Lamp / Light Bulb Symbol

The Lamp / Light Bulb symbol (IEC 60617 / ANSI Y32.2).

Definition: The Lamp / Light Bulb symbol represents a two-terminal light-emitting load (Terminals A and B, non-polarised), drawn per IEC 60617 as a circle with an inscribed X (diagonal cross) and per ANSI Y32.2 / IEEE 315 as a circle containing a looped filament.

Also known as: light bulb, bulb, lamp, filament lamp, incandescent lamp, indicator lamp, signal lamp, GCSE bulb symbol.

What the Lamp / Light Bulb symbol means

The lamp symbol denotes a device that converts electrical energy into light — classically an incandescent filament bulb, but in modern schematics the same symbol is used generically for any small indicator or illumination lamp where the light-source technology is not the point of the drawing. It is a pure load: current flowing through the two terminals produces light, and the symbol carries no polarity, so Terminal A and Terminal B are interchangeable (unlike the LED symbol, which has a defined anode and cathode).

In circuit analysis the lamp behaves as a resistive load, but a filament lamp is famously non-ohmic: the tungsten filament's resistance rises steeply with temperature, so a cold lamp draws an inrush current roughly 10 to 15 times its steady-state operating current. This is why the lamp's I-V curve — a curve that flattens as voltage rises — is a standard experiment in GCSE and A-level physics, and why filament lamps most often fail at the moment of switch-on.

How to identify the Lamp / Light Bulb symbol

In IEC 60617 the lamp is a circle with an X drawn corner-to-corner inside it — two diagonal lines crossing at the centre, with the two terminal leads leaving horizontally. This is the form taught in UK GCSE/KS3 physics and used across European schematics. A signal or indicator lamp variant fills or shades the crossed segments.

ANSI Y32.2 / IEEE 315 schematics instead draw a circle containing a small looped or zigzag filament (resembling the actual coiled tungsten wire), a form still common in North American automotive and appliance diagrams. Both forms are two-terminal and symmetric. Take care not to confuse the IEC lamp (circle with X) with the IEC motor (circle with the letter M) or a meter (circle with a letter such as A, V, or G).

Function in a circuit

In a circuit, the lamp is a load that limits and consumes current while producing light and heat. In series circuits, lamps share the supply voltage, so each glows dimmer as more are added; in parallel circuits each lamp receives the full supply voltage and glows at rated brightness independently — the core comparison exercise in introductory electronics and the reason household lighting is wired in parallel.

As an indicator, a lamp across a supply or load provides visual confirmation that power is present: pilot lamps on control panels, dashboard warning bulbs, and mains-presence indicators all use this symbol. Because a filament lamp's cold resistance is very low, designers must account for inrush current when the lamp is switched by small relay contacts or transistors.

Standards: IEC vs ANSI

IEC 60617	IEC 60617 (database symbol S00913 and related) defines the lamp as a circle with an inscribed diagonal cross; variants distinguish signal lamps and lamps with integrated indicators. IEC 60064 covers tungsten filament lamp performance; IEC 60432 covers safety.
ANSI/IEEE 315	ANSI Y32.2 / IEEE 315 section 12 defines the incandescent lamp as a circle enclosing a looped filament; a circle with an X is also recognised for indicating/pilot lights in North American industrial (NEMA/JIC) ladder diagrams, usually with a colour letter (R, G, A) inside.
Key difference	IEC uses the circle-with-X for all general lamps; ANSI traditionally shows the filament loop for incandescent bulbs and reserves the circle-with-letter form for panel pilot lights. Both are two-terminal, non-polarised, and functionally identical — the drawing style is the only real difference.

Terminals / pins

Pin	Name
a	A
b	B

Typical values

Common small-lamp ratings: 1.5 V, 2.5 V, 3.5 V, 6 V and 12 V flashlight/MES (E10) bulbs at 0.1–0.5 A; automotive bulbs at 12 V from 2 W (instrument) to 60 W (headlamp); mains GLS filament lamps at 230 V/120 V in 25–100 W ratings (now largely replaced by LED retrofits). Filament cold resistance is roughly 1/10 to 1/15 of hot resistance, giving inrush currents of 10–15× steady state. Typical incandescent efficacy is only 10–17 lm/W with about 90% of input power lost as heat.

