I can't hope to cover all types of component or failure modes that you are likely to encounter, so here is a short guide to the basic components that are most often encountered.

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Resistors.

These are the most basic of all the components we are likely to encounter. That said, they probably have the largest number of types, sizes and packages of all the components. For most of us, the resistor is a well defined device with linear characteristics, i.e. Ohms law applies. However, we should also be aware that resistors possess a few other characteristics that may not be so obvious:-

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1. Tolerance. The tolerance of a resistor is a measure of the variation in resistance over the resistors operating range. This figure is normally quoted as a percentage and dependant upon the type of resistor in question, can be between 10% and 0.001%. In other words, a 10kOhm 5% resistor, can be expected to have a resistance value of between 9500 Ohms and 10500 Ohms.

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2. Stability/Ageing. The stability of a resistor is its variation in value with time. Short term changes generally come under the heading of stability, while long term changes are generally termed ageing.

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3. Temperature Coefficient (TC) All resistors come with a temperature coefficient built in for free. The TC is usually quoted in Parts Per Million Per Degree Celcius, or PPM/ºC. So if our 10kOhm resistor has a TC of 200 ppm/ºC and it is subjected to a temperature change of say 20ºC, then its value will change by 4000PPM or 40 Ohms.

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4. Parasitic effects. Parasitic effects are those characteristics which are generally unwanted and are normally a by product of the resistors manufacture or packaging. For example wirewound resistors are generally highly inductive due to the wound nature of the resistive element. They also possess parasitic inductuance and capacitance due to the leads and body dimensions. At radio frequencies, these parasitic effects can become predominant and the resistor then behaves as a complex impedance.

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5. Non-linear effects. As the resistor is heated, either by its ambient surroundings or by its power dissipation, its value will change due to its TC. However the TC will also change with temperature and introduce a 'quadratic' term in the resistors otherwise linear characteristic.

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6. Noise. All components generate electrical noise and the resistor is no exception.

The above characteristics are all present to a greater or lesser extent in any resistor. The following are brief summaries of some common types of resistor. It is by no means a comprehensive list.

Carbon Composition.

Historically, these resistors were originally made from solid cylindrical pieces of carbon. In later years, the solid carbon was replaced by composite materials with variable but controlled resistivity. These resistors are generally cheap and have low parasitic inductance due the lack of any spiral wound element. Unfortunately, they have a high TC and poor stability. This type of resistor is often found in older equipment, especially that pre-dating the 1950s. When fault finding on old equipment, be suspicious of all carbon resistors. Their resistance tends to escalate with age.

Carbon Film.

These are an improved version of the old carbon composition types and have an improved TC. They are manufactured by depositing a thin carbon film onto a non conductive former which is then fitted with end caps to which the leads are welded. An epoxy coating is then usually applied to seal the device. TCs of 500ppm or more are typical for these resistors.Typical power ratings are from 1/8 to 2 Watts.

Metal Film.

Manufactured by depositing a resistive metal film onto a non-conducting ceramic core. The resistor value is trimmed by laser during manufacture by cutting a spiral groove through the resistive film. These resistors exhibit good stability but do have a small inductive component due to the spiral form of the resistive element. TCs are typically 50 to 200ppm.

Wirewound Power.

Available in a wide range of styles and power ratings from 1 Watt upto water cooled variants capable of handling many hundreds of watts. Typical construction is a wound resistive wire element from one or two turns for low value resistors upto hundreds of turns for high value resistors. Some wirewound resistors are encased in ceramic or aluminium and their power handling abilities are directly related to their physical size and surface area. Stability of a wirewound resistor isgenerally good, but has a highly inductive component. Generally not suitable for RF use unless constructed in a non inductive manner. TC generally low at around 50 to 100ppm and tolerances of 5 to 10% are typical.

Typical Failure Modes:

Resistors do not normally fail of their own accord and usually fail as a result of some other problem. e.g. overheating due to excessive applied voltage or excessive current. This type of fault is often accompanied by a burning smell and discolouration of the resistor body. Cases of less severe overload can often be detected with the tip of the finger. Once detected, it is important to find the fault which is causing the resistor to overheat. Changing the resistor will not cure the fault.

Overheated resistors behave in different ways. Carbon composition resistors in addition to discolouring will often change their value significantly. Use the colour bands as an indication of what the value should be, but beware of colour band changes on overheated resistors. In older equipment especially, remember to check any carbon resistors as a matter of course. They will often increase their value significantly with age. They rarely if ever reduce in value, so if your test meter is connected across a 10k resistor and you are getting a reading significantly higher, then check it out. If necessary, lift one end of the resistor and measure it again. Remember, that when you connect your test meter across any resistor in circuit, the meter reading cannot be higher than the value of that resistor. (Volt free circuit conditions assumed)

Carbon film resistors are generally quite reliable, but if subjected to overload will eventually fail open circuit due to the resistive film breaking down.

