We need some way to control the flow of
current from a voltage source, like a battery, so we do not
melt wires and blow up batteries. If you think of current,
charge flow, in terms of water flow, a good electrical
conductor is like big water pipe. Water mains and fire hoses
have their uses, but you do not want to take a drink from
one. Rather, we use small pipes, valves, and other devices
to limit water flow to practical levels. Resistors do the
same for current; they resist the flow of charge; they are
The value of a resistor is measured in ohms and represented
by the Greek letter capital omega. There are many different
ways to make a resistor. Some are just a coil of wire made
of a material that is a poor conductor. The most common and
inexpensive type is made from powdered carbon and a
glue-like binder. Such carbon composition resistors usually
have a brown cylindrical body with a wire lead on each end,
and colored bands that indicate the value of the resistor.
Fig - 1:
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.
The resistance between the two stationary contacts is, of course,
fixed, and is the value specified for the potentiometer. The
photo resistor or
photocell is composed of a light sensitive material.
When the photocell is exposed to more light, the resistance
decreases. This type of resistor makes an excellent light sensor.
The resistance value is specified in ohms, the standard symbol is
"R" or Ω. Resistor values are often stated as "k" (kilo, or times
1,000) or "M", (meg, or times 1,000,000) for convenience. There are
a few conventions that are followed, and these can cause problems
for the beginner. To explain - a resistor has a value of 2,200
Ohms. This may be shown as any of these:
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
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
- 4 : Resistor
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
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.
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).
: 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
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
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