LED Symbol
Definition: The LED (Light-Emitting Diode) symbol represents a polarised semiconductor device that emits light when forward-biased current flows from its anode to its cathode, drawn as a filled triangle pointing toward a vertical bar (identical to the standard diode symbol) with two short arrows angled outward from the triangle-bar junction to indicate light emission, per IEC 60617 and ANSI Y32.2 / IEEE 315-1975; it is assigned the schematic designator D or LED.
Also known as: LED symbol, light-emitting diode symbol, LED diode symbol, D symbol LED schematic, light emitting diode circuit symbol, LED indicator symbol.
What the LED symbol means
The LED symbol denotes a diode that converts electrical energy directly into light through electroluminescence. Like all diodes, it conducts current in only one direction — from anode to cathode when forward-biased — but the LED's defining characteristic is that this forward current causes the semiconductor junction to emit photons. The two arrows on the symbol represent emitted light rays, immediately distinguishing the LED from a standard rectifier diode on any schematic.
The polarity of the LED symbol is critical: the anode (pin id 'anode', marked '+') must be connected to the higher potential, and the cathode (pin id 'cathode', marked '-') to the lower potential for the device to emit light. Reverse-biasing the LED (cathode positive, anode negative) blocks current and produces no light; applying excessive reverse voltage will destroy the device. LEDs require a current-limiting resistor or constant-current driver in series to prevent overcurrent failure, because their forward voltage is nearly fixed while current rises steeply with small voltage increases.
How to identify the LED symbol
The LED symbol is drawn as a triangle (pointing rightward toward the cathode) with its right vertex touching a short vertical bar — identical to the basic diode symbol. Two short arrows (lines with arrowheads) extend diagonally outward from the triangle-bar junction, pointing away from the symbol body at roughly 45° angles, representing the light emitted by the device. These two emission arrows are the only feature that distinguishes the LED symbol from a plain rectifier diode symbol. The anode lead enters from the left base of the triangle; the cathode lead exits from the right of the vertical bar. The symbol is always drawn horizontally by convention but may be rotated 90° or 180° on a schematic — the triangle always points in the direction of conventional current flow (anode to cathode).
Function in a circuit
An LED is a semiconductor p-n junction diode that emits light when forward current flows through it. When the anode voltage exceeds the cathode voltage by the forward voltage threshold (V_F, typically 1.8 V for red, 2.0–2.2 V for yellow/green, 3.0–3.5 V for blue/white), the junction becomes forward-biased, carriers recombine across the junction, and photons are emitted at a wavelength determined by the semiconductor bandgap energy. The LED is a current-controlled device: light output (luminous intensity) is proportional to forward current (I_F), not forward voltage. Because V_F varies little with current, a current-limiting resistor or constant-current driver is always required to prevent thermal runaway and destruction.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 defines the LED symbol as the standard diode triangle-and-bar symbol with two arrows indicating light emission, pointing outward from the junction. The IEC symbol for a light-emitting diode is part of the semiconductor device family in IEC 60617-05. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2-1975 / IEEE 315-1975 defines the LED symbol identically: a triangle pointing toward a vertical bar, with two arrows indicating emitted light. The designator D (for diode) or LED is used; components may be labelled D1, D2 or LED1, LED2 depending on the designer's convention. |
| Key difference | The LED symbol is identical in IEC 60617 and ANSI Y32.2 / IEEE 315-1975. Both standards use the diode triangle-bar glyph with two outward arrows. No visual difference exists between IEC and ANSI LED symbols. |
Terminals / pins
| Pin | Name |
|---|---|
| anode | Anode (+) |
| cathode | Cathode (-) |
Typical values
LED forward voltage (V_F) varies by colour and semiconductor material: red (AlGaAs) ≈ 1.8–2.2 V; orange/yellow (AlGaInP) ≈ 2.0–2.4 V; green (standard GaP) ≈ 2.0–2.4 V; high-brightness green (InGaN) ≈ 3.0–3.5 V; blue and white (InGaN) ≈ 3.0–3.5 V; ultraviolet ≈ 3.5–4.0 V. Typical forward current (I_F) for indicator LEDs is 5–20 mA; high-power LEDs operate at 350 mA, 700 mA, 1 A, or higher. The current-limiting resistor value is calculated as R = (V_supply − V_F) / I_F. Maximum reverse voltage (V_R) for most signal LEDs is 5 V; exceeding this destroys the device.
