Resistors

Description 

fig.1 - Sample resistors

Resistor, component of an electric circuit that resists the flow of direct or alternating electric current.
An electric current is the movement of charged particles called electrons from one region to another.
The amount of resistance to the flow of current that a resistor causes depends on the material it is made of as well as its size and shape. Some resistors obey Ohm's law, which states that the current density is directly proportional to the electrical field when the temperature is constant.
The resistance of a material that follows Ohm's law is constant, or independent of voltage or current, and the relationship between current and voltage is linear. Modern electronic circuits depend on many devices that deviate from Ohm's law. In devices such as diodes, the current does not increase linearly with voltage and is different for two directions of current.Resistors are often made to have a specific value of resistance so that the characteristics of the circuit can be accurately calculated.
Most resistors used in electric circuits are cylindrical items a few millimeters long with wires at both ends to connect them to the circuit.

What can a resistor do ?

Resistors can help divide voltages, and when combined with other elements can help convert voltages for a specific electrical design.
Resistors can also be used to provide intense light or heat.
For example, the heating element in a household cooking range is a resistor, as is the tungsten filament in a common incandescent lamp.

Resistor types

Resistors may be made from carbon, metal oxide, or a coil of wire.
Carbon resistors are cheap but difficult to make with precisely specified resistance.
Wire resistors are useful for high power, since they dissipate heat more readily than other types.
Metal oxide resistors can be small, with an accurately specified resistance that does not change with time, and are suitable when high power is not required.

Fixed-value or variable resistors

fig.2 - Symbols

A fixed-value resistor has a series of coloured bands around its body, which signifies the resistor's value and tolerance.
Variable resistors are ones whose resistance can be varied between an upper and lower limit.

They may be adjusted by rotary or linear controls, and are often used in sound level and tone controls on radio sets and sound systems.
They need to be specified in terms of their resistance range and power rating, as fixed-value resistors are.

Some variable resistors are designed to provide equal increases in resistance for equal amounts of turn of the control knob-for example, 10 ohms for every 10° of turn.
Others are designed to double the resistance every time the knob is turned by a given amount.

Understanding Color Code

fig.3 - Color Codes

  First find the tolerance band, it will typically be gold ( 5%) and sometimes silver (10%). Starting from the other end, identify the first band - write down the number associated with that color; in this case Blue is 6. Now 'read' the next color, here it is red so write down a '2' next to the six. (you should have '62' so far.) Now read the third or 'multiplier' band and write down that number of zeros. In this example it is two so we get '6200' or '6,200'. If the 'multiplier' band is Black (for zero) don't write any zeros down. If the 'multiplier' band is Gold move the decimal point one to the left. 

If the resistor has one more band past the tolerance band it is a quality band. Read the number as the '% Failure rate per 1000 hour' This is rated assuming full wattage being applied to the resistors. (To get better failure rates, resistors are typically specified to have twice the needed wattage dissipation that the circuit produces)

1% resistors have three bands to read digits to the left of the multiplier. They have a different temperature coefficient in order to provide the 1% tolerance.

Connections 

Resistors can be connected in series or in parallel.

fig.4 - Connections

ER=Equivalent resistance

Connection in series
ER=R1 + R2 + R3

Connection in parallel
 

Measuring resistors (Wheatstone Bridge)

fig.4 - Wheatstone Bridge

The most accurate measurements of resistance are made with a galvanometer in a circuit called a Wheatstone bridge, named after the British physicist Charles Wheatstone. 

The Wheatstone bridge consists of four resistances connected together to form four arms of a diamond-shaped loop. Of the four resistances, the values of three (R1, R2, and R3) are known and one (Rx) is unknown. R1, R2, and R3 are variable resistors-the user can vary the value of each resistor's resistance. The resistors are arranged so that electric current splits at one corner of the diamond and flows through R1 and R3 on one side and R2 and Rx on the other side.
A galvanometer, a meter that measures electric current, connects the point of the diamond between R1 and R3 to the point of the diamond between R2 and Rx. Current flows out of the diamond at the point between R3 and Rx. The user varies the resistance of R1, R2, and R3 until the galvanometer reads zero. The ratio of the resistance of R1 to the resistance of R3 then equals the ratio of the resistance of R2 to the resistance of Rx. The values of R1, R2, and R3 are known, so the user can easily calculate the value of Rx.

Similar bridges, substituting known inductances and known capacitances for the resistance arms of the bridge, are employed in the measurement of the inductance and capacitance of circuit components.

Bridges of this type are usually known as AC bridges, because AC sources are used rather than DC sources. These bridges are often balanced by means of a telephone receiver rather than a galvanometer. When the bridge is unbalanced, a tone will be heard in the receiver corresponding to the frequency of the AC source, but when the bridge is balanced, no tone will be heard.

Useful Links

Resistance | Resistivity | Ohm’s Law | Media gallery (Capacitor) | Charles Wheatstone