Identifying faults in components

Biggest problem in electronics is identifying faults in components. You may came across many Breakout Boards, Shields or even the boards made by you. In all these it is necessary to check out the faults when these were malfunctioning as it may brick the SBC / Micro controller board connected with it. For example if there is no electrical isolation in H-Bridge / L293D board , then the 12VDC supply given to the Motor control Board , may affect the Raspberry Pi / Beaglebone Black / Arduino connected with it.

The troubleshooting of electronic circuits involves three steps, which should be done in a specific order. The first step is to identify the defect in the circuit. The second step includes fault analysis and determination of the possible causes. The third step is fixing the problem. First, it is important to identify the problem. In other words, we have to recognize the symptoms in the defective circuit. A defective circuit can be defined as one, where the output parameters are incorrect, although the input parameters are correct. For example, the input signal of the amplifier, but there is no signal at the output. In this case, the symptom is lack of voltage at the output.

identifying symptoms in a defective circuit

This particular symptom does not provide much information about the possible causes of the defect. The failure of various components in the circuit will result in the  same  symptom    (zero  voltage  at  the  output).  In  other  cases,  a  particular symptom points directly to a certain area where the fault is most likely to have occurred. For example, a dc voltage at the output with the level equal to the supply voltage indicates that there is a transistor in a cutoff condition in the circuit . Starting from the stage that is closer to the output and going backwards, all transistors have to be checked for an internally open pn-junction. The soldered joints and the values of the emitter resistors also have to be checked. If the amplifier is not defective, the amplified signal appears at the output. The amplitude of the output signal is approximately equal to the value of the rectified power supply. The waveform has to be an exact amplified replica of the input signal, without any kind of distortion.

 

 

Once the symptom is identified, the reasons that cause it have to be determined. The choice of which of several methods to use depends on the circuit complexity; on symptoms and on the personal preferences of the technician. The most common troubleshooting techniques are listed below:

 

  • Power check. This is the first thing you should do. It is amazing how many times a simple issue such as a blown fuse, or a flat battery is the cause of the circuit malfunction. So initially, ensure that the power cord is plugged in and that the fuses are not blown. If the circuit is powered from batteries, make sure that their voltage level is acceptable. If a power supply rectifier is present, check the level of the voltage at the output and make sure that the circuit is powered with the correct polarity.

 

  • Visual  inspection. This  inspection is  part  of  the  so  called  sensory  checks. Sensory  checks  rely  on  your  senses  to  detect  a  possible  fault.  The  visual inspection of the PCB is the simplest troubleshooting technique (which is very effective in half of the cases). The soldered joints have to be inspected thoroughly. If any doubt exists about the quality of a certain joint, it has to be re-soldered. The PCB has to be inspected visually for any burnt components. Sometimes, components that overheat leave a brownish mark on the board. They can be used as “starting points” in the troubleshooting process and the reasons why they overheat have to be determined. It is bad practice simply to replace such components, without trying to find out what actually caused the component to overheat. In many cases the reason is a faulty (or out of range) component in the vicinity of the failed component. It also has to be replaced.

 

  • Using a sense of touch. This is another sensory check. Overheated components can be detected by simply touching them. However, this check has to be performed with extreme caution. The circuit has to be turned off, and some time allowed for the biggest capacitors to discharge. Always touch the components with your right hand only! This is important because in the case of electric shock it is less likely that the current will pass through your heart. If possible, wear insulated shoes. In addition, care should be taken not to burn your fingers.  Using  the  sense  of  touch  is  a  very  useful  troubleshooting technique in circuits, where everything seems to work properly for a while, and then the circuit fails, due to overheating of a certain component. Identifying such components helps to detect the possible cause of the fault. Special freezing sprays are available, which allow instant freezing of components. If the circuit begins to operate properly immediately after the heated component is sprayed, this is an indication that this component is causing the circuit failure. Before replacing the component, further investigation is needed to determine what caused the overheating in the first place.

 

  • Smell check. When certain components fail due to overheating it is possible in most cases to detect a smell of smoke. This is usually the case, if the technician happens to be there at the time the accident occurred. If  not, it  is  usually possible to detect the failed component by visual inspection afterwards.

