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Flickering neon bulb, what causes the flicker?

  1. Oct 27, 2010 #1
    I have a power strip with a neon bulb that I would like to understand better. I can vary the voltage to the power strip by turning off and on an electric heater in the same room. With the heater on the voltage at the power strip reads about 125.0V on a digital volt meter. With the heater off the voltage at the power strip reads about 121.0V . In the "low" voltage state the neon bulb is off and in the "high" voltage state it is on, but the light flickers almost like Morse Code, on about one half the time.

    I understand that below a certain voltage the bulb won't work. What causes the random like operation when the bulb is on? If the voltage is near threshold is the bulbs light output effected by terrestrial radiation?

    I just put an 12 inch stack of books above the bulb, when I do the light decreases and the flicker slows down? Am I blocking radiation?

    Thanks for any ideas or help?
  2. jcsd
  3. Oct 28, 2010 #2


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    The voltage would normally drop on load so I guess that is a typo.

    However, the flickering is interesting. It could be acting like a Geiger–Müller tube. It contains a gas and it is barely conducting.

    What happens if you place a coin above the neon without holding it?
    Near it, but high enough so you can still see it.

    Is it affected by the room lights?
  4. Oct 28, 2010 #3
    I knew of a physics experiment (50 years ago) in which the experimenter recorded his data on photographic film by photographing a large array of neon bulbs which represented "1"s and "0"s of binary data. But they did not reliably light in total darkness (in ~ 100 microseconds), until he put a radioactive source nearby. Cosmic rays were not enough.

    Bob S
  5. Oct 28, 2010 #4


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    Yes, there must be a less exotic answer because Neons always seem to flicker and there usually isn't any radioactive material around.

    Maybe just mains interference, but the book experiment throws some doubt on that.

    Great question, though.
  6. Oct 29, 2010 #5
    Not a typo, the wiring in the room is via a 4 wire conductor, ground, neutral, red, black, with the red and black on opposite phases. The outlets in the room share a common neutral but some outlets are opposite phase. I have not thought out the reason for the higher voltage but was careful in my measurements.

  7. Oct 29, 2010 #6
    About the lights, it gets stranger, will report back after work. (A cigarette lighter seems to increase flicker rate, more experiments needed.)
  8. Oct 29, 2010 #7
    I can explain the opposite phase voltage increase....

    If you put a load one side of a balanced transformer the other side increases it's voltage to compensate.

    I guess that's more of an observation than an explanation, isn't it?
  9. Oct 30, 2010 #8


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    well they work off an AC voltage and from what I have always understood the flickering is due to the rising and falling of the AC cycles simple as that :)

  10. Oct 30, 2010 #9


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    Neons do give pulses of light at double the mains frequency, but this is at 100 Hz or 120 Hz depending on if you have 50 Hz or 60 Hz mains. This would be too fast for human eyes to see.

    "Flicker" is a random slow movement or pulsing of the light pattern of a neon. It may have a frequency of just a few Hz. It is common for neons to do this.
  11. Oct 30, 2010 #10
    There's always radioactive material around. Potassium-40 and carbon-14 in your own body, for example, and innumerable sources in the surrounding environment.

    In fact, neon lamps often contain small amounts of radioactive material in their electrodes to make them light more reliably/at lower voltages. Thorium, for example. And they are indeed sensitive to light. http://www.vf.utwente.nl/~ptdeboer/ham/neonclock/ [Broken] is a neon lamp clock project using neon bulbs as logic elements...the builder had to add blue LEDs to get reliable operation in darkness.

    The flicker...a steady glowing lamp has most of the current flowing through a specific path through the fill gas. Lighting and the presence of ionized neon will alter this path, though. If the current limiting series resistor is set to limit current low enough, the lamp can't establish one stable discharge path or even reliably light (and remember it's being fed by AC, so it must re-light 100 or 120 times a second). Aging of the resistor could cause the current to drop over time, or aging of the lamp itself (leakage of traces of air into it, for example) could cause its current requirements to increase. Changing illumination or nearby radiation sources would also influence the current required for a steady glow. The capacitance of your body might also have an effect.

    Potassium-40 decays by positron emission, and positrons annihilate with electrons to emit gamma, which might penetrate the bulb and make a difference. Potassium chloride is often used as a no-sodium salt substitute, you might obtain some and see if it influences the flicker. Or find something with a bright blue LED (shouldn't be hard these days) and see what effect that has.
    Last edited by a moderator: May 5, 2017
  12. Oct 30, 2010 #11
    Here is a simple circuit that uses a neon NE-2 glow lamp as a Geiger counter for detecting radioactivity.

    http://electronicdesign.com/article/test-and-measurement/simple-geiger-detector-uses-neon-glow-lamp1634.aspx [Broken]

    Bob S
    Last edited by a moderator: May 5, 2017
  13. Oct 31, 2010 #12
    Thanks for the replys!

