Resistors

Opposition to the flow of current is called resistance and the device or component used for this purpose is called resistor.

Example:

Circuit symbols:

Function

Resistors restrict the flow of electric current, for example a resistor is placed in series with a light-emitting diode (LED) to limit the current passing through the LED.

Connecting and soldering

Resistors may be connected either way round. They are not damaged by heat when soldering.

Resistor values - the resistor color code

Resistance is measured in ohms, the symbol for ohm is an omega Ω

1 Ω is quite small so resistors values are often given in kΩ (Kilo Ohm's) and MΩ (Mega Ohm's).

1 kΩ = 1000 Ω
1 MΩ = 1000000 Ω

Resistors values are normally shown using colored bands. Each color represents a number as shown in the table. Most resistors have 4 bands:

The first band gives the first digit.

• The second band gives the second digit.
• The third band indicates the number of zeros.
• The fourth band is used to shows the tolerance (precision) of the resistor, this may be ignored for almost all circuits but further details are given below.

This resistor has red (2), violet (7), yellow (4 zeros) and gold bands.

So its value is 270000 Ω= 270 kΩ

On circuit diagrams the Ω is usually omitted and the value is written 270K.

Small value resistors (less than 10 ohm)

The standard color code cannot show values of less than 10Ω. To show these small values two special colors are used for the third band: gold which means × 0.1 and silver which means × 0.01. The first and second bands represent the digits as normal.

For example:

red, violet, gold bands represent 27 x 0.1 = 2.7Ω

green, blue, silver bands represent 56 times 0.01 = 0.56 Ω

Tolerance of resistors (fourth band of color code)

The tolerance of a resistor is shown by the fourth band of the color code. Tolerance is the precision of the resistor and it is given as a percentage. For example a 390Ω resistor with a tolerance of ±10% will have a value within 10% of 390Ω, between 390 - 39 = 351Ω and 390 + 39 = 429Ω (39 is 10% of 390).
A special color code is used for the fourth band tolerance:
silver ±10%, gold ±5%, red ±2%, brown ±1%.
If no fourth band is shown the tolerance is ±20%.
Tolerance may be ignored for almost all circuits because precise resistor values are rarely required.

Resistor shorthand

Resistor values are often written on circuit diagrams using a code system which avoids using a decimal point because it is easy to miss the small dot. Instead the letters R, K and M are used in place of the decimal point. To read the code: replace the letter with a decimal point, then multiply the value by 1000 if the letter was K, or 1000000 if the letter was M. The letter R means multiply by 1.