DIODE

    

difference between conductors  insulators

         If you know a bit about electricity, you'll know that materials fall broadly into two categories. There are some that let electricity flow through them fairly well, known as conductors, and others that barely let electricity flow at all, known as insulators.metals such as copper and gold are examples of good conductors, while plastics and wood are typical insulators.

         What's the difference between a conductor and an insulator? Solids are joined together when their atoms link up. In something like a plastic, the electrons in atoms are fully occupied binding atoms into molecules and holding the molecules together. They're not free to move about and conduct electricity. But in a conductor the atoms are bound together in a different kind of structure. In metals, for example, atoms form a crystalline structure (a bit like equal-sized marbles packed inside a box) and some of their electrons remain free to move throughout the whole material, carrying electricity as they go.

                      How semiconductors work

          Not everything falls so neatly into the two categories of conductor or insulator. Put a big enough voltage across any material and it will become a conductor, whether it's normally an insulator or not. That's how lightning works. When a cloud moves through the air picking up electric charge, it creates a massive voltage between itself and the ground. Eventually, the voltage is so big that the air between the cloud and the ground (which is normally an insulator) suddenly "breaks down" and becomes a conductor—and you get a massive zap of lightning as electricity flows through it.

          Certain elements found in the middle of the periodic table (the orderly grouping of chemical elements) are normally insulators, but we can turn them into conductors with a chemical process called doping. We call these       materials semiconductors and silicon and germanium are two of the best known examples. Silicon is normally an insulator, but if you add a few atoms of the element antimony, you effectively sprinkle in some extra electrons and give it the power to conduct electricity. Silicon altered in this way is called n-type (negative-type) because extra electrons (shown here as black blobs) can carry negative electric charge through it.

       In the same way, if you add atoms of boron, you effectively take away electrons from the silicon and leave behind "holes" where electrons should be. This type of silicon is called p-type (positive type) because the holes (shown here as white blobs) can move around and carry positive electric charge.

                         junction diode works

Interesting things happen when you start putting p-type and n-type silicon together. Suppose you join a piece of n-type silicon (with slightly too many electrons) to a piece of p-type silicon (with slightly too few). What will happen? Some of the extra electrons in the n-type will nip across the join (which is called a junction) into the holes in the p-type so, either side of the junction, we'll get normal silicon forming again with neither too many nor too few electrons in it. Since ordinary silicon doesn't conduct electricity, nor does this junction. Effectively it becomes a barrier between the n-type and p-type silicon and we call it a depletion zonebecause it contains no free electrons or holes:

    Suppose you connect a battery to this little p-type/n-type junction. What will happen? It depends which way the battery is connected. If you put it so that the battery's negative terminal joins the n-type silicon, and the battery's positive terminal joins the p-type silicon, the depletion zone shrinks drastically. Electrons and holes move across the junction in opposite directions and a current flows. This is called forward-bias:

     However, if you reverse the current, all that happens is that the depletion zone gets wider. All the holes push up toward one end, all the electrons push up to the other end, and no current flows at all. This is called reverse-bias:

     That's how an ordinary diode works and why it allows an electric current will flow through it only one way. Think of a diode as an electrical one-way street.transistors, incidentally, take the junction idea a step further by putting three different pieces of semiconducting material side by side instead of two.

That's how an ordinary diode works and why it allows an electric current will flow through it only one way. Think of a diode as an electrical one-way street. Transistors , incidentally, take the junction idea a step further by putting three different pieces of semiconducting material side by side instead of two.

                                  RECTIFIER OUTPUTr

       While half-wave and full-wave rectification can deliver unidirectional current, neither produces a constant voltage. Producing steady DC from a rectified AC supply requires a smoothing circuit or filter. In its simplest form this can be just a reservoir capacitor or smoothing capacitor, placed at the DC output of the rectifier. There is still an AC ripple voltage component at the power supply frequency for a half-wave rectifier, twice that for full-wave, where the voltage is not completely smoothed.

     Sizing of the capacitor represents a tradeoff. For a given load, a larger capacitor reduces ripple but costs more and creates higher peak currents in the transformer secondary and in the supply that feeds it. The peak current is set in principle by the rate of rise of the supply voltage on the rising edge of the incoming sine-wave, but in practice it is reduced by the resistance of the transformer windings. In extreme cases where many rectifiers are loaded onto a power distribution circuit, peak currents may cause difficulty in maintaining a correctly shaped sinusoidal voltage on the ac supply.

     To limit ripple to a specified value the required capacitor size is proportional to the load current and inversely proportional to the supply frequency and the number of output peaks of the rectifier per input cycle. The load current and the supply frequency are generally outside the control of the designer of the rectifier system but the number of peaks per input cycle can be affected by the choice of rectifier design.

     A half-wave rectifier only gives one peak per cycle, and for this and other reasons is only used in very small power supplies. A full wave rectifier achieves two peaks per cycle, the best possible with a single-phase input. For three-phase inputs a three-phase bridge gives six peaks per cycle. Higher numbers of peaks can be achieved by using transformer networks placed before the rectifier to convert to a higher phase order.

      To further reduce ripple, a capacitor input filter can be used. This complements the reservoir capacitor with a choke (inductor) and a second filter capacitor, so that a steadier DC output can be obtained across the terminals of the filter capacitor. The choke presents a high impedance to the ripple current. For use at power-line frequencies inductors require CORES of iron or other magnetic materials, and add weight and size. Their use in power supplies for electronic equipment has therefore dwindled in favour of semiconductor circuits such as voltage regulators.

      A more usual alternative to a filter, and essential if the DC load requires very low ripple voltage, is to follow the reservoir capacitor with an active voltage regulator circuit. The reservoir capacitor must be large enough to prevent the troughs of the ripple dropping below the minimum voltage required by the regulator to produce the required output voltage. The regulator serves both to significantly reduce the ripple and to deal with variations in supply and load characteristics. It would be possible to use a smaller reservoir capacitor (these can be large on high-current power supplies) and then apply some filtering as well as the regulator, but this is not a common strategy. The extreme of this approach is to dispense with the reservoir capacitor altogether and put the rectified waveform straight into a choke-input filter. The advantage of this circuit is that the current waveform is smoother and consequently the rectifier no longer has to deal with the current as a large current pulse, but instead the current delivery is spread over the entire cycle. The disadvantage, apart from extra size and weight, is that the voltage output is much lower – approximately the average of an AC half-cycle rather than the peak.