Diodes and LEDs

How a diode works

A diode lets current flow in one direction only, from anode to cathode (the arrow in the symbol points the way current flows; the bar marks the cathode, and the physical part has a stripe at that end). In the forward direction a diode conducts once the voltage across it reaches the forward voltage V_f, about 0.6–0.7 V for silicon. In reverse it blocks, until it reaches its breakdown voltage.

• Forward voltage drop means power is lost: P = V_f × I. A silicon diode carrying 1 A dissipates about 0.7 W.
• Reverse voltage rating (V_RRM) is the largest reverse voltage it will block. Choose one well above the highest voltage that can appear, including spikes.
• A diode is not a resistor. The forward voltage is nearly constant, so the current is set by the rest of the circuit. Always limit the current with a resistor or a driver.

Types of diode

Type	Example	Forward drop	Rating (typical)	Use
Signal	1N4148	0.6–0.7 V	100 V, 200 mA, very fast	Logic, clamping, small flyback
Rectifier	1N4001–1N4007	0.7–1.0 V	50–1 000 V, 1 A, slow	Mains and low-frequency rectification, flyback
Schottky	1N5819, SS14	0.2–0.45 V	20–100 V, 1–3 A, very fast	Switching supplies, reverse protection, low drop
Fast / ultrafast	UF4007, MUR-series	0.9–1.3 V	To 1 000 V, 50 ns recovery	High-frequency rectifiers, snubbers
Zener	BZX55 (2.4–75 V)	reverse regulating	0.5 W, 1 W	Reference, simple regulator, clamping
TVS	SMBJ-series	reverse clamping	5–400 V, 600 W pulses	Surge and ESD protection
LED	many	1.8–3.4 V	10–20 mA (small)	Light; see below
Photodiode	BPW34	—	Reverse-biased	Light sensing, receivers

A Schottky diode has a metal-semiconductor junction. It drops much less voltage and switches faster than a silicon diode, but leaks more in reverse and has a lower voltage rating.

Rectifiers and power supplies

Rectifying turns AC into pulsating DC. A half-wave rectifier is one diode and passes one half-cycle. A full-wave bridge uses four diodes so that every half-cycle goes the same way through the load; the current always passes through two diodes, so the loss is about 1.4 V. A filter capacitor then fills the gaps between the peaks.

V_DC ≈ V_RMS × 1.414 − 1.4 V    ripple ΔV ≈ I ÷ (2 × f × C) (full-wave)

The result is a rough DC supply; follow it with a regulator (see Batteries and Power Supplies). Mains-powered circuits can kill: use a ready-made, certified supply unless you are qualified.

Zener diodes and TVS

A Zener diode is designed to be used in reverse. Above its rated voltage it conducts and holds the voltage nearly constant, which makes a simple reference or a clamp. A TVS (transient voltage suppressor) is a Zener-like part built to absorb large short pulses: put one across a supply or a data line to clamp surges and static.

R_series = (V_in − V_Z) ÷ (I_Z + I_load)

LEDs

A light-emitting diode gives off light when current flows forward. The color sets the forward voltage; the brightness follows the current, not the voltage. The longer lead is the anode (+), and the flat side of the lens marks the cathode.

Color	Forward voltage (typical)	Notes
Infrared	1.2–1.6 V	Remote controls, sensors
Red, orange, yellow	1.8–2.2 V	Lowest forward voltage
Green	2.0–3.3 V	Varies widely with type
Blue, white, UV	2.8–3.4 V	Little headroom on a 3.3 V supply
RGB	per color	Three LEDs in one package; common anode or common cathode

• Maximum current for a 5 mm indicator LED is about 20 mA. For normal use, 5–10 mA is plenty bright, and it lasts longer.
• Brightness is not proportional to current. Perceived brightness is roughly logarithmic, so halving the current looks only slightly dimmer.
• Power LEDs (1 W and up) need a heat sink and a constant-current driver, not a resistor.

Choosing the LED resistor

R = (V_supply − V_f) ÷ I_LED

The table gives the calculated resistance for 10 mA, the nearest standard (E24) value, and the actual current that results.