    After a little more experimentation, per suggestion, it seems that the bulb is indeed sensitive to visible light. In a room with the light on the bulb flickers at 125 volts and at 121 volts it is completely dark. Lights were turned out and at 125 volts the bulb is on maybe 10% of the time with the light on (it is partially shaded by a desk) the bulb is on maybe 60% of the time and if a flashlight is shined on the bulb it is on nearly all the time. Putting the books above the bulb seemed to effect the bulb because I was blocking light at the time, a more careful experiment with the books showed no noticeable change in the bulbs output.

    Thank you all for helping me to understand this better!
  14. Oct 14, 2012 #13
    Great fun! I have duplicated the light sensitivity of the neon bulb, I can turn it on and off by shining a flashlight at it!

    But I have no idea why a new functioning bulb can change to be voltage and light sensitive in a few weeks, yet still work OK (but dimly) with the photoionization from a flashlight? On my crockpot, the same bulb is normal on another setting ("high"), so it seems to have intact electrodes and neon.

    If it the cause is a deteriorating series resistor, why does photoionization still make it glow? The voltage to ionize the neon is getting through it - with and without the light. Dogma says this ionization potential is a property of the neon atoms...

    So why does is it photosensitive, and why does it flicker?
  15. May 9, 2014 #14


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    The reason neon bulbs flicker is that the gas is getting used to light the bulb it has a certain firing voltage that has to be controlled by limited current. This is accomplished by a series resistor. If the lamp fires at 67 volts then everything is ok. If the resistor that is used is a carbon composition type then age becomes a problem, they increase in resistance, and that reduces the voltage to a lower value. There are two options to consider, one is the voltage lines has spikes and overcomes the resistance to fire the neon, the other is gas is slowly leaking out of the contained glass envelope and is replaced with atmosphere thereby reducing the neon content and this effects the purity and firing voltage.

    Ambient Light itself does effect gasses through Ionization bombardment and at lower voltages on the electrodes would change triggering voltages.
  16. Nov 27, 2015 #15
    My neon light flickering experience involves an alarm clock.

    I took it apart, then powered the clock. When the bulb is exposed to ambient light, it glows steadily. However, when the room lights are turned out, it starts flickering, in a seemingly random pattern.

    Enclosing the bulb in an opaque covering (excluding ambient light from reaching the bulb) results in the flickering action. The voltage downstream of the inline resistor is about 106.7 VAC, but spikes to around 110-112 when the bulb is in the dark and flickering. I'm assuming (uh-oh…) the spikes in voltage are occurring when the bulb is in the 'dark' mode (no current being drawn).

    The bulb is about 30 years old, and only recently started it's flickering action. From the above post, it would appear the bulb is losing it's gas charge. I'm looking to put a new one in, but I'm not sure what bulb to purchase. It looks like the ones at this site, but there's no numbers visible on the bulb itself. The resistor is a 39.6K Ω 5% (orange-white-orange-gold), would that be any clue to the correct bulb?

    Thanks for the posts, you really CAN find anything on the internet!

    (In case the 'this' link doesn't work, the URL is http://www.ebay.com/itm/NEW-Box-of-...-125-Volts-AC-DC-FREE-RESISTORS-/131537647343)
    Last edited: Nov 27, 2015
  17. Nov 28, 2015 #16


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    The neon lamp has a negative resistance characteristic, whereby the striking voltage is more than the burning voltage. So it will oscillate in some conditions, depending on the circuit constants. All gas tubes have a tendency to instability, even the ordinary fluorescent tube will often contain striations being caused by audio frequency oscillation. Gas tubes rely on some residual ionisation in order to strike, and in most cases this is caused by background radiation/cosmic rays. So a gas tube is similar to a Geiger-Muller tube and will respond to radiation in a similar way when it is just on the point of striking. It will also respond to radio frequency radiation in a very sensitive manner.
  18. Nov 28, 2015 #17


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    I agree with Tech99. The following video shows another type of bulb which has similar, seemingly unpredictable, variations and flickers. I think its behavior is similar to the flickering neon light.

  19. Nov 28, 2015 #18


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    There could be some ambient light sensor that is trying to affect bulb brightness. When the room is dark it dims, and in your case starts flickering. You might try covering areas of the clock in a lit room to see if there is a sensor (just guessing - can't help it)
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