactions slow down until
the battery is no longer capable of supplying electrons. At this point the battery is discharged.
Recharging
The world of batteries divides into two major classes: primary and secondary batteries. Primary
batteries such as the common flashlight battery are used once and replaced. The chemical
reactions that supply current in them are irreversible. Secondary batteries (for example, car
batteries) can be recharged and reused. They use reversible chemical reactions. By reversing the
flow of electricity i.e. putting current in rather than taking it out, the chemical reactions are
reversed to restore active material that had been depleted. Secondary batteries are also known as
rechargeable batteries, storage batteries or accumulators.
There are two parameters that measure battery performance: voltage and capacity. In very
simple terms, the voltage is the force propelling each of the electrons coming out of a battery and
the capacity is the number of electrons that can be obtained from a battery. How these
parameters relate to batteries is explained below.
Voltage
The voltage of a battery cell is determined by the materials used in it. The reduction and
oxidation reactions mentioned in the "How a Battery Works" section, each produce a fixed
potential. The sum of the reduction and oxidation potentials is the voltage of the cell. For
example, the discharge reaction at the positive electrode for a lead-acid cell is PbO2 + SO42
+ 4Η+ + 2e- → PbSO4 + 2H2O which has a potential of 1.685 volts. The reaction at the negative
electrode is Pb + SO4-2 → PbSO4 + 2e- which has a potential of .356 volts. This means that the
overall voltage of a lead-acid cell is 2.04 volts. This value is known as the standard electrode
potential. Other factors, such as the acid concentration can also effect the voltage of a lead-acid
cell. The typical open circuit voltage of commercial lead-acid cells is around 2.15 volts.
Thus the voltage of any battery cell is established depending on the cell chemistry. Nickelcadmium cells are about 1.2 volts, lead-acid cells are about 2.0 volts, and lithium cells may be as
high as nearly 4 volts. Cells can be connected together so that their voltages accumulate. This
means lead-acid batteries with nominal voltages of 2v, 4v, 6v, etc. are possible.
Capacity
While the voltage of a cell is fixed by its chemistry, cell capacity is variable depending on the
quantity of active materials it contains. Individual cells may range in capacity from fractions of
an ampere-hour to many thousands of ampere-hours.
The capacity of a cell is essentially the number of electrons that can be obtained from it. Since
current is the number of electrons per unit time, cell capacity is the current supplied by a cell
over time and is normally measured in ampere-hours.
Battery vs. Cell
Voltage and Capacity
Batteries normally consist of multiple cells that are electrically connected. The way that the
electrical connections are made determines the voltage and capacity of the battery. If the
positive terminal of one cell is connected to the negative terminal of the next and so on through
the battery the result, as illustrated in Figure 2, is called a series-connected battery. The voltage
of this type of battery is the sum of the individual cell voltages. For example, a 12-volt
automobile battery consists of 6 2-volt lead-acid cells connected in series. Although the voltages
add, the cell capacity is fixed at the value for the individual cell.
The other way to connect cells within a battery is to connect the negative terminal from one cell
to the negative of the next cell and to connect the positive terminal to the positive terminal.
When this is done throughout the battery, the result is the parallel-connected battery shown in
Figure 3. Here the capacities of the individual cells add to make the battery capacity but the
battery voltage remains as the voltage of the individual cell.
Series-connected batteries are far more common than parallel-connected. Usually it is easier to
get added capacity by just using a larger cell rather than a parallel-connected battery.
All of the battery connections may be made internally so that it is difficult to determine the
number of cells by external examination. However, knowing the voltage of the basic cell, it is
easy to determine the number of cells by dividing the cell voltage into the battery voltage.
Cells used for batteries should always be identical. Mixing cells of different chemistry or
different size may be hazardous and should be avoided.
Most historians date the invention of batteries to about 1800 when experiments by Alessandro
Volta resulted in the generation of electrical current from chemical reactions between dissimilar
metals. The original voltaic pile used zinc and silver disks and a separator consisting of a porous
nonconducting material saturated with sea water. When stacked as sketched in Figure 4, a
voltage could be measured across each silver and zinc disk. Experiments with different
combinations of metals and electrolytes continued over the next 60 years. Even though large and
bulky variations of the voltaic pile provided the only practical source of elecricity in the early
19th century. They were the original primary battery.
Johann Ritter first demonstrated the elements of a rechargeable battery in 1802, but rechargeable
batteries remained a laboratory curiosity until the development, much later in the century of
practical steam-driven dynamos to recharge them.
During the first half of the 19th century experiments continued with a variety of electrochemical
couples (combinations of positive and negative electrode materials and electrolyte). Finally about
1860, the ancestors of today's primary and secondary batteries were developed.
On the primary side, in the 1860's George Leclanche' of France developed a form of the carbonzinc battery. The original version was a wet cell with the electrodes immersed in a pool of
electrolyte. It became popular because it was rugged, easy to manufacture, and had a good shelf
life. The original design was improved to incorporate the electrolyte into a wet paste. As a result
the cell could be produced as a sealed unit with no free liquid electrolyte. The carbon-zinc "dry"
cell is still the mainstay of the primary battery market.
