Why does Neon gas sometimes glow brilliant white ?

[DeepCut]

Hello,

I was recently pulsing a coil at fairly high voltages (200-600VDC) using DC squarewave, at fairly high frequencies (100Hz - 1.3KHz). The output of the coil had a Neon bulb attached.

I wasn't measuring current output at the time.

At one point, i don't remember the frequency or voltage, the Neon bulb glowed a brilliant white, with a purplish edging.

This lasted a few tens of seconds then the bulb blew.

Until this point, as far as i knew, the emission spectrum for Neon included only orange/red/green and purple at 450nm.

Can anyone explain this ?


Thankyou,

DC.

 

[Simon Bridge]

Very bright light of any color can look white to the eye.
This description, though, sounds like how you can get something glowing "white hot".

You'd see less of the reds if the gas cannot return to it's ground state and the mixture would shift to the blue end ... when you get a roughly even mix of reds, greens, and blues, you'd get something that looks white. See the available colors below:

 

[DeepCut]

Thank you Simon.

The colour it had was purplish, so I'm thinking around the 450nm range.

Here is a picture i took before the bulb blew :

I see what you mean now, of course, red, green and blue are its primary emissino spectra so a mixture of thtese is white.

Thank you very much for clearing that up :)

Gary.

 

[Simon Bridge]

That's a good pic - yeah: the actual color is mauvey implying you just have lots of the blue transitions - which would be expected for very bright light (you get lots of all the colors but you wouldn't normally get enough blue to notice.)

 

[DeepCut]

Ah, great, thank you again :)Gary.

 

[pumila]

I had a very similar experience years ago. There was no current limit on the neon bulb and it suddenly began to glow with an intense white light. I stopped it before it blew itself up, and found the electrodes heavily fused and damaged. From this I concluded that what I had seen was a high-pressure metal-ion plasma discharge from the electrodes. This explained the colour and intensity.

 

[DeepCut]

That's interesting, I've been wondering what material the electrodes are made of and can't find anything on the web.

My current was limited to 2A but i think it happened at around 1.6A.The seup i was testing with developed transient spikes of around 600V but the current was only 150mA on an analogue meter so I'm still puzzled.

I forgot to say i had two diodes (1N4001) on the coil output, one on each leg and both facing in the opposite direction to each other.All the best,

DC.

 

[pumila]

I am not an expert but even 150mA at 600 volts or more likely more is 90 watts in the electrodes and the space between them. And neon bulbs are tiny. Add to that the effect of that current hitting the tiny electrodes at 600 v. What did your neon bulb electrodes look like afterwards? I suspect if I had let mine run to failure either the glass would have melted or the electrodes would have fallen off at the junction with the glass.

 

[DeepCut]

I binned the bulb when i realized it wasn't working, it had blackened on the inside but i didn't examine the electrodes.

I'll do it again tomorrow and see what happens to the electrodes.All the best,

DC.

 

[NascentOxygen]

When the voltage is such that current jumps the gap the light is purple. The normal operation of a neon sees the gas around just one of the electrodes glowing orange. (One electrode if DC; alternating the electrodes if AC.)

I seem to recall that some gases give different colours, depending on the pressure in the tube. Can't say whether Ne is one such though.

 

[DeepCut]

Thanks Nascent :) So one electrode is active if on DC and that is orange, both electrodes are active alternately in AC and that is orange, but both active at the same time (arcing) gives purple.

I wonder if that's because the arc is effectively a short-circuit so the current is at it's maximum.

That would imply the colour changes only when the current exceeds a certain value.DC.

 

[pumila]

The blackened bulb implies that metal vapour was condensing on the coolish glass envelope, or the electrodes were sputtering the glass. Given short burn time both imply the electrodes were burning away. If the light was broad spectrum it implies a broadening of spectral lines commonly seen in high pressure discharge lamps such as sodium street lamps, not something the neon would have done by itself. Certainly when I experienced it the light was unbearably intense, lighting up the whole room brightly, and an actinic white. Seems unlikely it was from neon gas.

 

[NascentOxygen]

After I constructed my first ham radio receiver, I found I could elicit nice purplish flashes by discharging its HT condensers through a neon bulb with the current-limiting resistor shorted out.

Though hefty, these current bursts were of short duration. My mistreatment didn't seem to harm the tiny neon globe.

Further note to what I said earlier...
you can use the neon to determine the polarity of a DC voltage (of suitable volts), and to distinguish AC from DC.

 

[DeepCut]

I see what you mean about the HPS lamps.

And what a handy property of Neon with the AC/DC detection.

Seems that tungsten is the most common material for filament electrodes, it has a melting point of over 3K degrees though.