Where the LED symbol is used
- Power and status indicator LEDs on PCBs, panels, and consumer electronics (e.g. power-on LED, charging indicator, error warning)
- Optocoupler / optoisolator circuits where the LED provides galvanic isolation between a control signal and a high-voltage output stage
- Infrared (IR) LED transmitters in remote controls, proximity sensors, and IR communication links
- Seven-segment and alphanumeric LED displays for numeric readouts on instruments, clocks, and meters
- RGB LED modules in lighting control systems, where separate red, green, and blue LED dice are driven at individually controlled currents to mix any colour
- LED backlights in LCD screens, driven by constant-current PWM circuits to control brightness
- Automotive dashboard and exterior lighting, where high-brightness LEDs replace incandescent bulbs for efficiency and longevity
Example
In a 5 V microcontroller GPIO output circuit, a 330 Ω current-limiting resistor (R1) is placed in series between the GPIO pin (5 V logic HIGH) and the anode of a red LED (D1, V_F = 2.0 V). The LED cathode connects to GND. The LED symbol with its two outward arrows identifies D1 as a light emitter; the resistor limits current to (5 V − 2.0 V) / 330 Ω ≈ 9 mA — well within the LED's 20 mA maximum, giving stable light output without risk of damage.
Key facts
- The LED symbol is the standard diode symbol (triangle pointing to a bar) with two short outward arrows representing emitted light — the arrows are the sole visual distinction from a plain rectifier diode symbol.
- The LED is polarised: the anode (pin id 'anode', marked '+') must be at higher potential than the cathode (pin id 'cathode', marked '-') for the LED to emit light; reverse polarity blocks current and produces no light.
- The schematic designator for an LED is D (for diode, per IEEE 315-1975) or LED; LEDs are labelled D1, D2 or LED1, LED2 depending on the designer's convention.
- LED forward voltage (V_F) is colour-dependent: red ≈ 1.8–2.2 V, green/yellow ≈ 2.0–2.4 V, blue/white ≈ 3.0–3.5 V; LEDs are current-controlled devices requiring a series resistor or constant-current driver.
- LEDs are highly efficient light sources: they convert electrical energy directly into light (electroluminescence) with far higher efficiency than incandescent bulbs, producing minimal heat per lumen.
- The LED symbol is identical in IEC 60617 and ANSI Y32.2 / IEEE 315-1975 — no visual difference exists between the two standards for this symbol.
- The wavelength (colour) of light emitted by an LED is determined by the semiconductor bandgap energy, which is fixed by the material: InGaN for blue/green/white, AlGaInP for red/orange/yellow.
- A forward-biased LED can also function as a photodetector (light sensor) in reverse: when illuminated, it generates a small reverse current, a property exploited in some optical circuits.
Diagrams that use this symbol
- electrical connection diagram
- series and parallel circuit diagram
- series circuit diagram
- series connection diagram
- electrical circuit diagram
- electric circuit diagram
- electrical wiring diagram
- electrical diagram
Frequently asked questions
What does the LED symbol look like in a circuit diagram?
The LED symbol looks like a triangle pointing rightward toward a vertical bar, with two short arrows extending diagonally outward from the triangle-bar junction. The triangle represents the anode (positive terminal) and the bar represents the cathode (negative terminal). The two arrows are what distinguish the LED symbol from a plain diode symbol — they represent the light emitted by the device.
What does the LED symbol mean in a circuit?
The LED symbol means that component emits light when current flows through it from anode to cathode. It is a polarised, one-way current device: current and light emission only occur when the anode ('+') is at a higher voltage than the cathode ('−') by at least the LED's forward voltage (V_F). The two arrows on the symbol indicate that the device produces light, distinguishing it from other diode types.
How do I identify the anode and cathode on the LED symbol?
On the LED symbol, the anode (positive terminal, pin id 'anode') is the left lead entering the base of the triangle, and the cathode (negative terminal, pin id 'cathode') is the right lead exiting the vertical bar. Current flows from anode to cathode — in the same direction the triangle points. On a physical LED, the cathode is typically the shorter lead and is next to the flat edge on the lens rim.
What is the difference between the LED symbol and the diode symbol?
The LED symbol and the diode symbol share the same triangle-pointing-to-bar glyph. The only difference is that the LED symbol has two short arrows extending outward from the junction — representing emitted light. A plain rectifier diode has no arrows. If you see the triangle-bar symbol without arrows, it is a standard diode; with arrows, it is an LED or another light-emitting semiconductor (photodiode symbols also use arrows, but pointing inward to represent received light).
Does the LED symbol look different in IEC vs ANSI schematics?
No. The LED symbol is identical in IEC 60617 and ANSI Y32.2 / IEEE 315-1975 — a triangle-bar diode symbol with two outward arrows. This is one of the few schematic symbols that is completely standardised across both conventions with no visual variation.
What resistor value do I need with an LED?
The current-limiting resistor value is calculated as R = (V_supply − V_F) / I_F, where V_F is the LED forward voltage (≈ 2.0 V for red, ≈ 3.3 V for blue/white) and I_F is the desired forward current (typically 5–20 mA for indicator LEDs). For a 5 V supply with a red LED (V_F = 2.0 V) at 10 mA: R = (5 − 2.0) / 0.010 = 300 Ω; the nearest standard E12 value is 330 Ω.
What standard defines the LED symbol?
The LED symbol is defined in IEC 60617-05 (semiconductor device symbols, International Electrotechnical Commission) and ANSI Y32.2-1975 / IEEE 315-1975 (North America). Both standards define an identical glyph: the standard diode triangle-and-bar with two outward emission arrows.
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