The diode and its symbol
A diode is a single PN junction with two leads. The P side is the anode and the N side is the cathode. It conducts easily one way and hardly at all the other, so it acts as an electrical one-way valve. Its symbol is a triangle against a bar. The triangle points the way conventional current (positive to negative) flows when the diode is forward biased, and the bar is the cathode. Electron flow runs the other way, from cathode to anode, "against the arrow". FAA texts describe circuits with electron flow and most European texts use conventional current; the diode behaves the same either way.
Identifying diodes
- Cathode marking: a painted band at the cathode end on axial diodes, a line on surface-mount parts, a printed symbol on larger types. On an LED, the cathode has the shorter leg and a flat on the rim.
- Stud-mounted power rectifiers screw into a heat sink. In standard polarity types the stud is the cathode; in reverse polarity types (usually suffix R) the stud is the anode. Fitting the wrong one reverses the output of a rectifier assembly.
- Type numbers: the American JEDEC system starts single-junction devices (diodes) with 1N (1N4001, 1N4148), transistors with 2N and four-terminal devices with 3N. The European Pro Electron system uses a first letter for the material (A = germanium, B = silicon) and a second letter for the function — for example BY rectifier, BZ Zener, BA small-signal diode.
- Always confirm with the component data sheet or the illustrated parts catalogue — a correct-looking part with the wrong rating will fail.
The diode characteristic
The V–I characteristic plots current against voltage and summarises everything a diode does.
- Forward region — very little current flows until the voltage approaches the barrier potential. Around the knee it starts to rise steeply, and beyond that the current is set by the circuit while the diode's voltage stays near 0.7 V (silicon) or 0.3 V (germanium). For quick calculations, treat a conducting silicon diode as a fixed 0.7 V drop.
- Reverse region — only a tiny leakage current flows, almost independent of voltage, until the breakdown voltage (VBR) is reached. Then current rises abruptly.
- Temperature shifts the forward curve to the left (about −2 mV/°C) and increases the leakage.
An ideal diode would be a perfect closed switch forward and a perfect open switch in reverse. A real silicon diode adds about 0.7 V of forward drop, a little internal (bulk) resistance and a little leakage. That is close enough to ideal that diodes are used as automatic switches throughout aircraft wiring.
Diode parameters and ratings
A diode's data sheet lists the limits it must be kept within. The ones Module 4 expects you to know:
| Parameter | Meaning |
|---|---|
| Peak inverse voltage (PIV) — also peak reverse voltage, VRRM | The highest reverse voltage that may be applied, repeatedly, without breakdown. It must exceed the highest reverse voltage the circuit can produce, with a safety margin. |
| Maximum forward current — IF(AV) | The average forward current the diode can carry continuously at a stated temperature. |
| Surge current — IFSM | A brief, non-repetitive overload the diode survives, such as the charging current of a smoothing capacitor at switch-on. |
| Forward voltage — VF | The drop at rated current — about 0.7–1.1 V for silicon rectifiers. |
| Leakage current — IR | Reverse current at the rated reverse voltage, quoted at 25 °C and at a high temperature because it rises steeply with heat. |
| Power dissipation — PD | Forward voltage × forward current, turned into heat in the junction. It must be removed (heat sink, airflow) and is derated as the ambient temperature rises. |
| Temperature — Tj max | The maximum junction temperature (about 150–200 °C for silicon). Storage and operating temperature ranges are also given. |
| Frequency / switching speed — trr | When the voltage reverses, stored charge lets current flow backwards for a short reverse recovery time. Ordinary rectifiers (microseconds) are fine at 50–400 Hz. Fast-recovery and Schottky diodes are used at high frequencies, and junction capacitance limits radio-frequency use. |
The 1N4001–1N4007 family are general-purpose 1 A silicon rectifiers. They differ only in PIV, from 50 V (1N4001) to 1 000 V (1N4007). A 1N4007 can replace a 1N4001, because it has the same current rating and a higher PIV. A 1N4001 cannot replace a 1N4007.
Diodes in series and in parallel
When no single diode is big enough, diodes are combined — but they never share perfectly, because no two are identical.
- In series, to block a higher reverse voltage. The diode with the lowest leakage would take most of the voltage and break down first. The fix is voltage-sharing (equalising) resistors of equal value across each diode, carrying several times the leakage current, often with small capacitors across each diode to share fast transients during reverse recovery.
- In parallel, to carry more current. The diode with the lowest forward voltage takes most of the current, heats up, its forward voltage falls further, and it hogs even more: a form of thermal runaway. The fix is a small series (ballast) resistor in each branch, ideally with matched diodes on a common heat sink. Usually a single diode of adequate rating is the better choice.
- Forward-biased diodes in series simply add their drops (two silicon diodes ≈ 1.4 V). This is sometimes used to make a small fixed reference voltage.
Rectifier diodes and rectifier circuits
Rectifier diodes are built for power: large junctions, high current, high PIV, and packages that bolt to heat sinks. Their job is to turn AC into DC.
- Half-wave — one diode passes only the positive half cycles. The output is a series of pulses at the supply frequency, averaging only about 32% of the peak (Vpk/π), with heavy ripple.
- Full-wave, centre-tapped — two diodes fed from the two ends of a centre-tapped secondary conduct on alternate half cycles. The output pulses at twice the supply frequency, averaging about 64% of the peak. But each diode must block about twice the peak voltage of its half-winding.
- Bridge — four diodes, no centre tap. On each half cycle two diodes conduct in series, so the output loses about 1.4 V. Output frequency is again twice the supply, and each diode blocks only the peak voltage.
A smoothing (reservoir) capacitor across the output charges to the peak and fills the gaps between pulses, often with an inductor and a voltage regulator after it. With a capacitor-input filter, a half-wave diode can see up to twice the peak voltage in reverse, so the PIV rating must allow for it.
A transformer rectifier unit (TRU) turns the 115/200 V, 400 Hz three-phase supply into 28 V DC. The transformer steps the voltage down, and a three-phase full-wave bridge of six diodes rectifies it. Each of the three phases contributes two pulses per cycle, so the ripple is at 6 × 400 = 2 400 Hz and very small, and little smoothing is needed. Brushless generators carry a rotating rectifier of diodes on the shaft to feed the main field. Diodes also appear as blocking (isolation) diodes, which let one busbar feed another but not back, and as flyback (suppression) diodes across relay and solenoid coils, which absorb the voltage spike when the coil is switched off.
Functional testing of diodes
A digital multimeter's diode-test function drives a small current through the part and displays the voltage across it.
- Red lead on the anode, black on the cathode (forward): a good silicon diode reads about 0.5–0.8 V. Germanium and Schottky diodes read about 0.2–0.4 V, and an LED reads 1.6–3 V and may glow faintly.
- Leads reversed: a good diode reads OL (over-limit: no conduction).
- Shorted diode: near 0 V both ways. Open diode: OL both ways. Leaky diode: a reading in the reverse direction where there should be none.
- With an analogue ohmmeter, a good diode shows a low forward and a very high reverse resistance. The actual numbers change with the range, because the diode is non-linear. Also remember that on many analogue meters the black lead is positive on the resistance ranges.
- Test with the power off and capacitors discharged, and preferably out of circuit. Parallel components give false readings, so lift one lead if in doubt.
Power diodes rely on their heat sink. Clean the mounting faces, use the specified thermal compound and torque, and fit insulating (mica) washers and bushes where the stud must be isolated from the heat sink. When soldering small diodes, keep the heat brief and use a heat-sink clip on the lead.