General Electrical Information
Breadboards
Breadboards are used for constructing electrical circuits. They come in different sizes; the
most common size used in this laboratory has two rows of 50 holes along the top and bottom,
and 64 columns of ten holes, separated into two groups of five. Behind each hole is a small
spring which contacts any wire pushed into the hole. This spring is connected to neighbouring
holes so that there is an electrical connection between wires pushed into adjacent holes. The
diagram shows a section of the breadboard and which holes are connected together.
Figure 1: Diagram to show the connections between the holes in a breadboard.
Usually, you will connect only two or three wires together at each point. The rows of
holes at the top and bottom of the breadboard are normally used to provide high voltage
(+) and ground (−) connections, respectively.
Power Supplies
The low voltage power supplies used in this laboratory allow you to choose +5 volts, +12 volts
and −12 volts d.c. The maximum current which can be delivered is 1 A. Connection to the
breadboard is made by connecting a short wire from the +5 v or +12 v terminal to the top
row of breadboard holes, and another from “Ground” to the bottom row. The arrangement
of the terminals is shown in the following diagram:
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ON
+5 volts +12 volts
-12 volts
Ground
Tie Points
Power
Figure 2: Diagram to show the connections between the holes in a breadboard.
The four terminals labeled “Tie Points” are not connected to the power supply, but are
useful for connecting voltmeters and the like. You should, wherever possible, avoid using
these terminals for connecting components into your circuit. The above diagram refers to the
newest power supplies. The configuration is slightly different on the older power, however,
they are used in exactly the same way
From time to time you will be using larger power supplies which can deliver higher
currents. They will often incorporate a third “Ground” terminal which is normally not used:
electrical connections to the circuit will use the ‘+’ and ‘−’ terminals only. Take some time
to become familiar with a new power supply before starting your experiment.
Multimeters
A multimeter is the most useful measuring instrument which you will find in the laboratory;
it allows you to measure voltage, current and resistance, in both AC and DC circuits. Be
aware that in AC mode, the multimeter will give a null reading when connected to a DC
signal, since there is no time-varying component.
There are different kinds of multimeter. Some are digital, some are analogue, some use
push-button switches to change functions, some use large rotary switches, and some use
different jacks. It is important, therefore, to ensure that the switches are set
correctly before connecting the meter into the circuit to prevent damage to the
meter or to the circuit. The range switches set the maximum value of the quantity to be
measured. If you do not know the approximate value, start at a high value and work down,
finally using the scale which gives the most accurate value.
The current measurement often causes most trouble: the ideal current meter has zero
resistance, which means that if you put it directly across a battery or power supply, you will
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burn something up.
Always put the current meter in series with the element through which
the current you wish to measure is going.
This means that you have to break the circuit and insert the meter at the point of
disconnection so that the current can go through the meter as well as the circuit element of
interest. Also, you need to make sure that the ‘+’ and ‘−’ terminals are connected correctly:
when a digital meter displays a negative value, it means that the direction of the current in
the meter is opposite to what you thought.
To measure voltage, simply place the multimeter terminals across (or in parallel with)
the component of interest. However,using the multimeter as a voltmeter when the
ammeter function is selected must be avoided. If you attempt to use the meter in
this way, the internal fuse may blow.
A resistance measurement uses electrical power provided by the meter itself. The most
important thing to remember, therefore, when taking resistance measurements is not to
make this type of measurement on a circuit which has a voltage connected to it. To do so
would either give a meaningless result, or worse, damage the meter. You must disconnect
an individual component from the circuit before measuring its resistance, otherwise you will
measure the equivalent parallel resistance of the whole circuit.
Uncertainties in Multimeter Readings
The accuracy of the meter is often printed on the bottom side of the meter in the form
±([%of reading] + [number of least significant digits]).
For example, if the accuracy for resistance is given as ±(0.2%+1), then a reading of 0.673 kΩ
is uncertain by
0.673 × 0.002 (0.2%)
+
0.001 (1 digits)
≈
0.001 + 0.001
or
±0.002 kΩ.
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Thus the value to be recorded is 673 ± 2 Ω.
The Keithley 179A multimeters, when used in the 2 V range, can measure voltages with
an accuracy given by the numerical weight of the least significant digit plus 0.04% when
properly calibrated. For example, a reading of 1.9837 V has a least significant digit of
0.0001 V (0.1 mV); 0.04% of the reading is 0.8 mV. The resulting uncertainty in the reading
is actually about 1.0 mV and should be recorded as 1.984±0.001 V. Thus a 4 1/2 digit readout
is really only good to 3 1/2 digits. Be careful.
Resistor Colour Code
The resistance of a resistor is indicated by four coloured bands, as shown in the diagram.
Reading from left to right, the first two bands represent the first two digits, the third the
multiplier, and the fourth band, which is gold or silver represents the tolerance of the resistor.
A
B
C
D
Figure 3: Diagram to show the connections between the holes in a breadboard.
Denoting the bands by A, B, C and D, then the value of the resistor (in Ω) is
AB × 10C
where the numerical values of A, B and C are given by the colour code in the table.
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Colour
black
brown
red
orange
yellow
green
blue
violet
grey
white
Value
0
1
2
3
4
5
6
7
8
9
Multiplier
0
10 = 1
101 = 10
102 = 100
103 = 1000
104
105
106
107
108
109
Tolerance (D)
gold ±5%
silver ±10%
no band ±20%
For example, if the bands on a resistor are coloured yellow (4), violet (7), brown (1) and
silver, the nominal resistance is 470 Ω and the tolerance is ±10%. If the third band (band
C) is gold or silver, then the value of the multiplier is 10−1 or 10−2 , respectively.
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