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Resistors limit the amount of current that reaches a component such as an LED.

In some circuits different voltages need to be supplied to different parts of a circuit which can be done with resistors.  If two resistors are joined it forms a voltage divider, if the two resistors are of equal value the voltage in between the two resistors is half that of the rest of the circuit.

A resistor can control the voltage/current going into a component, so for instance placing a resistor at the input of a transistor controls how much the transistor amplifies a signal.

A resistor can protect the input of sensitive components.  If a resistor is placed at the input of a sensitive component the resistor limits the amount of current travelling to it protecting it from damage.  A 470 resistor is usually a sufficient one to protect LED’s.

A fixed resistor supplies a specific resistance depending on its value.  This is determined by a colour coding system that starts at the edge of the resistor and is combined of 4, 5, or sometimes 6 bands of different colours to give the resistor its value.

A resistors resistance is measured in ohms the bands of colour indicate what resistance it will provide.  The colour coding is standardized across the world and the amount of bands featured on the resistor dictates whether it is a standard precision or high precision resistor.

On a standard resistor bands one, two and three indicate its value, for example a resistor with a yellow, orange, red band would be 4,3 x 1000 so it would be 43000 ohms or 43k.  The fourth band represents its tolerance level which is usually between 5 and 10% of the resistors tolerance.

A resistors tolerance takes into account any variations that have taken place during manufacture.  If a resistor had a 2k marking for example the actual value may be slightly higher or slightly lower and this potential variation in the value is known as its tolerance and will be shown as a percentage so a +5 percent tolerance means that the value of the resistor may vary between 5 % above or below the stated value.  By knowing the tolerance I can decide on whether the resistor is going to be suitable for the circuit or whether I may have to change it for usually a higher value.

A high precision resistor may have the value printed on it or they may have five bands.  These will have a tighter tolerance than standard resistors where bands one through four represent the value and the fifth band represents its tolerance usually +1%.  If a circuit needs a specific value such as in timing than a high precision resistor would be used.

Resistors can also be measured by their power, this is measured in watts, the higher the watts the more heat there is generated by electrons travelling through the circuit.  Components can only stand so much heat before becoming damaged and the power rating tells you how many watts can safely pass through the resistor.  Watts are calculated by


P is the power measured in watts, I is the current measured in amps and V is the voltage measured across the resistor, so if the voltage is 5 volts, and 25 milliamps of current go through the resistor the watts are calculated by multiplying 5 by 0.25 to get 0.125.

To calculate the resistance of a single resistor in a circuit is easy using the table featured previously, but resistance will change if you add resistors in parallel or in series together.  To calculate resistors placed in series I just add the values of the resistors together.

For resistors laid in parallel it is a little more complex.  It is important to know the formulas though as resistors will only come in a limited number of common values, yet some circuits need a specific value that can only be created by inserting two or more resistors in series or parallel.  Also resistors are not the only components that have resistance, a motor for example may have some kind of resistance, and for some applications I might need to calculate the combined effect of having these various resistances in a single circuit.

To calculate the resistance of two resistors in parallel the formula is:  total resitance = R1 x R2 divided by R1 + R2

So if the values of the resistors were 1.2k (1200 ohms) and 2.2k (22o0 ohms) the total resistance would be:

1200 x 2200 =2640000

1200 + 2200 = 3400                         2640000 divided by 3400 = 776.47 total resistance

To calculate the total resistance of three resistors in parallel the formula is as follows

total resistance=                          1


1 divided by R1    +    1 divided by R2      +  1 divided by R3………………





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