Blinky bulbs

A reader writes:

What are those little LED-like, but flickery, orange lights, seen in nightlights, electric-blanket power lights, etc? I've seen them in antique radios and as indicator lights in other ancient gear, so I presume they're not actually LEDs.


They're neon bulbs. One giveaway is the colour; a plain neon tube, just low-pressure neon in a glass envelope, glows naturally with that orange-red colour when you put enough volts across it.

("Neon lights" that aren't orange-red may still contain neon, but have a phosphor coating on the inside of the glass that turns the light another colour. White fluorescent lights are all actually mercury-vapour tubes with a phosphor coating. The amount of mercury in even a large fluorescent lamp is very small.)

For a large neon tube, the voltage from end to end has to be up in the kilovolts. But if you make a little teeny neon bulb with electrodes only a few millimetres apart, you only need a bit more than a hundred volts to get it to glow.

This makes teeny neon bulbs a natural fit for indicator-light duty in countries with 115V-ish mains power. You still need to use a current-limiting resistor in series to discourage the lamp from zipping up past the C on this graph and burning up, but that's all you need. In countries with 230V-ish mains, you just need a larger resistor value.

If you run a neon lamp directly from 50 or 60Hz AC mains power like this, the bulb flickers at twice the mains frequency, because the two electrodes light up in turn, but only when the mains waveform is giving the bulb enough voltage to light. (From DC, the lamp won't flicker, but only one electrode will light up.) The older the lamp, the more flickery it will become, until eventually it doesn't light up at all. Little neon lamps ought to last 20,000 hours or more, but many modern ones seem to be of lousier quality.

(Incandescent bulbs don't visibly flicker when run from AC, because a tungsten filament has enough thermal inertia to keep glowing at very close to full brightness even when the mains waveform is crossing the zero-volts mark. A fluorescent tube driven almost-as-directly as a neon bulb from mains power will also flicker, which a lot of people hate. Modern high-frequency electronic ballasts solve this problem, for fluorescent tubes and compact-fluorescent lamps.)



4 Responses to “Blinky bulbs”

  1. charleso Says:

    You might even have one in reasonably modern equipment without even knowing it. As the strike voltage is near nominal mains voltage they crop up as cheap compact voltage protectors for low voltage circuits that might get accidental voltage spikes. They were particularly common on the on old coax based network cards. They are wired across a circuit. Normally they don't conduct as everything is around 5-12volts but if there is a voltage surge they strike and short circuit the two sides for the duration of the high voltage. Not much use in high currents but great for the short lived, low current high voltage spike you might get induced in a cable during a thunderstorm for instance. Hence they also crop up in telecoms quite a lot.

  2. veniceca Says:

    One small correction - "neon" tubes that have phosphor coatings are not filled with neon gas, but rather with argon and a small bit of mercury. In a clear tube Ar/Hg emits a pale blue glow; in a coated tube the color is dependent on the phosphor mix.

  3. Fallingwater Says:

    I have a multi-plug adapter that has to be older than myself (I'm 28), I remember it being in my dad's office when I was little and he'd already had it for years. The original neon bulb under the switch still emits something that vaguely qualifies as "light". It was flickery to begin with and it's now super-flickery, to the point it almost looks like there's a flame under the button instead of a lamp, but it still works.

  4. TwoHedWlf Says:

    Incandescents still flicker enough to be picked up by an optical RPM gauge though.

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