**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

orRKilo Ohm 1000 ohm

KorKMega Ohm 1000 000 ohm

MorMRefer 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.