Basics
The simplest way to think about an ordinary diode is as a one-way electrical valve. When its anode is sufficiently positive relative to its cathode, it is forward biased and conducts. When the polarity is reversed, it is reverse biased and normally conducts only a tiny leakage current.
The diode symbol
In the schematic symbol, the cathode is the side marked by the vertical bar. On many cylindrical diodes, a painted band on the body marks the same cathode end.
Forward voltage
A silicon PN diode does not suddenly switch from perfectly off to perfectly on, but in beginner calculations it is often approximated as having about 0.6 to 0.7 V across it while carrying ordinary forward current. The actual voltage depends on current, temperature, construction, and the individual part.
| Device | Very rough forward-voltage territory | Comment |
|---|---|---|
| Silicon PN diode | About 0.6–0.8 V in many ordinary circuits | Varies strongly with current and temperature. |
| Schottky diode | Often about 0.2–0.5 V | Usually lower drop, but greater leakage. |
| Red LED | Often around 1.6–2.2 V | Depends on chemistry, current, and temperature. |
| Blue/white LED | Often around 2.7–3.5 V | White LEDs normally use a blue-emitting junction plus phosphor. |
Reverse voltage
An ordinary diode can block reverse voltage only up to a limit. If that limit is exceeded, the junction enters breakdown. A normal rectifier may be damaged by excessive breakdown current. Zener and avalanche diodes are designed to use a controlled form of reverse breakdown.
Current limiting
A diode does not usually limit its own forward current to a safe value. The external circuit must do that.
R = (5 - 2) / 0.010 = 300 Ω
A nearby standard value such as 330 Ω would give somewhat less current.
Testing a diode with a multimeter
Most digital multimeters have a diode-test range. With the red lead on the anode and black lead on the cathode, a good silicon diode often displays its forward voltage, perhaps around 0.5 to 0.8 V. Reversing the leads should normally show over-range or no conduction.
- Remove power from the circuit.
- Discharge capacitors safely.
- Select diode-test mode.
- Measure in one direction, then reverse the meter leads.
- If surrounding circuitry affects the reading, lift one diode lead.
Half-wave rectification
A single diode in series with an AC source allows approximately one half-cycle to reach the load while blocking the opposite half-cycle. This produces pulsating DC.
Protection against reverse polarity
A diode can protect equipment from a reversed DC supply. A simple series diode blocks the wrong polarity but introduces forward voltage drop. Other circuits place a diode across the supply so a fuse or current-limited source responds when polarity is reversed. MOSFET-based protection is often used when low loss is important.
Flyback diode
When current through a relay or solenoid coil is switched off, the collapsing magnetic field can create a large voltage. A diode placed across a DC coil, reverse biased during normal operation, gives the current a safe path as the field collapses.
Temperature
The forward voltage of an ordinary silicon junction generally decreases as junction temperature rises at a fixed moderate current. Reverse leakage generally rises with temperature. These effects matter in precision circuits, power devices, and thermal design.