Diodes

Here is a charger. We need one of these to charge a mobile phone battery. But wait — the mobile uses current that only goes in one direction, direct current. And the current in the socket changes direction all the time — it is alternating current. We need the charger to convert the alternating current to direct current.

This little thing in the charger does just that. It's a diode. Let’s take a look at how it works. The diode consists of two pieces of silicon. Silicon conducts current less effectively than a conductor, but more effectively than an insulator.

Silicon is a semiconductor. By mixing in other materials, doping the silicon, we can make silicon conduct current better. The first piece of silicon is doped with a material that gives it more negative electrons; it is n-doped. The other piece of silicon is doped so it has fewer negative electrons; it is positively doped, p-doped. In the n-doped silicon, we have electrons that like to move and in the p-doped silicon there are holes where electrons are missing.

Now let’s look at the alternating current from the socket. What happens when the circuit is connected like this, and the current tries to go from the negative pole to the p-doped part of the diode, and from the n-doped part of the diode to the positive pole? The electrons in the diode do not want to move from the n-part to the p-part, but rather are drawn to the positive pole. No current passes through the diode. Now the alternating current has changed direction.

The plus pole has become the minus pole. Since the positive pole is now connected to the p-part, the negatively charged electrons in the diode want to move from the n-part to the p-part. And now there is current in the circuit. So the diode only conducts current in one direction. We are using the diode to convert alternating current to direct current.

Let’s draw a wiring diagram to see how this works. Here is an electrical circuit with alternating current from a socket. And here is the battery that wants direct current. First the current goes in this direction: From plus to plus on the mobile’s battery and then on from the battery’s minus to the socket’s minus. Then the current in the socket changes direction.

From plus to minus on the mobile’s battery and then on from the battery’s plus to the socket’s minus. In this way the current changes direction rapidly, 50 or 60 times every second. But the battery doesn’t charge. In order for it to charge, the current needs to go in the same direction all the time. We add a diode here.

The diode symbol is an arrow and a straight line. The arrow shows the direction in which the diode passes current. When the voltage goes in one direction, so that the electrons want to move clockwise, the diode lets them through. But when the voltage is reversed, and the alternating current wants to go in the other direction, the electrons stand still because the diode does not let any current through. We have created a rectifier!

We have converted alternating current to direct current. But the mobile is only charged half the time. The time the diode lets current through. Can we charge it more efficiently? Let’s look inside the charger.

Oh! Here there are several diodes, four of them. Hm, now that you know what one diode does, what do you think four diodes do?