Diodes
| Diodes | |
|---|---|
| What it does | Lets current flow in one direction only |
| Terminals | Anode and cathode; a band on the body marks the cathode |
| Forward voltage | About 0.7 V for silicon and 0.3 V for germanium; lower for Schottky types |
| Key ratings | Maximum forward current and peak inverse voltage (PIV) |
| Types | |
| Rectifier | Converts AC to DC |
| Zener | Holds a set voltage; regulators and references |
| LED | Gives off light |
| Schottky | Low forward voltage, fast switching |
| RF types | |
| PIN | RF switching and attenuators |
| Varactor | Capacitance that varies with voltage, for electronic tuning |
Diodes are semiconductor devices that let current flow easily in one direction and block it in the other, like a one-way valve. The basic diode converts alternating current into direct current, but specialized types regulate voltage, give off light, switch RF signals and tune circuits electronically. A ham station uses them everywhere: in power supplies, in the protection circuits of radios, in receivers and SWR meters, and on every front panel lit by LEDs.
How a diode works
A typical diode is a junction between two kinds of silicon: P-type, with a shortage of electrons, and N-type, with a surplus. Its two terminals are the anode (P side) and the cathode (N side), and the cathode is usually marked by a band on the body. When the anode is made more positive than the cathode, the diode is forward biased and conducts, once the voltage across it passes about 0.6 to 0.7 volts for silicon or about 0.3 volts for germanium. When the polarity is reversed, it is reverse biased and blocks current, until the voltage exceeds the diode's limit and it breaks down.
Two ratings matter most: the maximum forward current and the peak inverse voltage (PIV), the highest reverse voltage the diode can withstand. The familiar 1N4001 to 1N4007 family are all 1-ampere rectifiers, with PIV ratings from 50 volts for the 1N4001 to 1,000 volts for the 1N4007.
A diode does not respond in proportion to the voltage across it, and that nonlinearity is what lets it detect and mix signals. A diode connected across a relay coil, pointing against the supply, also absorbs the voltage spike the coil produces when it switches off.
Rectifiers and power supplies
Rectification is turning AC into pulsing DC.
- A half-wave rectifier is a single diode that 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.
On 60 Hz mains, a half-wave rectifier's output pulses at 60 Hz and a full-wave rectifier's at 120 Hz, which is easier to smooth with filter capacitors. The capacitor charges to the peak of the AC waveform, about 1.414 times its RMS value, less the diode drops (see RMS and peak values). A 12-volt RMS secondary feeding a bridge gives about 17 volts peak, or roughly 15.5 volts after the two diodes that conduct at any moment.
Many radios include a diode for reverse-polarity protection. It may be in series with the supply, costing a little voltage, or connected across the input so that it conducts and blows the fuse if the leads are reversed.
Zener diodes and LEDs
A Zener diode is designed to operate in reverse breakdown at a specified voltage, such as 5.1 or 9.1 volts. With a series resistor, it holds that voltage nearly constant, making a simple regulator or reference. Zeners also appear in overvoltage protection circuits.
A light-emitting diode (LED) gives off light when forward biased. Its forward voltage is higher than an ordinary silicon diode's, roughly 2 volts for a red LED and about 3 volts for white or blue. An LED needs a series resistor to limit its current: on a 13.8-volt supply, a red LED at 10 milliamps needs about (13.8 − 2) / 0.010, or roughly 1.2 kΩ. LEDs inside optocouplers pass keying and PTT signals between a computer and a radio while keeping the two electrically isolated, as in many digital mode interfaces.
RF diodes: Schottky, PIN and varactor
A Schottky diode uses a junction between metal and semiconductor. It has a low forward voltage and switches very quickly, which suits it to switching power supplies, low-loss battery and solar charging circuits, RF detectors in power meters, and the ring of four diodes in a double-balanced mixer.
A PIN diode has an undoped "intrinsic" layer between its P and N regions. At radio frequencies it behaves like a resistor whose value is set by the DC current through it: with current flowing, its resistance is low, and with reverse bias it is nearly an open circuit. PIN diodes switch RF paths and form variable attenuators. In transceivers and amplifiers they handle transmit-receive switching silently and much faster than a relay, which suits full break-in CW.
A varactor, or varicap, takes advantage of the capacitance of a reverse-biased junction. As the reverse voltage rises, the junction widens and the capacitance falls, giving a voltage-controlled capacitor. Varactors tune the voltage-controlled oscillators in frequency synthesizers, adjust electronically tuned filters, and can frequency-modulate an oscillator with audio.