Resistor

What they look like:

How they are shown on circuit diagrams

Their purpose

Resistors, as their name suggests, restrict the flow of electricity.  Restricting the flow of electricity is important for a number of purposes including:

• ensureing current flow does not exceed a components maximum ie. the component is not permanently damaged by excessive current
• assisting in timing circuits (generally with capacitors) by regulating the charging time which is directly related to current flow

How to identify them

Resistor values are measured in OHM's -     

The greater the value of resistance, the less current the resistor will allow to flow ie. a 10 ohm resistor will allow 10 times the current of a 100 ohm resistor.

Values of resistance can vary from tenths of ohms to millions of ohms and therefore the Sceintific Notation  system is used to describe their value:

Ohm              Single ohm                       or        R

Kilo Ohm       1000 ohm           K         or        K

Mega Ohm     1000 000 ohm    M        or        M

Refer Component Naming Conventions

Due to the small size of most resistors, a colour code is used to identify their value.  The code is described below, note that some resistors have 4 colour bands and some have 5 colour bands. 

All resistors have a tolerance which describes how accurately the resistor has been manufactured. 

• A resistor with a 5% tolerance will have a resistance within 5% of the described value ie the 560K resistor below could have an actual resistance anywhere in the range of 532K - 588K (95% - 105% of 560K)
• A resistor with a 1% tolerance will have a resistance within 1% of the described value ie the 39K resistor below could have an actual resistance anywhere in the range of 38.61K- 39.39K (99% - 101% of 39K)

In most cases a 5% tolerance is acceptable and only highly tuned circuits require the use of the lower tolerance (more accurate) resistors.

Combination of Resistors

Resistors can be connected in two ways, serial and parallel:

Serial

In the above schematic, two resistors are connected in serial (one after the other) fashion at point B.  The total resistance measured from A to C is the sum of the two resistors ie. 56K + 22K = 78K.

  Any number of resistors can be connected in this way ie. the total resistance is R1+R2+R3+...Rn (where n is the number of resistors).

Parallel

In the above schematic, two resistors are connected in parallel (side by side) fashion at points A & B.  The total resistance measured from A to B is calculated by the formula:

For the example above, where a 56K and a 22K are connected in parallel, the total resistance from A to B is:

Rtot = 1 / (1/56000 + 1/22000)

Rtot = 1/ (0.0000178 + 0.0000454)

Rtot = 15795 or 15.8K

Any number of resistors can be connected in this way ie. the total resistance is:

Rn (where n is the number of resistors).

Note that resistors connect in serial have a combined resistance greater than either single resistor while resistors connected in parallel have a combined resistance less than either resistor.

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