Neon Lamp Symbol

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

Definition: The Neon Lamp symbol represents a two-electrode gas-discharge glow lamp with non-polarised terminals A and B, drawn per IEC 60617 gas-filled-lamp convention as a circle containing two short parallel electrode lines with a filled dot inside the envelope — the dot signifying gas filling, which distinguishes it from vacuum devices.

Also known as: neon lamp, neon bulb, NE-2, glow lamp, neon indicator, mains indicator lamp, glow discharge lamp.

What the Neon Lamp symbol means

The neon lamp symbol denotes a miniature cold-cathode glow discharge tube: a glass envelope filled with neon (usually with a little argon) at low pressure, containing two electrodes with no filament and no heater. Below its striking voltage the lamp is effectively an open circuit; once the applied voltage reaches roughly 60–100 V the gas ionises, a warm orange glow forms around the negative electrode, and the lamp conducts while dropping a nearly constant maintaining voltage of about 50–60 V. This threshold behaviour — off until struck, then a constant-voltage glow — is what makes the neon useful as an indicator, a voltage reference, a surge indicator, and a relaxation oscillator element.

Because it strikes comfortably from rectified or AC mains and draws well under a milliamp, the neon lamp was for decades the universal mains 'power on' indicator, and it still appears in power strips, illuminated rocker switches, voltage testers and appliance panels. The symbol's dot is load-bearing: the same circle-and-electrodes without the dot denotes a vacuum device, so the dot is what says 'gas-filled'.

How to identify the Neon Lamp symbol

Draw a circle (the glass envelope) containing two short, straight parallel lines — the electrodes — each connected to a lead exiting the circle, plus a small filled dot placed inside the envelope between or beside the electrodes. The dot is the IEC marker for gas filling. Unlike the incandescent lamp symbol (circle with an X or filament loop) there is no filament drawn, reflecting the cold-cathode construction.

ANSI/IEEE practice is the same envelope-with-electrodes-and-dot form. The main confusion risks are the incandescent lamp (X instead of parallel electrodes), the gas-discharge surge arrester symbol (two electrodes in an envelope, but used in protection circuits and often drawn as a rectangle with a spark gap), and the xenon flash tube (which adds a trigger electrode and zigzag). Context — an indicator across the mains with a series resistor — usually settles it instantly.

Function in a circuit

In its dominant role as an indicator, the neon lamp is wired across the voltage being monitored with a series current-limiting resistor — always. Without the resistor the negative-resistance characteristic of the glow discharge lets current run away and the lamp destroys itself in seconds; with a typical 220 kΩ on 230 V mains the lamp draws a serene 0.5–1 mA and lasts tens of thousands of hours. On AC both electrodes glow alternately (appearing as both lit); on DC only the negative electrode (cathode) glows, a trick long used to indicate polarity.

The striking/maintaining voltage gap also enables the classic neon relaxation oscillator: a capacitor charges through a resistor until it reaches the striking voltage, the lamp fires and discharges the capacitor down to the maintaining voltage, then extinguishes and the cycle repeats — producing sawtooth waveforms and blinking indicators with a single lamp, resistor and capacitor. Neons additionally serve as crude ~60 V references and, historically, as protective spark gaps and logic elements.

Standards: IEC vs ANSI

IEC 60617	IEC 60617 represents gas-filled discharge lamps with an envelope, electrode strokes, and a dot indicating gas fill; the neon indicator falls under this family. IEC 60064/60155 relatives govern lamp hardware, while glow-lamp starters for fluorescent fittings are covered by IEC 60155.
ANSI/IEEE 315	ANSI Y32.2 / IEEE 315 shows the glow lamp as a circle with two electrodes and a dot for gas filling; classic North American type designations (NE-2, NE-51) originate from GE/industry registration rather than the drawing standard.
Key difference	Both systems agree on envelope + electrodes + gas-fill dot; differences are cosmetic (electrode shape, dot placement). The universal rule shared by both: a dot inside the envelope means gas-filled, no dot means vacuum. Neither standard draws a filament for the neon, which is the giveaway separating it from incandescent lamp symbols.

Terminals / pins

Pin	Name
a	A
b	B

Typical values

The ubiquitous NE-2 strikes at about 60–100 V (typically ~90 V DC), maintains at 50–60 V, and runs at 0.3–1 mA (design maximum a few mA). Series resistors: ~220 kΩ for 230 V AC operation, ~100 kΩ for 120 V AC, sized for roughly 0.5–1 mA. Power dissipation is on the order of 40–120 mW. Life ranges from ~15,000 to over 50,000 hours (long-life types trade brightness for lifespan); light output is a characteristic orange-red at 585–640 nm wavelengths. High-brightness argon-doped types strike slightly higher and glow with a violet tint.

