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P6H The diode A diode is a device that allows current to flow in one direction only. You should be able recognise and draw the symbol for a diode. Almost all diodes are made of silicon. Which direction? The electric current flows through a diode in the direction indicated by the triangle in its circuit symbol. When the diode is connected so that current flows, it is described as forward biased. When the diode is connected the other way round, so that current does not flow, it is described as reverse biased. The diode A diode contains two different layers, a positive p-type layer and a negative n-type layer. The p-type layer is missing electrons and contains positive holes where electrons would have been. The n-type layer contains extra electrons. The deletion layer where the two different layers are joined together acts as an insulator. This is what happens when the diode is connected in a circuit: if the positive terminal is connected to the n-type layer, holes and electrons move away from the depletion layer, making it wider and preventing current from flowing if the positive terminal is connected to the p-type layer, holes and electrons move into the depletion layer, making it disappear and allowing current to flow Refraction AC and DC AC is alternating current and DC is direct current. These oscilloscope traces show the difference between them. Current-voltage graph The graph shows how the current varies with voltage for a silicon diode. AC electricity is supplied by the mains supply. DC electricity is supplied by batteries and solar cells. Many devices need a DC supply rather than an AC supply. A rectifier changes AC into DC. The process is called rectification and it uses diodes. Half-wave rectification A single diode can produce half-wave rectification. One half of the AC wave is removed because it cannot pass through the diode. Full-wave rectification Half-wave rectification is achieved using one diode, but full-wave rectification needs four diodes in a bridge circuit. Note that in full-wave rectification current passes all the time, rather than just every half cycle. Full-wave rectification The diagrams below show the bridge circuit needed for full-wave rectification. The two diagrams show how the DC (direct current) output is only in one direction, even when the direction of the AC (alternating current) input is reversed in the second diagram. You should remember that one diode can produce half-wave rectification, and you should be able to recognise half-wave rectification from a voltage-time graph (as seen in the last diagram). Capacitors A capacitor is an electrical device that stores charge temporarily. As a capacitor discharges, the voltage across its plates falls, so the current in the circuit falls too. The lower the resistance of the circuit, the faster a capacitor will discharge. A simple capacitor can be made using two parallel metal plates separated by an insulating material, and this arrangement is reflected in the circuit symbol for a capacitor. You should be able to recognise and draw the circuit symbol for a capacitor. Smoothing circuits The DC (direct current) output from a rectifier is not smooth – it varies a lot with time. A capacitor with a high capacitance can be used to smooth the output so that it varies very little. This is important for some electronic devices, such as radios. Charging and discharging A capacitor is charged by connecting it to a DC (direct current) supply. As charge is stored, the voltage across the capacitor gradually increases to a maximum. The maximum voltage is the same as the voltage of the DC supply. Smoothing circuits As the voltage in the rectifier circuit increases, the capacitor stores charge. When the voltage begins to fall, the capacitor begins to discharge, keeping the DC more constant. When the voltage rises again, the capacitor begins to store charge again. This process repeats and keeps the DC supply smoother.