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Appendix
1 Fault Finding
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Most circuits
do not work first time! |
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If your
circuit does not work straight away, don't worry; just
find out why it does not work. |
| 1. |
BEFORE
connecting the power supply to the circuit |
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Check that everything is in the right
place (then check again and again and again ...etc).
Check that you have made all the
necessary cuts to the vero board.
Check that all soldered joints are
good; a well soldered component should not move very
much when you give it a little push.
CHECK THAT
THERE ARE NO ACCIDENTAL SHORT CIRCUITS FROM ONE TRACK TO
THE NEXT (this is the most common cause of
problems); if necessary, use a magnifying glass.
N.B. look also at the ends of the vero
board; sometimes even unused board has short circuits at
the ends of the tracks.
The diagram below shows some of the possible faults to
look for. |
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a - well soldered joints but
"filament" of solder from one track to the next
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b - thin piece of copper joining the
end of one track to the end of the next track
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c - badly soldered joint; the solder
is not making good contact with the copper track
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d - well soldered joint but wire too
long; it might reach the next track
- e - badly soldered joint; the solder is not making
good contact with the wire.
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2. |
When connecting the
power supply |
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As soon as possible after the power supply has been
connected to the circuit, see if any of the transistors or
chips are overheating.
If they are switch off immediately and do not
switch on again until the reason for the overheating
has been found. |
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N.B. |
You now have two problems i) you must find the original
cause of the overheating and ii) you must find out whether
the overheated components have been damaged. |
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To detect overheating components touch them lightly with
the back of your finger but
DO NOT USE
THIS METHOD FOR ANY COMPONENTS SUCH AS TRIACS, THYRISTORS
ETC WHICH ARE CONNECTED TO THE 220v SUPPLY. |
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3. |
Using a Voltmeter |
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In general, if we refer to the
voltage "at a point" in a circuit, we mean the voltage
shown by a meter connected to that point and the circuit
"ground" line. In most cases, the circuit "ground" line
is the negative supply connection.
First check that the supply voltage
is reaching all parts of the circuit board
Measure the voltage across base and
emitter (Vbe) of any transistors in the
circuit; Vbe must be between zero and
(about) 0·7v (for silicon transistors); a voltage
significantly outside this range means that the
transistor will have to be replaced. N.B. if the voltage
is zero, the transistor might be faulty.
In analogue circuits (amplifiers,
radios etc) you can also measure Vce (the
voltage across the collector and emitter). Vce
should usually be greater than zero but less
than the supply voltage.
In digital circuits (using CMOS
chips, that is chips with numbers starting with 40...)
all points should be either at zero volts or a voltage
very nearly equal to the supply voltage.
However, if the voltage at a point in a circuit is
oscillating the voltmeter will either give a
regularly varying reading (for low frequency
oscillations) or will read a sort of average voltage
(for high frequency oscillations). For example, if the
voltage at a point is varying as shown below (at high
frequency), a voltmeter would read about 4·5v because
the voltage spends as much time at zero as it spends at
spends at 9v.
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..... |
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If, on the other hand,
the voltage varies as shown in the next diagram, the
voltmeter would read less than half the maximum because it
spends a longer time at zero than at 9v. In this case we
would expect a reading of about 3v. |
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Voltages in Op. Amp.
Circuits |
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Op. Amp.
used as an amplifier |
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Consider the amplifier
circuit shown below. |
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The potential divider formed by the two resistors, R, is
intended to ensure that, when no other input is applied to
the amplifier, its output voltage will be equal to half
the supply voltage. So, in this case, the voltmeter
will read 4·5v. |
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Op. Amp.
used as comparator |
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Consider the
comparator circuit shown below. |
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In comparator
circuits, the actual output voltage depends on which op.
amp. you use. |
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The following table shows the output voltages to be
expected when using some of the most common op. amps. |
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Output voltage when V+ < V- |
Output voltage when V+ > V- |
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741 |
2v |
Vs - 0·6v |
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081/4 |
1·5v |
Vs - 0·6v |
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3130 |
zero |
Vs |
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3140 |
zero |
Vs - 2v |
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