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                               Fig - 1: Resistor

There are other types of resistors. The potentiometer is a variable resistor. When the knob of a potentiometer is turned, a slider moves along the resistance element. Potentiometers generally have three terminals, a common slider terminal, and one that exhibits increasing resistance and one that has decreasing resistance relative to the slider as the  shaft is turned in one direction.
 

• 2,200 Ohms

• 2,200 Ω

• 2,200R

• 2.2k

• 2.2k Ω

• 2k2

The use of the symbol for Ohms (Omega, Ω is optional, and is most commonly left off, since it is irksome to add from most keyboards.  The letter "R" and the "2k2" conventions are European, and not commonly seen in the US and other backward countries :-)  Other variants are 0R1, for example, which means 0.1 Ohm

The schematic symbols for resistors are either of those shown below.  I use the Euro version of the symbol exclusively.

  Fig - 2 :Resistor Symbols

The basic formula for resistance is Ohm's law, which states that ...

1.1.1   R = V / I   Where V is voltage, I is current, and R is resistance

The other formula you need with resistance is Power (P)

1.1.2   P = V² / R
1.1.3   P = I² * R

The easiest way to transpose any formula is what I call the "Transposition Triangle" - which can (and will) be applied to other formulae.  The resistance and power forms are shown below - just cover the value you want, and the correct formula is shown.  In case anyone ever wondered why they had to do algebra at school, now you know - it is primarily for the manipulation of a formula - they just don't teach the simple ways.  A blank between two values means they are multiplied, and the line means divide.

     Fig - 3 : Transposition Triangles for Resistance


                                                                  Fig - 4 : Resistor Colour Code

Tolerance : The tolerance of resistors is mostly 1%, 2%, 5% and 10%.  In the old days, 20% was also common, but these are now rare.  Even 10% resistors are hard to get except in extremely high or low values (> 1M or < 1R), where they may be the only options available at a sensible price.

A 100R resistor with 5% tolerance may be anywhere between 95 and 105 ohms - in most circuits this is insignificant, but there will be occasions where very close tolerance is needed (e.g. 0.1% or better).  This is fairly rare for audio, but there are a few instances where you may see such close tolerance components.

Power Ratings : Resistors are available with power ratings of 1/8th W (or less for surface mount devices), up to hundreds of watts.  The most common are 1/4W (0.25W), 1/2W (0.5W), 1W, 5W and 10W.  Very few projects require higher powers, and it is often much cheaper to use multiple 10W resistors than a single (say) 50W unit.  They will also be very much easier to obtain.

Like all components, it is preferable to keep temperatures as low as possible, so no resistor should be operated at its full power rating for any extended time.  I recommend a maximum of 0.5 of the power rating wherever possible.  Wirewound resistors can tolerate severe overloads for a short period, but I prefer to keep the absolute maximum to somewhat less than 250% - even for very brief periods, since they may become open circuit from the stress, rather than temperature (this does happen, and I have experienced it during tests and repairs).

Resistance Materials : Resistors are made from a number of different materials.  I shall only concentrate on the most common varieties, and the attributes I have described for each are typical - there will be variations from different makers, and specialised types that don't follow these (very) basic characteristics.  All resistors are comparatively cheap.

• Carbon Composition:  Low to medium power.  Comparatively poor tolerance and stability.  Noisier than most others.

• Carbon Film:  Low power.  Reasonable tolerance and stability.  Reasonably quiet.

• Metal Film:  Low to medium power.  Very good tolerance and stability.  Quiet.

• Wirewound:  High to very high power.  Acceptable to very good tolerance, good stability.  Quiet.  May have inductance.

Resistors make noise.  Everything that is above 0K (zero Kelvin, absolute zero, or -273 degrees Celsius) makes noise, and resistors are no exception.  Noise is proportional to temperature and voltage.  Low noise circuits will always use low resistor values and low voltage wherever possible.

Resistors may also have inductance, and Wirewound types are the worst for this.  There are non-inductive Wirewound resistors, but are not readily available, and usually not cheap.


                                                Two Resistors in Parallel

                                                                  Two Resistors in Series