Where the Lamp / Light Bulb symbol is used

GCSE, KS3 and class 6–10 physics circuit diagrams demonstrating series and parallel circuits, brightness comparison, and I-V characteristics
Battery-and-bulb educational kits and science-fair projects using 1.5–6 V MES screw or bayonet lamps
Automotive wiring diagrams for headlamps, tail lamps, indicator (turn signal) and instrument-cluster bulbs
Control-panel pilot and indicator lamps showing motor-running, power-on, or fault states in industrial ladder diagrams
Appliance schematics for oven, refrigerator and range-hood interior lamps
Torch (flashlight), lantern and decorative string-light circuits

Example

In a simple two-cell torch circuit, the Lamp symbol's Terminal A connects to the positive terminal of a 3 V battery pack through an SPST slide switch and Terminal B returns directly to the battery negative; the 3 V 0.3 A MES bulb draws about 0.9 W, and because the symbol is non-polarised the A and B connections could be swapped with no change in operation.

Key facts

The IEC 60617 lamp symbol is a circle with a diagonal cross (X) inside; the ANSI/IEEE 315 incandescent form is a circle with a looped filament.
The symbol has two non-polarised terminals (A, B) — a lamp works either way round, unlike an LED.
A filament lamp is non-ohmic: resistance increases with filament temperature, producing the curved I-V characteristic studied in GCSE physics.
Cold-filament inrush current is typically 10–15 times the steady operating current, which is why filament bulbs usually blow at switch-on.
Lamps in series share the supply voltage and dim as more are added; lamps in parallel each see full supply voltage — the reason building lighting is parallel-wired.
IEC reference designators: E for lamps used as luminaires, H (or HL) for signal/indicator lamps; ANSI ladder diagrams commonly use PL or LT for pilot lights.
Incandescent lamps convert roughly 90% of electrical input to heat, with luminous efficacy around 10–17 lm/W versus 80–150+ lm/W for LEDs.
The same generic circle symbol is often used for a bulb of any technology when the schematic only needs to show 'a light' rather than a specific device.

Frequently asked questions

How do you draw the light bulb symbol in a circuit diagram for GCSE physics?

Draw a circle and place an X inside it — two straight lines crossing corner-to-corner — then bring one connecting wire out of each side of the circle. This is the standard IEC lamp symbol used on GCSE, KS3 and IGCSE exam papers. Do not confuse it with the motor symbol (circle with the letter M) or an ammeter (circle with the letter A), and remember examiners expect the cross to touch the circle's edge.

What is the difference between the lamp symbol and the LED symbol?

The lamp symbol is a circle with an X (IEC) or a filament loop (ANSI) and has no polarity — either terminal can go to positive. The LED symbol is a diode triangle-and-bar with two small arrows pointing away from it, and it is polarised: current only flows from anode to cathode, and it needs a series resistor. Use the lamp symbol for filament bulbs and generic indicators, and the LED symbol when the device is specifically a light-emitting diode.

Does the light bulb symbol have polarity?

No. A filament lamp is a simple resistive load, so terminals A and B are interchangeable and the symbol is drawn symmetrically. This distinguishes it from polarised light sources such as LEDs. In AC circuits the lamp works identically in either orientation; in DC circuits swapping the leads has no effect on brightness or lifetime.

Why is a filament lamp non-ohmic?

As current heats the tungsten filament toward 2,500–3,000 K, the metal's resistivity rises, so resistance increases with applied voltage. The result is an I-V graph that curves and flattens rather than a straight line through the origin, meaning the lamp does not obey Ohm's law over its operating range. Practically, this causes a large inrush current at switch-on — the filament's cold resistance is roughly a tenth of its hot resistance.

What letter designator is used for a lamp on a schematic?

In IEC/European practice, lamps for illumination are designated E and signalling/indicator lamps H (for example H1 for a fault lamp), following IEC 81346 conventions. North American ladder diagrams commonly mark pilot lights PL or LT, often with a colour letter inside the circle such as R (red), G (green) or A (amber).

Related symbols

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