In other words, "If it normally gets hot it normally fails". When considering an item of faulty equipment, always remember that the reliability of any component is proportional to the power it dissipates. These failures can affect transistors just as easily as any other component. However such faults as do occur usually follow a distinct pattern, and careful recording of previous faults can be a good indication of future ones. our homes) and over which the designers have no control, is not a precise science. Designing an electronic product for a particular life span, under conditions that will be very variable (e.g. Some faults will occur due to the product exceeding its "designed life" whilst others will occur prematurely. Manufacturers are able to produce products that perform to carefully worked out strategies. Home products are designed to be produced at a profitable price, and to give trouble free operation for a time. The reason these recurrent faults occur is basically down to design. Many faults can be found, especially in equipment produced for the home user, by referring to databases of recurrent faults published in technical magazines on the Internet. In most cases this protection works well, but it is rarely 100% effective. ![]() Most mains powered (and even some low power) circuits liable to such damage have some sort of protection built in that prevents damage. Also voltage spikes caused locally by such events as inductive equipment such as motors starting or stopping. In mains (line) powered equipment very short lived high voltage pulses can occur from time to time, caused by such events as lightning strikes (even some distance away from the damage location) can damage semiconductors. What is more sinister is that sometimes such discharges do not cause immediate destruction of the device, but do damage the insulation to such a degree that the device fails sometime (hours or years) later. ![]() This can be because the very thin insulating layers within the device have broken down completely due to high voltage static electricity, carelessly applied to the terminals. ![]() Sometimes this will result in a transistor (or a circuit board) not working when fitted within a system. Bad handling of FETs can lead to damage by electrostatic discharge. Sometimes external causes may damage or even destroy transistors. Always check the voltages at the transistor terminals after replacement to make sure there are no abnormal readings. In such circumstances it is wise to investigate the reasons for the failed transistor rather than just replacing it. This may eventually result in a transistor operating outside its normal parameters, for example running at a higher than permitted temperature. The reason for this is that other components such as resistors may change their values with age, especially if they are subject to heating effects caused by current flow. However older systems containing transistors do begin to give more problems. A slice of silicon 10 years old should be the same as a 1-year-old slice. There is no real reason that transistors should suffer from aging. Items that survive these tests can confidently be put into regular use. That is running it on a test bench for a number of hours to make sure no early failures occur. A large proportion of manufacturing faults can be detected by "soak testing" new equipment. If it operates correctly for this period then the chances are that it will continue to do so. If there is a fault in a new transistor, it will often show up in the first few hours of use. Manufacturing faults do (very occasionally) occur, usually in new equipment. ![]() Provided they are operated correctly there is no reason for them to fail at all but of course they do fail and this can be for a variety of reasons.
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