Selasa, 25 Januari 2011


resistor is a two-terminal electronic component having a resistance (R) that produces a voltage (V) across its terminals that is proportionalto the electric current (I) flowing through it in accordance with Ohm's law:
V = IR
Resistors are elements of electrical networks and electronic circuits and are ubiquitous in most electronic equipment. Practical resistors can be made of various compounds and films, as well as resistance wire (wire made of a high-resistivity alloy, such as nickel-chrome).
The primary characteristics of a resistor are the resistance, the tolerance, the maximum working voltage and the power rating. Other characteristics include temperature coefficientnoise, and inductance. Less well-known is critical resistance, the value below which power dissipation limits the maximum permitted current, and above which the limit is applied voltage. Critical resistance is determined by the design, materials and dimensions of the resistor.
Resistors can be integrated into hybrid and printed circuits, as well as integrated circuits. Size, and position of leads (or terminals), are relevant to equipment designers; resistors must be physically large enough not to overheat when dissipating their power.

The behavior of an ideal resistor is dictated by the relationship specified in Ohm's law:
V=I \cdot R
Ohm's law states that the voltage (V) across a resistor is proportional to the current (I) through it where the constant of proportionality is the resistance (R).
Equivalently, Ohm's law can be stated:
This formulation of Ohm's law states that, when a voltage (V) is maintained across a resistance (R), a current (I) will flow through the resistance.
This formulation is often used in practice. For example, if V is 12 volts and R is 400 ohms, a current of 12 / 400 = 0.03 amperes will flow through the resistance R.

Series and parallel resistors

Resistors in a parallel configuration each have the same potential difference (voltage). To find their total equivalent resistance (Req):
A diagram of several resistors, side by side, both leads of each connected to the same wires

\frac{1}{R_\mathrm{eq}} = \frac{1}{R_1} + \frac{1}{R_2} + \cdots +  \frac{1}{R_n}
The parallel property can be represented in equations by two vertical lines "||" (as in geometry) to simplify equations. For two resistors,

R_\mathrm{eq} = R_1 \| R_2 = {R_1 R_2 \over R_1 + R_2}
The current through resistors in series stays the same, but the voltage across each resistor can be different. The sum of the potential differences (voltage) is equal to the total voltage. To find their total resistance:
A diagram of several resistors, connected end to end, with the same amount of current going through each

R_\mathrm{eq} = R_1  + R_2 + \cdots + R_n
A resistor network that is a combination of parallel and series can be broken up into smaller parts that are either one or the other. For instance,
A diagram of three resistors, two in parallel, which are in series with the other

R_\mathrm{eq} = \left( R_1 \| R_2 \right) + R_3 = {R_1 R_2 \over R_1 + R_2} + R_3

Resistor values - the resistor colour code

The Resistor
Colour Code



















Resistance is measured in ohms, the symbol for ohm is an omega ohm.
1 kohm = 1000 ohm     1 Mohm = 1000000 ohm.Resistor values are normally shown using coloured bands.1 ohm is quite small so resistor values are often given in kohm and Mohm.
Each colour 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.
resistorThis resistor has red (2), violet (7), yellow (4 zeros) and gold bands.
So its value is 270000 ohm = 270 kohm.
On circuit diagrams the ohm is usually omitted and the value is written 270K.
Find out how to make your own Resistor Colour Code Calculator

Small value resistors (less than 10 ohm)

The standard colour code cannot show values of less than 10ohm. To show these small values two special colours are used for the third bandgold which means × 0.1 and silver which means × 0.01. The first and second bands represent the digits as normal.For example:
redvioletgold bands represent 27 × 0.1 = 2.7 ohm
greenbluesilver bands represent 56 × 0.01 = 0.56 ohm

Tolerance of resistors (fourth band of colour code)

The tolerance of a resistor is shown by the fourth band of the colour code. Tolerance is the precision of the resistor and it is given as a percentage. For example a 390ohm resistor with a tolerance of ±10% will have a value within 10% of 390ohm, between 390 - 39 = 351ohm and 390 + 39 = 429ohm (39 is 10% of 390).A special colour 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%.