Resistance
Resistance is a property of materials which determines how easily charge can flow through them at a particular voltage.
Resistance is measured in units called ohms.
The resistors you haver seen so far (illustrated below) are known as Fixed Resistors and they are made in a range of values known as the preferred value series. With such a wide range of potential values, it would be very expensive and uneconomic to manufacture every single value. To overcome this a range
of preferred values has been created. The most common ranges are known as the E12 and E24, although others are available.
  • E12 range: 10R, 12R, 15R, 18R, 22R, 27R, 33R, 39R, 47R, 56R, 68R, 82R and their multiples.
  • E24 range: 10R, 11R, 12R, 13R, 15R, 16R, 18R, 20R, 24R, 27R, 30R, 33R, 36R, 39R, 43R, 47R, 51R, 56R, 62R, 68R, 75R, 82R, 91R and their multiples.
When choosing a resistor we always choose the preferred value that is higher than that calculated. For example if our calculated value for a resistor was 45R, we would use a 47R resistor. (Based on the E12 range pictured below)
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Most of the fixed resistors you are likely to use will only be suitable for use with small currents. They are likely to be 0.25W, 0.33W, 0.5W resistors, this is the measure of the maximum power-handling capacity.

Resistor Colour Codes
Most resistors are labelled with four coloured bands to show the resistance value and tolerance. This is known as the resistor colour code. You should know how to use it to check resistor values.

external image itouch.jpgIf you have an iTouch or iPhone you can download applications for calculations of Ohm's Law and for working out resistor codes.
Here are two links to these applications:
And the same application can be accessed online at:
Ready for you to check your resistor codes and how to calculate resistors in series and in parallel.


Tolerance
Tolerance is a measure of the accuracy of the colour coding by indicating the actual resistance of the resistor.
  • For example 1K gold band resistor - 5% tolerance could have a resistance of 100 ohms ± 50ohms - therefore the range of accuracy of the resistor is between 950 ohms and 1050 ohms

Variable Resistor
The resistance value is changed by moving the contact (wiper) across the resistance track. The maximum value is usually printed on the body, examples of these in use at school include a 555 timer, used to adjust the time. And the mini-amp project, used to adjust the volume.
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Resistors in Series
When you start to look at circuits, you will often see resistors connected together one after another as if they were in a continuous chain. We say they are connected in series. When resistors are connected in this way, the overall resistance can be found by R (total) = R1 + R2 +R3 and so on.
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All you have to do is simply add all the resistances together.
Example
What is the overall resistance of a circuit if R1 is 22R and R2 is 56R?
  • R = R1 + R2
  • R = 22 + 56
  • R = 78R
Resistors are often connected in series because we require a resistor value that is not manufactured or within the preferred range. By combining resistors, we can often obtain the required value. Another use is to drop or reduce voltages in a circuit.

Resistors in Parallel
When two or more resistors lay side by side with their ends connected together, we say they are connected in parallel. When resistors are connected in this
way, the overall resistance can be found by R total = R1 x R2 / (R1 + R2)
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0r R total = 1/R1 + 1/R2 + 1/R3
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Examples:
Two resistors are connected together in parallel. R1 is 8R and R2 is 8R. What is the total resistance?
  • R total = R1 x R2 / (R1 +R2)
  • R total = 8 x 8 / (8 + 8)
  • R total = 64 / 16
  • R total = 4R

Two resistors are connected together in parallel. R1 is 8R and R2 is 6R. What is the total resistance?
  • R total = R1 x R2 / (R1 +R2)
  • R total = 8 x 6 / (8 + 6)
  • R total = 48 / 14
  • R total = 3.43R

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Check out this site: http://mhuss.com/Resistors


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