Typical resistorsIn electricity, a resistor is an electrical device that is used to restrict the flow of electricity in a circuit, or to drop the voltage of a supply down to a lower value.


Symbols for resistor

Resistance can be thought of as the property of a circuit that converts electrical energy into heat energy. This is because the circuit opposes the flow of electrical current.

Where ever you can feel heat in an electrical circuit, that is a sure sign of resistance.

A resistor is a component that we place into an electric or electronic circuit for one or more of the following reasons:

  • We can use resistors to restrict the amount of electricity that flows in a circut- we can limit the current.
    For example, in a time delay circuit, we can use fixed resistors, or a variable resistors to control the rate at which a capacitor discharges.
  • We can use resistors to restrict the amount of voltage that acts on a component - we can limit the voltage.
    2V LED with resistor wired in series to allow the diode to be used in a 12V circuitThis explains why we can use 2V LEDs in 12V and 24V plant vehicles.
    The resistors cause a voltage drop across themselves and limit the voltage and current that act on the LEDs.
    Never connect an ordinary LED into a plant vehicle unless you have it supplied in series with the appropriate resistor. If you don't have this resistor, you will destroy the LED by connecting 12V across it. 
  • Another example is using a resistor to protect the base emitter circuits in transistors. In this role, a resistor is known as a base limiting resistor. Check out the time delay circuit to read more about this:

    Base limiting resistor

» Course Glossary


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