Nixie Fabrication specs

[nixie]

Hi,

I recently stumbled across some nixie tubes and also this forum.  Lot's of good info.

Does anyone out there have any source of technichal info on NE glow bulbs?  I'm interested in attempting to fabricate some larger scale bulbs and wondering about electrode materials, gas pressures, and voltages.

also,  what physically happens  so that NE glow bulbs get zapped with no current limiting?  How could one determine ideal current for a bulb?

thanks,

nixie

[Max]

Hello

quote:Originally posted by nixie:
I'm interested in attempting to fabricate some larger scale bulbs and wondering about electrode materials, gas pressures, and voltages.

In commercial glow lamps the electrodes are made of pure iron or nickel and sometimes are doped with rare earth in order to ease the electron (secondary) emission process occurring at the cathode. The neon pressure is around 10 mbar, but the higher this pressure, the higher the current needed for the lamp to operate correctly (see below). About 1% of argon is usually added to neon in order to decrease the starting voltage (this forms a so-called Penning mixture).
Concerning the voltage, it depends on the pressure and purity of the neon gas, current, chemical nature and surface state of the electrodes. So this is hard to give a ?standard? value, especially if you are going to make the lamp by yourself.

quote:Originally posted by nixie:
also,  what physically happens  so that NE glow bulbs get zapped with no current limiting?  How could one determine ideal current for a bulb?

If a glow (or any discharge) lamp is connected to the mains without any current limiting device (resistor, capacitor or inductor), the current flowing through the lamp will increase until something blows up (The lamp, or the fuse).
The reason for this (in the case of glow lamps) is the fact that any electrical discharge do not self-regulate their current, because the process of electron and ion production is unstable: electrons create ions by impact on atoms, and these ions turn out to generate electrons upon impact on the cathode surface. In one word, we have a ?feedback loop? reaction.
If this process is left unregulated, the electron and ion density will increase exponentially, the impedance of the discharge decrease and the current increase. Since the power from the mains is voltage-stabilized , it is thus necessary to control the current by a ballasting device.

Now, concerning the ideal current. Once you have made your lamp, you connect it in series to a DC generator and a variable resistor. Set the latter such as the lamp voltage-drop is constant (normal glow regime) and the negative glow covers the whole surface area of your cathode. Stop at this point, because if you further increase the current the lamp voltage-drop will rise and the electrodes will suffer from severe sputtering (you will enter the abnormal glow regime).

A++

Max

[nixie]

Max,

thank you much for the info.  It may be about 6 months before i get to do something with it. busy life in grad school.  I still have to learn a bit about glass blowing and talk the people in our glass lab into letting me use their stuff.  I'll post some pics some day...

btw is there some advantage to using nickel over steel?  or perhaps nickel steel (like 308 stainless)?

also, a larger glow lamp would seem to draw more current than a smaller one.  would this in turn cause it to blacken the glass faster?
do you think using some sort of tungsten and halogen mix could work to prevent this like in halogen incandescents?  If it worked it would mean a loooooong life, I would think.

also, how hot do the electrodes get?  i guess that would depend on current to some extenent.  solder connections probably would be inferior to welded?

thanks again,

nixie

[Max]

quote:Originally posted by nixie:

is there some advantage to using nickel over steel?  or perhaps nickel steel (like 308 stainless)?

To the point of view of the discharge, there are no significant differences, but there may be some concerning the sputtering rates. I have no data concerning neon ions, but I know that iron is usually prefered.

quote:
also, a larger glow lamp would seem to draw more current than a smaller one.  would this in turn cause it to blacken the glass faster?

It depends on your bulb surface area. If you have a large cathode folded in a small buib, you will have a faster blackening than in a larger bulb.

quote:
do you think using some sort of tungsten and halogen mix could work to prevent this like in halogen incandescents?  If it worked it would mean a loooooong life, I would think.

We cannot use the tungsten-halogen cycle in glow lamps, because this cycle relies on the high temperature of the filament that dissociate tungsten halide molecules, thus relasing free tungsten atoms on the filament surface. In glow lamps the cathode temperature is not very high (at least no more that 60C) and I don't think you can have a chemical cycle due to the temperature difference between the electrodes and the lamp wall.
Still, it is possible to use a gas that will form a molecule with the sputtered atoms from the cathode. And either this molecular gas will deposit by forming a transparent film, or its vapor pressure is high at room temperature so it will remain in the gas phase. But the major problem is that the added gas will surely affect the discharge characteristics. Either it will tremendously increase the breakdown/operating voltage (electronegative gasses, metastable quenching, dissociation etc..) or change the light emission spectrum so the lamp will be barely visible (most molecules have lower excitation energies than rare gas atoms).
The other problem is the reactions that will change the chemical composition inside the lamp, thus changing its electrical characteristics.


 

quote:
also, how hot do the electrodes get?  i guess that would depend on current to some extenent.  solder connections probably would be inferior to welded?

As far as the solders are concerned, the electrode temperature is not a concern. The problem in fact is the different chemical nature of the solder. This solder may have a higher secondary electron yield than the electrode itself, and in that case the discharge will be brighter in these regions (or even worse, the discharge will occure only in front of the solder). The other problem is the sputtering yield, if you plan to use tin-based solders they will sputter much faster that iron. The result will be a nice shiny tin-mirrored nixie lamp (it may look good anyway!)
More seriously, I strongly advise you to spot-weld your electrodes.

[James]

Hi Nixie,

About 5 years ago I made a number of the beehive spiral type neon glow lamps, consuming about 5 watts or so.  I made mine with the glassblower at University and I'm sure you'd have no problem to do the same

The electrodes were made from essentially pure iron wire (which all glassblowers should have a stock of - its used for numerous glass cutting applications) and the gas filling I used was 1.5% argon in neon, at a pressure of 10 torr.  Swedish iron wire is the classical material used for neon lamp electrodes, but I'm not sure how this differs to ordinary pure iron - do you know Max?  In any case, the pure iron worked fine in the lamps I made.

The most critical operation I found was to do a very thorough job of outgassing the electrodes if you plan to make large lamps.  Of course, you will bake out the glass when it is under vacuum, but just doing this alone is not enough.  Even though the electrode temperature is low, ion bombardment causes it to outgas and release impurities into the lamp.  The best way to prevent it from doing this is to thoroughly outgas the electrodes while the tube is still on your vacuum system.

The easiest way is simply to drive the device at a much higer current than you would normally use - and don't be afraid to increas it to the point where the electrodes just begin to glow red-hot.  Don't maintain it too long though or you may begin to sputter some material onto the glass!  In mass production, the electrodes were usually heated by induction in an RF field to avoid sputtering, but for hand made lamps you'll be fine by just overdriving the discharge.  For this reason, solder is not good to use!  Also tin/lead solders sputter very heavily and will blacken the bulb rapidly.  Someone at your university must have a spot-welding machine you can use

By the way, should you need glass stems with the lead wires etc and glass bulbs to seal the assemblies into just let me know - I can either send you a few of what I have here or put you in touch with a local factory.

And if you send me an email, I will reply with the technical specs of the NE series neon glow lamps for you.

Best regards!

James.

[Max]

quote:Originally posted by James:

Swedish iron wire is the classical material used for neon lamp electrodes, but I'm not sure how this differs to ordinary pure iron - do you know Max?  

No, I have no idea.
perhaps that this kind iron has an impurity concentration (coper, chrome, carbon) that makes it particularly resistant to incident neon ions...

Max