Diodes and LEDs: rectifier, Schottky, Zener, TVS and LED resistors
Diode types explained (rectifier, signal, Schottky, Zener, TVS/ESD, LED), forward voltage, reverse polarity protection, flyback diodes and how to calculate an LED series resistor.
One-way valves
A diode conducts from anode to cathode (the end with the stripe) and blocks the other way. When it conducts it drops a roughly constant forward voltage (Vf), and it dissipates Vf × I as heat.
The family
| Type | Vf | Used for |
|---|---|---|
| Rectifier (1N400x) | ≈ 0.7–1 V | Converting AC to DC, reverse-polarity protection |
| Signal (1N4148) | ≈ 0.7 V | Small, fast signals |
| Schottky | 0.2–0.45 V | Low loss in power paths; leaks more current, especially when hot |
| Zener | Breaks down at a set reverse voltage | Simple voltage references and clamps |
| TVS / ESD | Clamps fast surges | Protecting connectors and inputs from static and transients |
| LED | 1.8–3.3 V depending on colour | Light; needs current limiting |
Driving an LED
An LED does not limit its own current. Connected straight to a supply it burns out, or burns out the pin driving it. The series resistor sets the current: R = (Vsupply − Vf) / I. For an indicator, 2 to 5 mA is enough with modern LEDs.
Diodes as protection
- Reverse polarity: a series diode (or better, a MOSFET) stops damage when a battery or supply is connected backwards.
- Flyback: a diode across a relay coil or motor absorbs the voltage spike when it switches off. See the transistor lesson.
- ESD: TVS diodes at connectors take the hit from static discharge instead of the chip.
Input protection is where many products fail certification or come back from the field. Choosing TVS parts, reverse-polarity schemes and surge ratings for real cables and real users is part of our schematic and pre-compliance work.
Parts and tools to practise
| Item | What to look for | Buy |
|---|---|---|
| Diode assortment | 1N4148 signal, 1N400x rectifier, Schottky and Zener values. | View on Amazon ↗ |
| LED assortment | 3 mm and 5 mm in several colours, plus a few RGB. | View on Amazon ↗ |
| Resistor assortment, 1 % metal film | E24 or E12 values from 10 Ω to 1 MΩ, 1/4 W. Sorted in labelled bags or a box. | View on Amazon ↗ |
| Solderless breadboard kit with jumper wires | Full-size breadboard, pre-cut jumpers and flexible Dupont wires. | View on Amazon ↗ |
| Digital multimeter | Auto-range with continuity beeper, CAT III rated. You will use it every day. | View on Amazon ↗ |
Links go to Amazon. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. We only list tools of the kind we use on our own bench.
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