# » When you have completed this module you will know, what components do, what they physically look like and how they are represented in a circuit diagram.

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» When you have completed this module you will know, what components do, what they physically look like and how they are represented in a circuit diagram. The following components are covered: » Resistors » Capacitors » Diodes » Transistors » Integrated circuits

» In an electronic circuit, electricity flows through the wires and components, like water flows through pipes. » The purpose of a resistor is to limit the flow of current. » When comparing it to water the resistor is like a short piece of thin pipe in the middle of a wide length of pipe. The thin pipe restricts the amount of water that flows along the pipe.

» Resistors are available in different values. The bigger the resistance, the more it limits the current that flows through it. » The value of the resistor is indicated by a number of colour bands printed on the side of the component. » Resistance is measured in ohms. » The symbol to represent a resistor on a circuit diagram is a rectangle, as shown below. » Variable resistors, pre-set resistors and resistive sensors all take the basic rectangle and add to it, so that when ever you see a rectangle you know it's a resistor of some type.

» A capacitor is a device, that allows electricity to be stored. Once a capacitors has been charged it can then be discharged. » It is like having a bucket with a tap on it, the bucket can be filled, then emptied later. » The image on the left shows the two most common types of capacitor.

» Just as you can get different sized buckets, which hold different volumes of water, you get different sized capacitors that hold different amounts of charge. Considering a capacitor as a bucket, the area of the buckets base is equivalent to the capacitance (C) and the height of the bucket is equal to its voltage rating (V).

» Capacitors are made of two metal plates that are next to each other. The bigger the plates the more charge they hold and hence the bigger the capacitance of the capacitor. » So that capacitors with a high capacitance don't take up loads of space the plates in an electrolytic capacitor are rolled up like a toilet roll. » The amount of charge that a capacitor can hold, it's capacitance, is measured in Farads.

» With electrolytic capacitors like the one below, it is important to connect the capacitor up the right way around. If you look at this capacitor you will see that there is a white band on it with a minus sign. This indicates which leg on the capacitor should be connected to the most negative part of the circuit. » You should be careful when using electrolytic capacitors as they can be permanently damaged if connected the wrong way around. » Capacitors The symbol used to represent a capacitor on circuit diagrams is two parallel lines. If you can remember the capacitor has two parallel plates inside it you should be able to remember the symbol is two parallel lines.

» Diodes Diodes are used to control the direction of flow of electricity. » In one direction they allow the current to flow through the diode, in the other direction the current is blocked. » Diodes can be used to protect electronics against the power being connected the wrong way around and can also be used to regulate mains electricity.

» As the diode only allows current to flow in one direction it's important that we can work out which way the electricity will flow. » On the circuit diagram this is shown with the direction of the arrow. On the part a band marks the end that the current flows out of. » On both the symbol and the part (in the picture) the current will flow from left to right.

» An LED is a special diode, LED stands for Light Emitting Diode. It is like normal diodes, in that it only allows current to flow in one direction. However, when the current is flowing the LED lights. The symbol for an LED is the same as the diode, but with the addition of two arrows to show there is light coming from the diode.

» A transistor allows a small amount of current to control a big amount of current. » You can think of it as a switch, when a voltage is applied to the input it turns on and current flows through the device. » The way the transistor is packaged depends upon how much current it allows to be switched, the part shown left is a low current part, bigger are available. » The circuit diagram symbol for a transistor and the part are shown in the drawing.

» The circuit diagram (left) shows a transistor driving a load. The input to the transistor (the base) is connected to the output of an IC, which is currently at 0 volts. » No current is flowing through the load. » When the output of the IC is taken to 5 volts (or any voltage above 0.7 volts) a small amount of current flows into the base of the transistor (labelled Ib). » At this point a large amount of current flows through the transistor, flowing from the collector into the emitter (labelled Ice). At this stage the load is switched on.

» Integrated circuits or IC for short, are lots of transistors packed into one device. There are lots of different types of IC, they can provide basic logic functions and timing functions or even be as complex as a computer processor. » As each IC is different the circuit diagram for it is also different, but they all tend to be a box with connections around the edge.

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