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Resistors are probably the most simple and common component that is used in electronics. They limit maximum current flow through themselves based on Ohm's Law. This means that a resisitor can be used to run a low voltage device from a high voltage power supply. Resistors are non-polar meaning they have no specific orientation.
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Resistance [Top]


Resistance refers to how much a substance impedes the flow of electricity. Good conductors such as metals, have a relatively low resistance (does not impede the flow of electrons) compared to that of a bad conductor such as wood, which has an extremely high resistance.

Resistor Tolerance [Top]


The tolerance of a resistor is a measure of how accurate the rating of a resistor is. This depends on what material the resistor is made out of. Typical values for resistor tolerance are 1%, 5% and 10%.

Resistor Wattage [Top]


Different circuits have different power requirements, and to this extent you must ensure that you have resistors which can handle the amount of power without over heating or blowing up. Typical values for wattage sizes (measured in watts) are 1/4 W, 1/2 W, 1 W and 5 W.

An easy way to calculate the wattage of a circuit is using the following formula with P = Power (W), V = Voltage (V) and I = Current (amps). To establish what formula you need to use, cover the symbol whose value you wish to know. This leaves a diagramatic representation of the required formula (eg: cover the P and your are left with V x I).

P = I x V or V = P/I or I = P/V

A useful variation of this formula is P = I2.R where R is resistance in ohms. This is a combination of the Power formula and Ohm's Law.

Resistors in Series [Top]


When you place resistors in series (i.e: one after the other), the total resistance changes.
R(total)= R1 + R2 + R3 + ...
Note that the resistance will always be higher.

Resistors in Parallel [Top]


When you place resistors in parallel (i.e: connecting several resistors to the same points), the total resistance changes.
R(total)= 1/((1/R1) + (1/R2) + (1/R3) + ...
Note that the resistance will always be lower.

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This page was last modified on the 3rd October, 1997 by Ian Clarke.
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