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Fault Finding
If your circuit does not work straight away, don't worry; just find out why it does not work.
 
   
1. Before the power supply is connected  
For projects constructed using vero board  
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.  
   
The diagram below illustrates some of the possible faults to look for.  
   
 
   
A   Well soldered joints; no problem.
B   Well soldered joint but wire too long; it can make contact with the adjacent track.
C   Bad joint; the solder makes contact with the component but not the copper track.
D   Bad joint; the solder makes contact with the copper track but not the component.
E   Thin strip of copper joining one track to the next; often near where you made a cut in the track.
    Occasionally even unused board has short circuits at the ends of the tracks.
 
   
2. When connecting the power supply for the first time  
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.  
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.  
   
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... duh, obviously!
 
   
3. Using a voltmeter  
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 many cases, the circuit ground line is the negative supply connection.  
   
First, check that the supply voltage is reaching the appropriate 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.  
If the voltage is zero, the transistor might be faulty but the problem could, of course, be elsewhere.  
   
In analogue circuits (amplifiers, radios etc) you can also measure Vce (the voltage across the collector and emitter).  
Vce should be greater than zero but less than the supply voltage.  
   
In digital circuits, using CMOS chips, 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.  
   
 
   
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.  
   
Voltages in Op Amp Circuits:  
Consider the following circuit in which an op amp used as an (inverting) amplifier.  
   
 
   
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.  
   
In the next circuit, the op amp is being used as a comparator.  
   
 
   
The open loop gain of an op amp is so high that, in a circuit like this, as the slider of the variable resistor is moved, the output will suddenly switch from one extreme to the other.  
In the case shown, the switching would occur when the voltage on pin 2 passes half the supply voltage.  
The output voltage depends on which op amp you are using.  
The following table shows the approximate output voltage to be expected when using some common op amps.  
   
op amp when V+ < V- when V+ > V-
741 2 Vs - 0.6
081/4 1.5 Vs - 0.6
3130 zero Vs
3140 zero Vs - 2
 
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