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Electronic Fault Finding Support Booklet The Rules of Electronic Fault Finding are: Study the circuit schematic and break the circuit down into a series of blocks to form a block diagram, draw the block diagram Label each block, list its inputs and outputs and provide a brief description of the function of the block Use data sheets to work out the pin-out, parameters, tolerances and values of the components on the schematic so you can compare them with the physical components With all of the information gathered you can carry out a risk assessment to ensure you are working safely at all times Devise your plan, which essentially is a list of things to check using the fault finding methods you will learn in this unit, the test and measuring equipment you are going to use to carry out the checking, the expected result of each check and the actual result Carry out your plan!! Consider the circuit below After some study the circuit can be broken down into a series of blocks Now we can draw a block diagram of the circuit. This helps us understand the function of each block and its inputs and outputs. Block 1 Block 2 Block 3 Bridge Rectifier Voltage Regulator LM35 Temperature Sensing Chip 15 volt AC power input 13.6 volt DC power input 5 volt DC power input 13.6 volt DC power output 5 volt DC output Output depends on temperature Function Function Function (Transducer) Converts AC to DC Provides power for the IC’s @ 5 volts DC Converts temperature to voltage This process needs to be repeated for each block in the system!! Always remember, this is an aid to understanding what the circuit does. If you do not understand what a circuit does, you will never be able to carry out fault finding on it. Fault Finding Techniques In electronic systems there are two major methods for tracing faults within a system, they are called: Input to Output Half Split Input to Output This method works very well on small circuits. Basically you trace the electronic signal from the input of the circuit to the output. It is one reason why we break large circuits down into smaller sub-circuits or blocks. If we know what the electrical inputs and outputs to a block should be then we can work out if the block is functioning properly after taking measurements with suitable electronic measuring equipment. Let’s demonstrate this with the bridge rectifier block. Here is the bridge rectifier circuit can you tell where to measure the inputs and outputs? If you are completely inexperienced and have no idea where to start I would advise that the very first thing you do is to list each component in a table, along with a physical image and a brief description of what the component does. This is a similar exercise to the block diagram except we are taking each sub-circuit in turn and breaking it down to component level. At first it takes time but as you gather experience and knowledge things will begin to go much faster. Component Symbol/Name Function Physical Image This component transforms the mains supply down from 240 volts AC to 15 volts AC. The AC input provides all of the power for the circuit but it has to be turned into DC first. V1 15 volts AC input A general purpose diode, current flows in the direction of the arrow. An electrolytic capacitor Recall that a diode only allows electrical current to flow in one direction (Direct current DC) So if we apply a current that flows back and forth (Alternating current AC). The diode will stop the current flowing in one direction, this is called Rectification Capacitors have many uses in circuits and come in all shapes and sizes. This capacitor is used to smooth out the rectified DC waveform before it is fed to the input of the voltage regulator Current always flows to earth, ground or neutral potential. Electrical Earth To summarize The information in our table tells us that: The input to the circuit is V1 the transformer That the electrical current flows in the direction of the arrow That electrical current always flows to ground And that the capacitor is used to smooth the output before it is fed to the voltage regulator From this we can deduce the input to the circuit and the output of the circuit The expected waveform measured at the input using an oscilloscope should be 15 volts AC The expected waveform measured at the output using an oscilloscope should be 13.6 volts DC Ok so we have worked out what the components look like, we have a basic idea of the system function and we know what measurements to expect at the input and output of the system. We switch the system on and it does not work. Congratulations, you have just arrived at the point where fault finding skills are needed. In a simple system like this we can use a variety of skills to trace the fault with the circuit. I am going to do this in the style of a report card. Fault Technique – Input to Output No voltage measured at the output of the bridge rectifier “” Tests carried out Comments Is the system switched on at the mains? Yes Measure the output of the transformer before it is connected to the bridge rectifier using an oscilloscope “” Connect the transformer to the bridge rectifier and switch on circuit “” Use a multi-meter to measure the resistance between the inputs of the bridge rectifier No voltage measured at the output of the transformer. However it was discovered that the transformer had an over-current protection trip, this was reset and a voltage was then measured at the output of the transformer. When the circuit was switched on there was an audible click from the transformer. It was discovered that the over-current trip had operated again Over-current trips are usually caused by the presence of a short circuit. In this case a short circuit was found at the AC input to the circuit. The short circuit was fixed and now the circuit functions correctly This is the essence of fault finding. Using a range of techniques you will gain both knowledge and experience of finding and fixing circuit faults. The list such as the one shown here is but a small snippet of a complete fault finding routine. It is your task to prepare and use a fault finding routine on the circuit shown as a part of this unit. It won’t be perfect at first but each time you apply the things you have learned your skills will improve.