Where the Neon Lamp symbol is used

Mains power-on indicators in power strips, extension leads, kettles and illuminated rocker switches (neon plus integral series resistor)
Non-contact and contact voltage testers / mains test screwdrivers, where the glow confirms live voltage
Fluorescent-lamp glow starters, where a neon glow discharge heats a bimetal contact to strike the tube
Relaxation oscillator, sawtooth and flasher circuits built from one neon, a resistor and a capacitor
Vintage equipment: Nixie-adjacent panel indicators, stroboscope timing lights and radio dial pointers
Crude overvoltage/surge indication and spark-gap protection across telephone and signal lines in legacy gear

Example

In a mains-presence indicator, the Neon Lamp symbol's Terminal A connects through a 220 kΩ series resistor to the 230 V AC line conductor and Terminal B connects to neutral; the lamp strikes each half-cycle once the instantaneous voltage exceeds ~90 V and glows steadily at about 0.7 mA — and because terminals A and B are non-polarised on AC, the lamp and resistor may be swapped end-for-end with no change, though the resistor must never be omitted.

Key facts

The symbol is an envelope (circle) with two parallel electrodes and a filled dot — the dot is the IEC/ANSI marker meaning 'gas-filled'.
A neon lamp must always be used with a series current-limiting resistor; its negative-resistance glow discharge otherwise self-destructs.
Typical NE-2 characteristics: strikes at ~90 V, maintains ~55 V, runs at 0.5–1 mA — microscopic power for a visible indicator.
On DC only the negative electrode glows; on AC both electrodes glow alternately — a built-in polarity indicator.
There is no filament: it is a cold-cathode device with essentially no inrush current and a life of 15,000–50,000+ hours.
The striking/maintaining voltage hysteresis enables single-lamp relaxation oscillators and flashers with just a resistor and capacitor.
Neon indicators dominated mains 'power-on' duty for decades and still fill power strips and rocker switches; LEDs need ~30× more current for the same job at mains voltage.
The neon glow is orange-red (585–640 nm); other colours in 'neon' signs come from different gases or phosphor coatings, not neon.

Frequently asked questions

Why does a neon lamp need a series resistor?

Once struck, a glow discharge exhibits negative differential resistance: as current rises its voltage drop falls, so on a stiff supply the current would increase without limit until the lamp sputters and destroys itself. A series resistor (typically 100–220 kΩ on mains) converts the supply into a current source of roughly 0.5–1 mA, holding the lamp in its stable glow region. Panel-mount neon indicators almost always have this resistor built into the leads or housing.

What voltage does a neon lamp need to light?

A standard NE-2 strikes (ignites) at roughly 60–100 V — commonly quoted as ~90 V DC — and once struck maintains conduction at about 50–60 V. It therefore works directly from 120 V or 230 V mains through a resistor, but will not light from ordinary battery or logic voltages. This high striking threshold is precisely why it makes an unambiguous mains-presence indicator: if it glows, line voltage is genuinely there.

What is the difference between the neon lamp symbol and the regular lamp symbol?

The incandescent lamp symbol shows a filament — an X inside the circle (IEC) or a filament loop (ANSI). The neon symbol shows no filament: instead the circle contains two separate parallel electrode lines plus a dot marking the envelope as gas-filled. If you see the dot and two unconnected electrodes, it's a gas-discharge glow lamp; if the marks inside the circle join the two leads, it's a filament lamp.

Can a neon lamp work on DC, and does polarity matter?

Yes, neon lamps run happily on DC above their striking voltage with an appropriate resistor. Polarity does not affect whether it lights — terminals A and B are interchangeable — but it changes what you see: only the negative electrode (cathode) is sheathed in glow on DC, while AC lights both electrodes alternately. Technicians historically exploited this to check DC polarity by eye. Long-term DC operation causes slow sputtering of the cathode, so lamp life is somewhat shorter than on AC.

How does a neon relaxation oscillator work?

Connect a resistor from a DC supply (above ~100 V) to a capacitor, with the neon lamp across the capacitor. The capacitor charges exponentially until it reaches the lamp's striking voltage (~90 V); the lamp fires, rapidly discharging the capacitor to the maintaining voltage (~55 V), then extinguishes, and charging begins again. The result is a sawtooth across the capacitor and a rhythmic flash from the lamp, with frequency set by R×C — a complete oscillator from three passive parts.

Related symbols

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