 

  • Component replacement. This troubleshooting method relies mostly on the operator’s skills and experience. Certain symptoms are an obvious indication of a particular component failure. This statement is especially true for an experienced electronic technician. For example, some TV service technicians can unmistakably identify the failed component in a TV set (even before opening it), by just briefly examining the symptoms. Component replacement is a good troubleshooting technique for an experienced electronics technician, as it saves a lot of time and money. Moreover, this technique guarantees the success of the repair, because if enough components are replaced, eventually the faulty one will be replaced too. However, it is recommended that the amateur technician initially applies some logical thinking to the troubleshooting process.

 

  • Signal tracing. This troubleshooting technique is not the most common one, but it is the most desirable as it requires intelligent and logical thinking from the troubleshooter. This method is based on the measuring of the signal at various test points along the circuit. A test point in the circuit is the point, where the value of the voltage is known to the operator. This troubleshooting technique relies on finding a point, where the signal becomes incorrect. Thus, the operator knows that the problem exists in that portion of the circuit, between the point where the signal becomes incorrect, and the point where the signal appeared correct for the last time. In other words, the operator constantly narrows the searched portion of the circuit, until he finds what causes the fault. There are two basic approaches in conducting the signal tracing. In the first approach, the signal check  starts  from  the  input,  checking  consecutively  the  test  points towards the output. The checks are carried out, until a point, with an incorrect signal is found. The second approach is to start from the output and to work backwards towards the input in the same manner until a correct signal appears.

 

Fault Analysis

Fault  analysis  requires  a  good  knowledge of  the  theory and  a  good  analytical thinking. It  is  not  something, which can  be  studied from books, but  it  can  be acquired through constant troubleshooting and experimenting. The basic question in fault analysis is: “What would the symptoms in the circuit be, if the component X is faulty?” For each specific application, there are no ready answers to this question. If they were, many books devoted to industrial electronics would be meaningless, anyway. However, there are certain rules, which can be adhered to, during the troubleshooting process. One of the tasks of this manual is to teach you some of these basic rules. As an example, let us examine a bridge rectifier, to illustrate the process of the fault analysis.  It consists of a transformer, a rectifier and a filter. The voltages, taken with an oscilloscope at each test point are depicted in the figure. A signal trace is conducted commencing from the output and working towards the input. An analysis of all possible faults in this circuit are given below:

good rectifier

symptoms of a capacitor

  • Faulty capacitors C. There are three possible problems. The capacitor could be shorted, opened or leaky. If the capacitor is shorted, it effectively brings both terminals of the load resistor together and therefore the output voltage is zero. If the capacitor is open , it does not filter the output voltage supplied from the rectifier. The waveform of the voltage at the output remains the same as the waveform of the voltage after the rectifier. Therefore, the waveforms at points C and D are identical. The only difference is that the amplitude of the voltage at the point D is smaller due to the voltage drop across the resistor Rsurge. Finally, if the capacitor is leaky the output voltage will appear with increased ripples on the output. A leaky capacitor appears as if there is a leakage resistor, connected to it in parallel. The leakage resistor decreases the time for a discharge, thus the voltage ripples increase at the output.

 

 

 

  • Faulty resistor R surge. There is only one possible faulty condition, namely a blown resistor Rsurge (Rsurge appears as an open circuit). This occurs, when an excessive current flows through it. An excessive current flows through Rsurge if the output terminals are short-circuited or if the capacitor is shorted. In both cases when Rsurge blows, it brakes the circuit and prevents the diodes (which are more expensive than the resistor) from burning too. The output voltage in this case is zero. Before replacing Rsurge make sure that the capacitor or the output terminals of the circuit are not shorted and that the conductive paths of the PCB are not shorted out.

 

  • Shorted diode. A shorted diode appears as a jumper between the points of the connection, as it conducts the current in both directions. Figure 8.14 illustrates the current that flows in the circuit, when the diode D4 is shorted out. During the positive half-period, the current flows through D3  and D4  as normal. The shortened diode exhibits zero resistance in both directions and it appears for the circuit as if it is simply forward-biased. Thus, the positive half-period appears as normal at the point C. However, during the negative half-period the picture changes.  The current now flows through D1 and D4   instead of through the rest of the circuit, because these two diodes, connected in series provide a path of less  resistance. Effectively the  secondary winding  is  short-circuited and  an excessive current flows through it. Thus, the diode D4 can be damaged fairly quickly, due to overheating. The increase in the current in the secondary winding increases the current in the primary winding. If the circuit is properly fused, the fuse on the primary winding should blow. If this is not the case the diode D1 overheats (and even possibly burns) and the voltage at the test point C has the form. With some thought you can analyze what happens in the circuit when some other diode shorts out, or when two or more diodes short out simultaneously.

 

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