Metal Film resistors are a very popular style of resistor, used in many different types of equipment. As with carbon film resistors, when subjected to overload, they tend to fail open circuit. This is due to the spiral resistive film breaking down On occaisions when a short term overload condition has existed, there may not be any visual indication that the resistor has failed. Again, check it with your test meter if in doubt.

Metal film resistors are often used in circuits where a known failure mode is required to prevent fire hazard. In other words, our resistor is now being used as a fuse. This is one instance where the metal film resistors unique characteristics are put to good use. If you replace one of these resistors, make sure you replace it with an identical type.

Wirewound power resistors as the name suggests, are generally used where the resistors ability to dissipate significant amounts of power is important. Wirewound resistors are generally very robust and rarely fail open circuit unless subjected to massive overloads. Aluminium encased power resistors, under these extreme conditions can often 'vent their innards' at speed. Circuit board tracks and surrounding components can often be damaged during these exteme overloads and generally speaking are easy to spot. Always check the soldered joints on any resistor that shows signs of overheating. It is by no means unusual for the heat to be conducted along the resistor leads and melt the soldered joint. This generally will result in either partial or complete failure of the joint. The cause of the overload is often not obvious, and a bit more detective work is then required.

Miscellaneous faults:

The above failure modes are probably the most common that we are likely to encounter, but there are a few odd failure modes we should also be aware of:-

If the resistor has been subjected to sufficient mechanical stress, the ceramic body of the resisor may be cracked. This may not be immediately apparent, but will eventually reveal itself through intermittent or noisy operation. A hot soldering iron tip in close proximity to the damaged resistor will sometimes find this one.

Mechanical damage may also break through the protective covering of the resistor, resulting in damage to the conductive film which may cause a shift in resistance value (generally upwards) or even open circuit in extreme cases.

There are many other types of resistor that I have not mentioned, but I hope you now have a better insight into the role of the humble resistor and resistor related faults.

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Capacitors:

Next to resistors, capacitors are probably one of the commonest components, found in practically all electrical and electronic equipment.

Capacitors are manufactured in a huge variety of materials, styles and sizes, but the good news is, we can basically divide them into two large groups i.e. Those which are electrolytic and those that aren't.

Electrolytics are basically polarised i.e. they are designed to be used with a d.c. voltage on one end. If you connect the voltage around the wrong way, they tend to go bang and frighten the cat. There are quite a few different types of electrolytic capacitor, and they are generally not used where a precise value of capacitance is required. They do however, all basically offer large values of capacitance in a small physical size. They also tend to exhibit relatively large leakage currents and have fairly large tolerances. It is not unusual for an electrolytic capacitor to have a specified tolerance of ± 50% or even more. Electrolytics are typically used, but not restricted to use in low frequency energy storage applications. e.g. Power supply filtering. At high frequencies ESR (Equivalent Series Resistance) renders the standard electrolytic capacitor useless. This is worth remembering and is the reason that for decoupling, electrolytics are often found in combination with ceramic capacitors which have better high frequency characteristics. In switch mode power supplies capacitors with low ESR are normally specified.

When a capacitor is suspect, it is not as easy to check as say a resistor with a multi meter. But there are simple ways that relative checks can be carried out. If you are really keen, connect the capacitor in series with a resistor and apply a known dc voltage. Time the voltage rise across the capacitor and knowing the time constant (C x R) compare the times. It should take CxR seconds for the capacitor voltage to reach 63% of the supply voltage. Or, alternatively, it takes approximately 5 times C x R for the voltage to reach 99% of the supply voltage. This method though is only of use for large values of capacitor.

Another method uses the dynamics of a moving coil or analogue multimeter on the ohms range. Connect the multimeter across the capacitor and watch the 'kick' as the capacitor charges. Reverse the connections to see the effect again. You can compare the rate and size of the 'kick' as the capacitor is charged and discharged, with that of a known good capacitor. Be careful using this technique on low voltage tantalums as a small reverse voltage is applied to the capacitor. Again this technique is really only of use with relatively large capacitors and gives an approximate comparison only.

For small capacitors it is probably easiest to substitute a suspect capacitor with a known good one. Some of the better DVMs can also measure capacitors over a useful range of values.

To summarise, capacitors generally fail by becoming leaky or eventually failing short circuit.The primary causes of this are old age or extremes of temperature. Excessive voltages are usually the prime cause of complete capacitor failure. Between the onset of abnormal leakage and complete failure, different symptoms might be exhibited depending on the capacitors function within the circuit. A capacitor used for timing may result in an incorrect time period, it may even cause the timing circuit to latch up completely. Excessive ripple on a dc supply or hum on an audio output stage can both be signs of capacitor degradation or failure.