Now that we know how to step down AC voltages to a manageable level, we are left with the problem of converting 12VAC into our desired 5V DC supply. We’ll approach this in two steps: first we’ll convert this AC voltage into a DC one via a process called rectification. Then we’ll step this down to 5V using a regulator. Fortunately, you have already been exposed to the key component needed for recti- fication: the diode. Recall that diodes are two-terminal devices which allow current to pass only in one direction. Unlike LEDs, silicon diodes typically drop about 0.6 or 0.7V when conducting. But for our purposes in this chapter, we’ll tend to think of them as ideal, one-way current valves.
The simplest possible circuit for converting AC into DC is a half-wave rectifier. It basically consists of a single diode which only allows current flow in one direction. A possible circuit is shown below. Note the symbol for the transformer.
The operation of this circuit is straightforward. When VAC is in the positive part of the cycle, a positive voltage is produced on the secondary side of the transformer. This turns on the diode, allowing most of that voltage to be seen across the load. When VAC goes negative, the secondary side is also negative, and the diode is off. No current flows in the load during the negative half of the cycle. Hence the name half-wave rectifier. The figure below shows typical waveforms. Diodes used in these sorts of circuits are specially constructed to handle the large currents, and are often called rectifier diodes, or just rectifiers for short.
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Looking at the output voltage, you should be struck by how little it resembles the output of a battery. While this output voltage is now positive only, it certainly is very bumpy, and not even on half the time. What we need is a way to smooth things out - something which will store some of the energy during the on times, and release it during the off times. The ideal solution is a capacitor.
In the circuit below, we have added a large capacitor (labelled Cmoby, after the whale of enormous dimensions) which can store energy during the on times, and release it during the off times. What happens now is that the diode only turns on
when the voltage on the cap is about 0.7V below that coming out of the transformer. The transformer only charges the cap upwards - the diode insures this. Meanwhile, the load discharges the cap with our standard RC time constant. The circuit must be carefully designed so that the time constant is much longer than the AC cycle time. In any case, there will be some amount of ripple on the output as the cap discharges between pulses. The sketch below illustrates what happens.
There is something else new in this circuit. Notice how the bottom plate of the capacitor is shown with a curve, and the top is marked with a plus sign. That is
VAC Vout VAC Rload 1:n Vout Transformer Cmoby VAC Vout
because special capacitors are required to get a very high capacitance in a reason- able space. In particular, you will be using electrolytic capacitors which are con- structed using a paper soaked with an electrolyte. This gives enormous capacitances in a small volume, but also results in the cap being polarized - i.e. it only works with one polarity of voltage. If you reverse the polarity, hydrogen can dissociate from the internal anode, resulting in a nasty tendency to explode. Therefore, it is imperative that you guarantee via circuit design that electrolytic caps never see the wrong polarity. Electrolytic caps always have their polarity clearly marked, often with a bunch of minus signs pointing to the negative terminal. You should have a 1000µf electrolytic cap for use in power supply circuits. Caps have a maximum voltage rating which should never be exceeded.
(A side note on electrolytic caps: there are now specially constructed, nonpolarized electrolytic caps available. These are somewhat more expensive, but at least elimi- nate the problem of accidentally inserting caps backwards.)
While the half-wave rectifier has the virtue of simplicity, it lacks efficiency and ele- gance. A better solution would use the power available from the transformer on both sides of the cycle. Systems of this type are called full-wave recitifiers. In the days when transformers were cheaper than diodes (shortly after the Jurasic period), center-tapped transformer circuits using two diodes were particularly common. A center-tapped transformer adds a connection in the middle of the secondary coil. This allows you to treat it as two separate secondary coils, with a common middle connection. In the circuit shown below, each of these halves are used on opposite parts of the cycle, basically stringing two half-wave rectifiers together.
The output of this power supply has significantly less ripple because the capacitor is refreshed twice as often. If we were to remove the cap and look at the waveform, it would look something like the figure shown below. The AC waveform is said to have been rectified. However, this is hardly a constant voltage, and while the addi- tion of a cap helps cleans things up, there may still be a fair amount of ripple. The
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next figure shows how this is cleaned up by the capacitor. You should compare this with the ripple of the half-wave circuit.
In an era where diodes sell for pennies, a better solution is a full-wave bridge recti- fier. This particularly clever circuit uses four diodes to always turn the output volt- age the right direction and is shown in the figure below. If you examine this closely,
you will notice that which ever side of the transformer output is positive has a path through a diode to the positive side of the cap. Similarly, which ever side is negative has a path to the negative side of the cap. At any given moment, only two diodes on opposite sides are conducting, and this switches every half-cycle.
VAC Vout VAC Vout VAC Rload 1:n Cmoby Vout diode bridge