The resistor is a component that offers resistance to the flow of electrons(current), this resistance is measured in ohms. A resistor's value is indicated by a colour code or numbers.

Linear resistors: A perfect linear resistor has a constant resistance independent of current, voltage, temperature and other factors. In reality no resistor is prefect and even two resistors with the same marked value may have several percent difference if measured. Therefore, in addition to their value, resistors are marked with a indication of inaccuracy, called tolerance, measured in percent. The most used resistors have a tolerance of 5%, but if needed resistors down to 1% are available.

The first resistors were made of a solid rod or tube of carbon, with a metal clamp on each end. This resistor has a low inductance and can withstand overload for a short time, but has a high capacitance(0.2pF-1 pF) and the resistance is higly dependent on temperature and voltage. This resistor has been replaced by other types in most applications, but is sometimes used in pulse circiuts due to the low inductance.

The carbon film resistor is made of a ceramic core with a thin film of carbon on the outside. To vary the resistance of these resistors, the film is cut into a spiral by a diamond edged blade, or by a LASER cutter. This resistor type have a high capacitance(~0.2pF), medium to high inductance and low temperature stability. But they are cheap to manufacture.

The metal film resistor is similar to the carbon film resistor, but uses a more stable metal film as the resistive layer. These resistors have low capacitance, high voltage and temperature stability. But their pulse resilience is low, and therefore metal film resistors should not be used instead of carbon resistors in pulse circiuts.

The Thick film/cerment resistor, is made from a mix of metaloxides and ceramics that is fastened on a ceramic base. This is the most common type of surface mounted resistors. They have medium capacitance, a good long time stability and can 'survive' relatively high temperatures.

The wirewound resistor: This type of resistor consists of a metal wire that is wound around a ceramic core. The wire is made from a highly stable alloy. This resistor is usually used in high effect applications, but have a high inductance.

Nonlinear resistors:
Some times the nonlinearety of a resistor is desirable. This can be like the thermistor and the light dependent resistor(LDR) that is used as sensors Or as voltage or current dependent resistors(VDR/CDR)that is used to protect against high voltages or currents.
Here are some resistor color codes:
The first and second band specifies the first two digits. Their codes are as follows:

Black 0
Brown 1
Red 2
Orange 3
Yellow 4
Green 5
Blue 6
Violet 7
Gray 8
White 9

The third band is a multiplier for the first two bands to find the final number of ohms:

Black * 1
Brown * 10
Red * 100
Orange * 1k
Yellow * 10k
Green * 100k
Blue * 1m
Silver / 100
Gold / 10

The final band(4th) represents the tolerance or accuracy of this measurement:

Gold 5%
Silver 10%
None 20%


Example:
Yellow - Violet - Orange - Gold
is 4.7k ohms with 5% accuracy. (4 - 7 - *1k - 5%)

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