Power transformers can be categorized as step-down or step-up. As you remember, the output, or secondary, voltage of a step-down unit is lower than the input, or primary, voltage. The reverse is true for a step-up transformer.
Most solid-state electronic devices, such as radios, need only a few volts. The power supplies for such equipment use step-down power transformers. The physical size of the transformer depends on the current. Some devices need only a small current and a low voltage. The transformer in a radio receiver, for example, can be physically small. A ham radio transmitter or hi-fi amplifier needs more current. This means that the secondary winding of the transformer must consist of heavy-gauge wire, and the core must be bulky to contain the magnetic flux.
Some circuits need high voltage. The cathode-ray tube (CRT) in a conventional home television set needs several hundred volts. Some ham radio power amplifiers use vacuum tubes working at more than 1 kV dc. The transformers in these appliances are step-up types. They are moderate to large in size, because of the number of turns in the secondary, and also because high voltages can spark, or arc, between wire turns if the windings are too tight. If a step-up transformer needs to supply only a small amount of current, it need not be big. But for ham radio transmitters and radio or television broadcast amplifiers, the transformers are large, heavy, and expensive. Transformer Ratings Transformers are rated according to output voltage and current. For a given unit, the volt-ampere (VA) capacity is often specified. This is the product of the voltage and current.
A transformer with 12-V output, capable of delivering 10 A, has 12 V × 10 A = 120 VA of capacity. The nature of power-supply filtering, to be discussed later in this further topics, makes it necessary for the power-transformer VA rating to be greater than the wattage consumed by the load. A high-quality, rugged power transformer, capable of providing the necessary currents and/or voltages, is crucial in any power supply. The transformer is usually the most expensive component to replace.
Rectifier diodes are available in various sizes, intended for different purposes. Most rectifier diodes are made of silicon, and are known as silicon rectifiers. Some are fabricated from selenium, and are called selenium rectifiers. Two important features of a power-supply diode are the average forward current (Io) rating and the peak inverse voltage (PIV) rating.
Average Forward Current
Electric current produces heat. If the current through a diode is too great, the heat will destroy the P-N junction. When designing a power supply, it is wise to use diodes with an Io rating of at least 1.5 times the expected average dc forward current. If this current is 4.0 A, for example, the rectifier diodes should be rated at Io = 6.0 A or more.
Note that Io flows through the diodes. The current drawn by the load is often different from this. Also, note that Io is an average figure. The instantaneous forward current is another thing, and can be 15 or 20 times the Io, depending on the nature of the filtering circuit. Some diodes have heatsinks to help carry heat away from the P-N junction. A selenium diode can be recognized by the appearance of its heatsink, which looks something like a baseboard radiator built around a steam pipe.
Diodes can be connected in parallel to increase the current rating over that of an individual diode. When this is done, small-value resistors should be placed in series with each diode in the set to equalize the current. Each resistor should have a value such that the voltage drop across it is about 1 V under normal operating conditions.
Peak Inverse Voltage
The PIV rating of a diode is the instantaneous reverse-bias voltage that it can withstand without the avalanche effect taking place. A good power supply has diodes whose PIV ratings are significantly greater than the peak ac input voltage. If the PIV rating is not great enough, the diode or diodes in a supply conduct for part of the reverse cycle. This degrades the efficiency of the supply because the reverse current bucks the forward current.
Diodes can be connected in series to get a higher PIV capacity than a single diode alone. This scheme is sometimes seen in high-voltage supplies, such as those needed for tube-type power amplifiers. High-value resistors, of about 500 Ω for each peak-inverse volt, are placed across each diode in the set to distribute the reverse bias equally among the diodes. In addition, each diode is shunted by (that is, connected in parallel with) a capacitor of 0.005 μF or 0.1 μF.