Lead Batteries
Secondary batteries date back to 1860 when Raymond Gaston Planté invented the lead-acid
battery. His cell used two thin lead plates separated by rubber sheets. He rolled the combination
up and immersed it in a dilute sulfuric acid solution. Initial capacity was extremely limited since
the positive plate had little active material available for reaction. About 1881, Faure and others
developed batteries using a paste of lead oxides for the positive plate active materials. This
allowed much quicker formation and better plate efficiency than the solid Planté plate. Although
the rudiments of the flooded lead-acid battery date back to the 1880's, there has been a
continuing stream of improvements in the materials of construction and the manufacturing and
formation processes.
Since many of the problems with flooded lead-acid batteries involved electrolyte leakage, many
attempts have been made to eliminate free acid in the battery. German researchers developed the
gelled-electrolyte lead-acid battery in the early 1960's which was a major improvement. Working
from a different approach, Gates Energy Products developed a sealed-lead battery which
represents the state of the art today.
While there are various choices for a rechargeable system, lead-acid batteries are still the
workhorses. They represent about 60% of all batteries sold worldwide. Lead-acid batteries are
usually more economical and have a high tolerance for abuse. The fact that all of the batteries
used for starting, lighting, and ignition (SLI) service on automobiles and trucks are lead-acid
indicates their ability to withstand varied forms of maltreatment. Lead-acid batteries also provide
motive power for everything from forklifts to submarines. Lead-acid batteries are also mainstays
of the backup systems that provide power when the electrical grid fails. Now, development of the
sealed-lead battery has allowed lead technology to be used in applications such as electronics
that need a clean power source.
All lead batteries work on the same set of reactions and use the same active materials. At the
positive electrode, lead dioxide (PbO2) is converted to lead sulfate (PbSO4) and at the negative
electrode, sponge metallic lead (Pb) is also converted to lead sulfate (PbSO4). The electrolyte is a
dilute mixture of sulfuric acid that provides the sulfate ion for the discharge reactions.
Sealed-Lead
Development work on the sealed-lead battery was
begun in 1967 by subsidiaries of The Gates
Corporation with first commercial uses occurring
in the early Seventies. It has become an accepted high-performance power source for clean
applications including computer power and power backup, telecommunications, emergency
lighting, security alarms, and consumer products. It is also becoming popular in cordless tools
and appliances, electric vehicles, and other applications which require frequent discharges.
The sealed lead cell, shown in Figure 5, consists of positive and negative electrodes and their
accompanying separators that are wound in a spiral pattern. The electrodes consist of pure lead
grids pasted with mixtures of lead oxides. These oxides are converted to the proper active
materials when the cell receives its first charge in a process called formation. The pure lead
supporting grids allow the flexibility needed for winding the plate and also give excellent
corrosion resistance to prolong cell life. The separator consists of a fibrous glass mat. The cell
works as a starved electrolyte system where the quantity of electrolyte is limited to the amount
that is either absorbed in the plates or wets the fibers in the separator. The result is open gas
paths between the plates that allow gases evolved during overcharge to diffuse from one plate to
the other where they are recombined. This recombination provides a closed system reducing
venting of gases under normal overcharge conditions. A resealing safety vent is provided to
handle pressure buildup during abusive overcharges. Since the electrolyte is recycled, the water
loss that requires routine maintenance or limits life is minimized. The sealed-lead system has
proven to provide high performance and long life in a clean, compact package.
Flooded
The basic flooded lead-acid battery as shown in Figure 6 has changed little in concept since the
1880's although improvements in materials and manufacturing methods continue to bring
improvements in energy density life, and reliability. The major markets for flooded lead-acid
batteries are SLI service for autos, trucks, and boats and motive power for prime movers ranging
from fork-lift trucks to submarines. All flooded batteries consist of flat pasted plates immersed in
a pool of electrolyte. The batteries are not recombining and vent flammable gases on overcharge.
Regular water addition is required for most forms of flooded batteries although low-maintenance
types come with excess electrolyte that is calculated to compensate for water loss during a
normal lifetime.
Batteries come in all sizes and shapes to fit a diversity of applications. However, there are three
major parameters that work to determine the suitability of battery types for an application.
The first major concern is whether the battery is used in high-rate or low-rate service. Some
batteries are much better suited to handling a long-term drain at a low rate than they are a highrate load for a short interval. Examples of low-drain situations include memory backup for
electronic circuits and clocks or watches. High-rate loads include most cordless appliances and
engine start.
The second concern pertains to rechargeable batteries. It is the relative amount of time the battery
spends being charged versus the time it spends on discharge. Some batteries are used in "float"
applications where they spend most of their time on charge with only rare discharges. Most
power backup applications fall into this category.
"Cyclic" applications are those where the battery is used regularly and it gets relatively little time
to recharge between uses. Most battery-powered portable equipment falls into this category.
The last major concern regarding battery applications is the environment in which it is used,
specifically the temperatures at which the battery is required to operate. Batteries and people like
approximately the same temperature range. If the temperature gets too warm, the chemical
reactions within the battery are accelerated and its life may be shortened. If the battery gets too
cold, the chemical reactions are slowed down which reduces the battery output.
Of course in matching the battery to the application, there are a variety of other concerns. But
those listed above are the ones that can have dominant influence on battery selection and, even,
battery feasibility in the application.