A diode is a part that lets current flow in only one direction. It is widely used for protection that guards a circuit even if power is connected backwards, for tidying signals, and for rectification that turns AC into DC, and the Zener diode adds the property of "holding the voltage at a set value" to make a reference voltage. This article covers the direction and forward voltage of ordinary diodes, their typical uses, and the resistor calculation for a Zener circuit.
Basic properties of an ordinary diode
A diode has two terminals, anode (A) and cathode (K), and current flows only from the anode toward the cathode. The band drawn on the body marks the cathode side.
- Forward: when the anode is higher than the cathode by a certain voltage or more, current flows. That voltage is the forward voltage (Vf).
- Reverse: applied the other way, almost nothing flows. But if it exceeds the reverse voltage it can withstand (the rating), it fails.
| Type | Forward voltage (approx.) | Characteristics |
|---|---|---|
| Silicon small-signal (1N4148, 1S1588) | 0.6–0.8 V | Fast switching, for signal processing |
| Rectifier (1N4001–1N4007) | 0.7–1.0 V | 1 A-class power rectification, reverse voltage 50–1000 V |
| Schottky | 0.2–0.4 V | Small voltage drop and fast, reverse voltage on the low side |
| LED | 1.8–3.4 V | Emits light (see LED resistor calculation) |
Actual values change with the current flowing, so check the datasheet for exact values.
Common uses
Reverse-polarity protection
If you put a diode in series at the power entry, current is blocked even if you connect the power backwards. In exchange, about 0.7 V is dropped even in the forward direction, so about 4.3 V is left in a 5 V circuit. If voltage headroom is short, use a Schottky. The power entry arrangement is in Power Rails, Grounding, and Decoupling Capacitor Placement and Making 5V from a 9V Battery.
Flyback (freewheeling) diode
Inductive loads such as relay coils and motors produce a high back-EMF when you cut the current. Attaching a diode in reverse, in parallel with the load to absorb this voltage protects the transistor. For details, see Driving Motors, Relays, and Solenoids.
Signal OR and clamping
Put a diode in series with each of two signal lines and join the outputs, and you get a simple OR circuit where the higher voltage comes out. They are also used as clamps that limit overvoltage by connecting diodes from an input line to the supply and ground.
Rectification
One diode (half-wave) or four (bridge) are used to turn AC into one-directional current. Unless you are building your own power adapter, it is hardly used at the beginner stage. AC mains carries a high risk of shock, so it is not covered within this article.
Zener diodes
A Zener diode is a diode that, when applied in reverse, conducts once it reaches a set voltage (the Zener voltage, Vz) and holds that voltage. In the forward direction it behaves like an ordinary diode. Typical products are below, and the 1 W 1N47xx series and the 0.5 W BZX55 series are widely used.
| Product | Zener voltage |
|---|---|
| 1N4728A | 3.3 V |
| 1N4732A | 4.7 V |
| 1N4733A | 5.1 V |
| 1N4735A | 6.2 V |
| 1N4742A | 12 V |
Resistor calculation for a Zener circuit
Always use a Zener with a series resistor. The resistor limits the current through the Zener and takes up the leftover voltage.
R = (input voltage - Vz) / (Zener minimum current + load current)
For example, to make 5.1 V from 12 V with a 10 mA load and a Zener minimum current of 5 mA, R = (12 - 5.1) / 0.015 = 460Ω, so use 470 Ω.
- Resistor heat:
(12 - 5.1)² / 470 ≈ 0.10W→ 1/4 W is enough. - In the worst case where the load is removed, the Zener sees
5.1V × 14.7mA ≈ 75mW, so a 0.5 W or larger part has plenty of room.
Using them in Patterndo
Patterndo has 1S1588 and 1N4148 diodes and a Zener diode (5.1 V by default, 1N4733), and you can change the Zener's voltage in its properties. If you connect a diode backwards, no current flows and it does not work, but no separate error item appears, so if the voltage and current in simulation differ from what you expect, check the anode and cathode direction first. For an electrolytic capacitor, a polarity error is shown in diagnostics if the polarity is reversed (Reading Diagnostics Messages).
Using them on perfboard
- Place the band in the same direction as in the layout diagram, and check once more before soldering.
- Glass-body diodes are weak to heat, so solder briefly, and when bending the leads, hold the glass part and do not force it.
- The 1N400x rectifier is slightly bigger, and often uses 4 holes at 2.54 mm spacing (about 10 mm).
Summary
For a diode, just remember "the banded side is the cathode, and current goes from anode to cathode". Use it in series for power protection and in reverse parallel for inductive loads, and a Zener is for making a reference voltage by limiting its current with a series resistor. Next, see load driving with diodes in Transistor Switch Circuits.