Passive Components

You are required to be able to identify and describe the function and application of the following passive components.

  • Resistor (including the colour code)
  • Variable resistor
  • Polarized capacitor
  • Non polarized capacitor
  • Diode
  • Relay

The Resistor

The resistor is probably the most basic of all electronic components, the basic function of a resistor is to limit or restrict the current flowing in a circuit. Resistors are also used in voltage dividers as we found out in the section on sensors and switches. Electrical Resistance is measured in Ohms (Ω)

There are a few various types of resistor. You can learn all about them here.

The following slide gives some information on the construction of probably the most common resistor known as the carbon composition resistor.

Carbon Resist

Circuit Symbols and the Resistor Colour Code

In order to be able to identify a resistor in an electronic circuit diagram you need to know the circuit symbol. You also need to know the resistor colour code so that the resistor you select for a real circuit matches up with the value on the circuit diagram.

The following slide gives an example reading the resistor colour code and the circuit symbols used for a resistor.

info sheet Resistance

The Variable Resistor

As its name implies, a variable resistor is one where we can change the value of resistance by adjusting a slider or dial for example. Variable resistors can also be used as potentiometers. The next slide shows some of the common applications for a variable resistor.


The next slide shows some common types of variable resistor and the difference between a variable resistor and potentiometer, the circuit symbols and examples of both components being used in real circuits. Potentiometers will have their ohmic value stamped or printed somewhere on the body of the component.

Variable ressies

Potentiometers make for good voltage dividers as well. The are more expensive than standard resistors and less stable (generally speaking). You can fine tune the output without having to change any other components though which makes them a good choice for circuits where flexibility is needed.

The Capacitor

The capacitor is a more complicated component than the resistor. We will begin by using a slide to demonstrate the basic construction and operation of a capacitor. Construction and operation a similar (in basic terms) for both polarized and non polarized capacitors.

Capacitor info

The following video also gives some excellent information on capacitors and examples of reading the values which can be difficult for some to grasp.

Capacitors behave very differently when connected to different power supplies such as ac or dc. To keep things as simple as possible we are only going to deal with capacitor behaviour in dc circuits.

Polarized Capacitors

There are two major differences between a polarized and non polarized capacitor. The first is a safety issue. If you connect a polarized capacitor the wrong way round it will explode as demonstrated by this video.

The other major difference lies in the amount of charge that can be stored by a polarized capacitor. Electrolytic capacitors which are the most common example of this type can typically store charges which are thousands of times greater than for example the charge stored by a ceramic capacitor. Thus the type of dielectric used for a capacitor does play a significant role in the way that capacitor will behave in a circuit.

One of the most common uses for a polarized capacitor is in a rectifier circuit. The next slide demonstrates how such a capacitor functions in this circuit. We will also use the slide to introduce the diode.

A rectifier circuit changes an ac current to a dc current. For more information on this you can check the video below.

The next slide shows the basic half wave rectifier circuit. Half wave simply means that only half of the sine wave input is converted to a dc output.


The next slide clearly shows the action of the diode only allowing half of the current to pass through. I this slide the capacitor has been removed so you can clearly see the action of the diode.


Now with the capacitor added back into the circuit we can clearly see the charge and discharge effect. The capacitor charges to each peak of the incoming waveform then discharges more slowly through the resistor.


When we zoom at to the same resolution as the input we can clearly see the dc output.


This concludes the section on diodes. We have looked at a diode in forward bias mode. In the next section on relays we will look at a diode in reverse bias mode.


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