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Electronics >> Basics of Electronics >> Resistors


What is a Resistor?

A Resistor is an electrical device that resists the flow of electrical current. It is a passive device used to control, or impede the flow of, electric current in an electric circuit by providing resistance, thereby developing a drop in voltage across the device.

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 poor conductors.

How are Resistor Made?

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.

Resistor
                               Fig - 1: Resistor

What is a Potentiometer?

A 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.

Measurement of Resistance

The value of a resistor is measured in ohms and represented by the Greek letter capital omega (Ω).

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:

  • 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

Schematic Symbol of Resistor

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


Resistor Symbols
 
Fig - 2 :Resistor Symbols

How to Calculate Resistance?

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

  • R=V/I: Where V is voltage, I is current, and R is resistance.

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

  • P=V2/R

  • 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.

Transposition Triangles for Resistance
  
  Fig - 3 : Transposition Triangles for Resistance

Resistor Colour Code

Resistor Colour Code
                                                                
 Fig - 4 : Resistor Colour Code

What is Tolerance of a 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 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 Rating of Resistor

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. 

Wire wound 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 specialized 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 Parallel
Two Resistors in Series
                                
                                 Two Resistors in Series

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