AC and DC
| AC and DC | |
|---|---|
| DC | Direct current: flows in one direction only |
| AC | Alternating current: reverses direction in a repeating cycle |
| Unit of frequency | Hertz (Hz), cycles per second |
| US household power | 120 V at 60 Hz (240 V for large appliances) |
| In the shack | |
| Typical DC sources | Batteries, solar panels, 13.8 V power supplies |
| Typical AC signals | Household power, audio, radio frequency |
| AC to DC | Rectifier, filter and regulator |
| DC to AC | Inverter or oscillator |
AC and DC are the two basic forms of electric current. Direct current (DC) flows steadily in one direction, as it does from a battery. Alternating current (AC) reverses direction over and over, as it does from a wall outlet. Ham radio uses both all the time: transceivers run on DC, while household power, audio and radio signals themselves are all AC. The difference between them, and how one is turned into the other, is part of the basic electronics on the Technician exam.
Direct current
DC has a fixed polarity. One terminal is always positive and the other negative, and current flows around the circuit in one direction. Batteries, solar panels and the power supplies that run most transceivers all deliver DC. Most amateur equipment is designed for about 13.8 volts DC, the voltage of a car's electrical system with the engine running. Polarity matters: connecting a radio backward can blow its fuse or damage it, which is why DC leads are color-coded, red for positive and black for negative.
Alternating current
AC changes direction in a repeating pattern. One complete repetition is a cycle, and the number of cycles each second is the frequency, measured in hertz (Hz). The time one cycle takes is the period:
period = 1 / frequency
US household power is 60 Hz, so each cycle lasts 1/60 of a second, about 16.7 milliseconds; much of the rest of the world uses 50 Hz. Audio signals run from roughly 20 Hz to 20,000 Hz. Radio signals are AC at far higher frequencies: a signal on 7.074 MHz completes more than 7 million cycles every second. How frequency relates to the length of a radio wave is covered under Wavelength and frequency.
AC won out for electric power distribution in the late 19th century largely because transformers can raise and lower its voltage easily. Power travels long distances at high voltage, which wastes less energy in the wires, and is stepped down near homes. A transformer does not work on steady DC.
Sine waves
The natural shape of AC is the sine wave, a smooth curve that rises to a positive peak, falls through zero to an equal negative peak, and returns. A pure sine wave contains just one frequency. Other shapes, such as square and sawtooth waves, are made of a fundamental frequency plus harmonics, whole-number multiples of it. A transmitter must keep its output close to a pure sine wave so that it does not radiate on harmonic frequencies; see Harmonics and spurious emissions.
Points within a cycle are measured in degrees, 360 to a full cycle. Two waves of the same frequency that are shifted in time relative to each other are said to differ in phase; two waves 180 degrees apart are exact opposites.
Because an AC voltage is always changing, it can be described several ways. Household "120 volts" is an RMS value, and the actual peaks reach about 170 volts. See RMS and peak values.
Rectification
Turning AC into DC begins with rectification, using diodes, which let current through in only one direction.
- A half-wave rectifier uses one diode and passes only one half of each cycle.
- A full-wave rectifier uses both halves, either with two diodes and a center-tapped transformer or with four diodes in a bridge arrangement.
The result is pulsating DC, which still rises and falls. A large filter capacitor charges on each peak and supplies current between them, smoothing the pulses. What remains is called ripple. On 60 Hz power, a half-wave rectifier gives ripple at 60 Hz and a full-wave rectifier at 120 Hz, which is easier to filter. Ripple that reaches a transmitter is heard as hum on the signal.
Power supplies
A traditional linear power supply chains these steps together: a transformer lowers the voltage, a rectifier converts it to pulsating DC, filter capacitors smooth it, and a regulator holds the output steady as the load changes. A switching supply rectifies the line voltage first and then switches it on and off at a high frequency, which allows a far smaller transformer and a lighter, more efficient supply.
Conversion also runs the other way. An inverter turns DC from a battery into AC to run household equipment in the field, and an oscillator is, in effect, a circuit that turns DC into AC at a chosen frequency.
Household AC can kill, and the filter capacitors in a supply can hold a dangerous charge after it is unplugged. Many supplies include a bleeder resistor across the capacitors to drain that charge when the power is off.