Neon tube question

[dhaen]

A colleague has a "Dapy" radio (no jokes please ).
This incorporates a neon tube for decoration. Over time the neon tube has failed to strike. At first, raising the ambient light level would make it work. Now it doesn't strike at all.
It's a phosphor coated neon with one connection at each end. There appears to be about 600v across the thing.
Should that be enough to make it strike?
There seems to be mercury in the tube. What is it for?

TIA

[SY]

How do you know it's neon? Does it glow orange when on?

[dhaen]

The phosphor coating used to glow blue. The internal gas type is an assumption

[SY]

Mercury vapor tube, perhaps. Sounds like the tube has slowly gone leaky and/or the phosphor has exhausted itself. That would explain why hitting it with light might have helped ignition along in its declining years.

[Sch3mat1c]

Typically, neon tubes (and other gas discharge tubes, gas regs for instance) ignite with help from light. The extra EM radiation helps kick off the discharge. In fact, you can set up a relaxation oscillator and cup your hands around the tube: frequency drops!

In a more dangerous persuit, Marx generators, a series of capacitors (with spark gaps joining adjecent caps in series) are charged by resistors; when the voltage of *all* the caps reaches threshhold, the gaps fire, connecting them suddenly in series and producing very high voltages (there are some in existance giving a few million volts). Well obviously the gaps have to discharge at the same time: apparently, the first spark's UV radiation causes a chain reaction of dropping nearby gap firing voltages.

Tim

[dhaen]

Thanks for the interest

So it's probably a mercury vapour + argon tube.
But how to determine the striking voltage?

[dhaen]

The tube is about 12 inches.
It seems unlikely that the 600v is going to strike it, so perhaps the fault lies in the power supply...

[Da5id4Vz]

Is the Hg in liquid state?

How long is the tube?

Its likely not phosphors in side the tube. Colored "neon" lamps are usually tinted with a combination or colored glass and mixtures of other inert gasses.

Keep in mind that a favorite means of building a Jacobs Latter is to use an old neon transformer arcing between two diverging conductors.

Kilovolts, and yup a little dangerous.

http://www.gilway.com/pdf/appl-neonlamps.pdf

Safe way to fix the radio might be to buy a $10 computer or car neon accent light that has a built in power supply.

[dhaen]

The Hg is liquid.
The tube is round..ish

[Da5id4Vz]

The first link I sent you was, well, crap.

A little more googling led to sites like this:

http://www.neonpro.com/docc/product-01.htm

Here I learned that yes, there are indeed colored powders used to tint “neon” tubes. They have a gadget listed there used for measuring proper voltage for tube length.

There seem to lots of smallish DC powered supplies available for this sort of thing.

One last thought, before the Mods, appropriately, cut us off. (Were dealing with non audio KV here so it’s clearly a safety issue)

A sign maker once told me that surplus inverters used in cold cathode lighting can be used to light small neon signs. You might try checking the trading post to see if anyone who has just dismantled an LCD display wants to part with the lamp and power supply module.