LED (Vf)	3.3 V supply	5 V supply	12 V supply
Red (2.0 V)	130 Ω → 130 Ω (10.0 mA)	300 Ω → 300 Ω (10.0 mA)	1 kΩ → 1 kΩ (10.0 mA)
Green (2.2 V)	110 Ω → 110 Ω (10.0 mA)	280 Ω → 270 Ω (10.4 mA)	980 Ω → 1 kΩ (9.8 mA)
White / blue (3.2 V)	not enough headroom	180 Ω → 180 Ω (10.0 mA)	880 Ω → 910 Ω (9.7 mA)

• Blue and white on 3.3 V is fragile. With V_f = 3.0 V the resistor is only 30 Ω; because V_f varies from part to part (2.8–3.2 V), the current could be anywhere from 3.3 to 16.7 mA. Use a higher supply or a constant-current driver.
• Several LEDs in series need the supply to exceed the sum of the forward voltages, with one resistor: three red LEDs on 12 V need R = (12 − 6) ÷ 0.010 = 600 Ω → 620 Ω.
• LEDs in parallel should each have their own resistor, because they do not share current evenly.
• Power in the resistor: P = (V_supply − V_f) × I. For 12 V and a red LED at 10 mA, that is 0.1 W, so a ¼ W part is enough.

Driving LEDs well

• From a microcontroller pin: the pin supplies only 10–20 mA, so use one LED per pin through a resistor. For more, switch with a transistor (see Transistors).
• Dimming: use PWM (switch on and off fast, at least 200 Hz so it does not flicker). The color stays true, unlike lowering the current.
• LED strips and modules usually have the resistors built in and run at a fixed 5 V, 12 V or 24 V. Use a supply rated 20 % above the strip’s total current and check the wire gauge for long runs.
• Reverse polarity damages some LEDs: they tolerate only about 5 V in reverse. Mind the orientation.
• Addressable LEDs (WS2812, “NeoPixel”) contain a driver chip in each package, are chained on one data line and need a 5 V supply with decoupling and a small series resistor on the data line.

Related

• Electronics Fundamentals: the overview of this guide.
• Resistors: picking the standard value and power rating.
• Transistors: driving bigger loads.
• Inductors, Transformers and Relays: flyback diodes and transformers.

FAQ

Can I connect an LED straight to a battery?

Not safely. The LED’s current rises steeply once it turns on, so with no resistor it draws as much as the battery can give and burns out. A coin cell (CR2032) has enough internal resistance to limit the current, which is why some LED throwies work, but it is not good practice.

Why use a Schottky diode?

It drops less voltage than silicon (0.3 V against 0.7 V), so it wastes less power and leaves more voltage for the load, and it switches faster. It is the usual choice for reverse-polarity protection of low voltages and for the diode in a switching regulator.

What is the difference between a Zener and a TVS?

Both clamp voltage. A Zener is built to hold a voltage steadily and carry a modest current; a TVS is built to absorb a short, large energy pulse such as a static discharge or a motor spike. Use a TVS for protection and a Zener for reference.

Why does my LED glow faintly when it should be off?

A tiny leakage current, from a pull-up, a transistor’s off-state current or a long wire coupling mains noise, is enough to give a faint glow. Add a resistor in parallel with the LED, or a bleeder at the switch.

How do I tell which side of a diode is the cathode?

Look for the stripe on a rectifier or signal diode; on an LED, the flat side of the lens or the shorter lead is the cathode. Or use the diode-test range of a multimeter: it reads about 0.6 V with the red lead on the anode.

References

• Horowitz, P. and Hill, W., The Art of Electronics, 3rd ed., Cambridge University Press: chapter 1 (diodes) and chapter 2.
• Manufacturer datasheets for 1N4148, 1N4001–1N4007, 1N5819, BZX55 and SMBJ series (for example Vishay, onsemi, Diodes Inc.).
• LED manufacturers’ datasheets (for example Kingbright, Lite-On, Cree): forward voltage and current ratings by color.
• IEC 60063, preferred number series (E24) for resistor selection.

Last reviewed: October 2026. Values are typical: check the datasheets of the parts you use. Mains-powered rectifiers can kill: use certified supplies unless you are qualified.