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Resistance is Measured in Ohms

In a simple circuit, the resistor limits the current to a
small steady safe level.

A Simple Resistor Circuit

Without a current limiting resistor, the current is very
large and possibly dangerous. There is a fire risk.
This is called a short circuit.

Short Circuit


What Resistors Do

Resistor Types.jpg

Working from the top down in the image above.


Resistor Construction

Resistor Structure.gif

A typical resistor structure is shown above. The whole item is coated in paint/plastic and the colour code stripes added last.


Power Ratings

Resistors can get hot. They must be able to dissipate (get rid of) this heat. Some commonly available power ratings are 1/8, 1/4, 1/2, 1, 2.5, 5, 10, 25 and 50 Watts.



Resistor Symbols.gif



Type Advantages Disadvantages
Carbon Film

Resistor Carbon Film.jpg

  • Cheap
  • Unstable because resistance changes over time
  • Resistance changes with temperature
  • Can cause noise in circuits. The noise is caused by fluctuations in contact resistances between the carbon granules.
  • Not very accurate (poor tolerance)
Metal Oxide

Resistor MO.jpg

  • More accurate
  • More stable
  • Less noise
  • More expensive (still remarkably cheap though)
Wire Wound

Resistor WW.jpg

  • Very stable
  • Accurate
  • Good at dissipating heat

On the left is a damaged resistor where you can see the wire windings.

  • Most expensive
  • Wire winding is inductive so these can't be used in circuits involving radio frequencies.
Wire Wound Aluminium Coated
  • Like normal wire wound but they can be bolted to a heat sink.
  • Rated at up to 50 Watts.
  • Most expensive
  • Wire winding is inductive so these can't be used in circuits involving radio frequencies.
Light Dependent Resistors

Resistor LDR.jpg

  • Resistance varies with the light level.
  • LDRs are used to measure the light level.
  • They are used in light meters for photography and for systems that turn on lights when it gets dark.
  • The LDR is normally used with a fixed resistor. This prevents too much current flowing when the light is very bright. It also converts the varying light level into a varying voltage.
  • Must be protected in bright light. The current must be limited to a safe level with a second fixed resistor.
  • They are slow to react so they are not used with fibre-optic or other optical communications systems.

Resistor Pot.gif

Here is a flash animation for a potentiometer.

  • Uses include light dimmers, volume controls, tone controls and voltage adjusters.
  • They can be pre-set and adjusted with a small screw driver or adjusted with a knob.
  • Potentiometer - uses all three pins and works as a voltage divider.
  • Variable Resistor - uses only two pins.
  • Corroded tracks can create nasty noises when the pot is adjusted.
Surface Mounted

Resistor SM.jpg

  • Surface Mounted - 1 or 2 mm across.
  • Small (minute)
  • These can be too small for the average amateur constructor to use!
  • Too small to colour code or label. If you mix them up, you are in trouble.
  • Capacitors look the same as resistors.

Zero Ohm Resistors

Zero ohm resistors exist. These are used instead of wires because robotic circuit board assembly machines are already able to work with standard resistor shapes. A whole new robot would be needed to work with wires so zero ohm resistors were invented.




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