[sdiy] how to fix a blinking/blank neon power indicator?

[Tom Corbitt]

Neon lights are like diodes, it takes a certain amount of voltage to
turn them on and one they're on they're pretty much a short, which is
why you need to resistor to limit current. When the voltage gets high
enough, it generates a plasma inside the glass which gives off light.
Like all things electronic, neon bulbs have a finite lifetime, looking
at various ne-2 datasheets scattered around the web it looks like they
are commonly rated around 25k hours, which is around 3 years if left
on 24/7.

I'm sure there are a lot of factors involved in computing the wear and
tear inside the bulb, but like you I'm guessing that a big part of it
is electrode wear which probably increases the distance between the
two poles and outgasses various things into the envelope, which
increases the voltage requirements for plasma generation. As an aside,
light changes the firing characteristics as well, which is why you
sometimes see neon bulbs shielded from light in circuits.

As for your cap question, placing a cap across the leads will have the
opposite effect. The lamp will always blink, as you just have just
created an relaxation oscillator circuit (assuming you have kept the
resistor in series). If you build a line of these and set them off
running, every so often they appear to be synchronized together, in
the old days people used to call them "mystery boxes" and the like,
since it appeared to have some method to it's blinking madness (when
in fact the method is the brain's quest for patterns)

The only way to "fix" the bulb would be to apply a higher voltage to
the tube, which can be done, but it's probably not worth the effort.
If you don't mind the work and it's mechanically possible to get at,
changing the bulb is the easiest solution (you should be able to find
them just about anywhere (hamfests, surplus, digikey/mouser) for
almost nothing. Just hit google with  "ne-2"

Back in the pre-transistor days, people used neon bulbs in all kinds
of amazing ways. It was the 555/2n2222 of it's day.

Cheers,
Tom





On Tue, Apr 8, 2008 at 1:32 PM, anthony <aankrom at bluemarble.net> wrote:
> Anybody know why sometimes just out of the blue, a neon bulb will stop
> working or begin to blink intermittantly?
>
>  My guess is that there wasn't an adequate current-limiting resistor and the
> gap betwwen the electrodes widened. This would seem to explain the blinking
> ones too: the gap hasn't widened enough to keep it from firing, but it has
> to wait for enough charge to build to "jump" across.
>
>  I've also noticed that heavy inductive loads, say a variac controlling a
> 1000 watt heating element, can cause the neon bulb to go out if the setup is
> (unwisely) connected to a power strip that has a neon power indicator (the
> one I was using didn't have a surge protector so it wasn't anything to do
> with that...). But I don't know what this is about unless it somehow causes
> a voltage drop locally across the system. But it couldn't have to the
> electrical system to the whole building in a modern university.
>
>  So I had an idea to "fix" a failing bulb: connecting something like a
> polypropylene X2 rated capacitor across the neon bulb leads. I haven't tried
> this yet: I was wondering if it'd work. I have a HUGE collection of a wide
> range of MKP X2 caps taken from various power supplies.
>
>  So does anyone think this'd work?  If so what would be a good capacitance
> rating? Is there a better fix? I'd just replace the bulb, but it is a row of
> switches with the neon bulbs right inside them and these is a number of them
> that do not work. Would something like an oil-filled motor-start cap work
> better? I have a 3.5uF one rated for 450VAC and a 0.5uF one rated for
> 2500VAC (probably too small: might as well use a MPK X2...). Or a 10uF,
> 160VAC metallized paper motor-start cap maybe?
>
>  And gosh what you do with one of those organs that's chock FULL of neon
> bulbs?
>
>  aa
>
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