Now we really pull together what we have learnt in this section by looking at alternating currents and intridce oscilloscopes and transformers.
Youtube video, suitable whether of not you have the board with you.
See board images.
Tutorial guide for when you have the board with you.
Before we start, please ensure the two small switches are down in the off position before you start and the oscilloscope is unplugged from the board.
So, this is a bit of a crazy looking board – so let’s start going through it in a moment. But first I will explain what it’s about. So far, all the power supply for every board has been provided by batteries. These produce something called Direct Current or DC for short. That means the current flows round the circuit in just one direction and the voltage provided by the supply doesn’t really change (until a battery goes flat). We think of the current flowing from the positive terminal of the battery, round the circuit and back to the negative terminal.
All that changes here, this board uses Alternating Current of AC as it’s known. This is how the household electricity works which is why this board is plugged into the mains as we call it via the connection on the left side of the board. As you can see by the left voltmeter, the voltage we are getting is about 12 volts, but this time is not steady – it’s not actually always 12 volts, this is the average.
In fact, the voltage starts at zero, climbs up to about 15 volts, the slides back to zero and then does the same but in the opposite polarity (direction). That happens 50 times a second. It other words it’s constantly changing! It’s a sort of shaking back and forwards with the average being 12 volts firstly one way and then the other. The box on the right is an oscilloscope, it’s like a graph with voltage going up and down and time going from left to right. It’s great for showing what happens then the voltage is changing. Plug the leads into the two yellow sockets (doesn’t matter which way round) and see the way the voltage goes up and down.
One of the reasons why this type of electricity is used is because the voltage can be changed really easily using something called a transformer. This is two coils of wire wrapped round some iron (ferrite) so it can be turned into an electromagnet. When this AC electricity is passed through the first coil it induces a voltage in the second one because of the constantly changing magnetic field.
If the coils are the same size, the voltage will be the same, but if not, the voltage will be different. Less wire on the second coil means less voltage. With the transformer on this board, there is a 4.5 ratio in the size of the coils, so the 12 volts going in, is turned about 2.5 volts coming out. You can see this output voltage in the second voltmeter.
The speaker is connected to the circuit and by pressing the yellow button you will hear the low note created by the voltage changing 50 times a second. Please only hold this down for 3 seconds.
You will see the four diodes wired up as a bridge rectifier. The AC voltage is going into it and what comes out can be seen by connecting the oscilloscope to the red and black sockets (make sure you get them round the right way this time). Now there is no negative voltage, but instead it goes from zero, up to 15 voltages and back down 100 times a second.
But if we switch in the lefthand capacitor, then we can see it flatten out as some of the energy stored in the capacitor (when it’s giving a high voltage) is releasing and stops the voltage dropping down. Now switch on the bigger capacitor on. The line is flat because there is enough energy stored to cover the time when the supply voltage drops. So, what we have done is create a direct current (DC) from an alternating current (AC) supply. This is how your mobile charger works!