AC Electrics - Transformers — Page 231, Lesson 252

I want to walk you through the next topic in AC Electrics: transformers and rectification. Let's start with a special type of transformer called an autotransformer. An autotransformer is different from the two-coil transformers we've looked at. Instead of having separate primary and secondary windings, it uses a single continuous winding with a tap somewhere along it. The key advantage is cost: autotransformers are less expensive than two-coil transformers because they use less wire. However, there's a critical limitation — they do not electrically isolate the primary and secondary windings. That means the input and output circuits are directly connected electrically. Because of this lack of isolation, autotransformers cannot be used in many circuits where safety or signal separation is required. Now, let's move into rectification of alternating current. A rectifier is a device that converts AC into DC. The most common type is the diode rectifier, which I'll describe in more detail. The operation of the diode rectifier is covered in the Semiconductor chapter, but for now, know that it's a very common device in modern aircraft solid-state circuits. A diode can serve two main purposes: it can convert AC to DC, or it can act as a blocking diode — think of it as an electrical non-return valve — to prevent reverse current flow in a DC system. There's also a special type of rectifier called a Zener diode. Zener diodes are designed to conduct at a predetermined voltage. So instead of just blocking or passing current, they start conducting only when the voltage across them reaches a specific threshold. How does a diode work electrically? A diode has a high resistance in one direction and a low resistance in the other. That's what makes it a one-way valve for current. The accepted symbol for a diode rectifier, and the direction of conventional current flow, is shown in Figure 14.4. Now let's look at half-wave rectification. If you insert a diode in the secondary circuit of a transformer, it will allow current to flow through the load in only one direction. This is called half-wave rectification. What happens to the AC waveform? The bottom half of the AC waveform is blocked — it gets cut off. The frequency of the output is the same as the input frequency. So if you put 50 Hz AC in, you get pulses of DC at 50 Hz out, but only the positive half of each cycle passes through. Figure 14.4 illustrates this single-phase half-wave rectification. So to summarise: autotransformers save wire but lack isolation; rectifiers convert AC to DC using diodes; and half-wave rectification blocks half the AC waveform, giving you a pulsed DC output at the same frequency as the input.