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Shaun

Fri Feb 15 2008, 02:43AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Hello everyone,

I just bought a 15 Watt CO2 laser tube on Ebay, becuase I've always wanted to get my hands on one and it was going pretty cheap as far as laser tubes go.

Specs for the tube are 15kV ignition, 9.3kV operation, 20mA tube current. I don't think it will be too hard to put together a power supply, but there are a few questions I have.

Can I just put a flyback or something else outputting 15kVDC directly across the tube? What I mean is, once the tube "ignites," will the voltage across the tube drop to 9.3kV? Or do I need a 9.3kV supply and then pulse 15kV through it to light the tube? Either is do-able, I just want to be sure.

Next, will I need a smoothing capacitor or limiting resistor, or does the tube's impedance limit the current as long as it is kept cool?

As for cooling the tube, from what I've read a 15W tube should be easy to air-cool, but I'll see how that turns out once it comes in.

My actual plan is to power it with my ZVS driver as it is more than capable of powering this tube, plus its DC. I have an HV divider I can use to precisely calibrate the output voltage of the flyback vs. input voltage to the driver.

Does anyone see any problems here?

Thanks,
Shaun

...

Fri Feb 15 2008, 04:48AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

the 15kv for ignition will probably need to be applied in series with the supply, because if you tried to apply if directly across the tube you would just fry your power supply.

You should not use a smoothing capacitor with the tube, as it will probably show some 'negative resistance'--meaning that if you used a capacitor it would act more like a strobe light than a laser tube. A big fat inductor would be a superior choice, although you could probably get away with just full wave rectified ac.

Also, you will definitely want to water cool the thing, or else you will melt the tubes bore.

plazmatron

Sat Feb 16 2008, 01:26AM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

As far as I know the CO2 laser tube you describe, needs water cooling. At 10% efficiency a 15W CO2 laser is generating 150W of heat, which will happily melt the bore, and damage the mirrors.
Low pressure tubes like this exhibit `negative resistance` and will continue drawing current until they melt! So, you need a ballast resistance in series with the anode, to limit the current.
A flyback driver seems quite reasonable for a 15W tube. On modern diode split LOPTs, the voltage will drop a few kV under load, so you should be OK voltage wise. The most important parameter is current. Too much and the tube overheats, too little, and the tube will wink on and off, and this causes spluttering, which shortens tube life.

An invaluable resource for stuff like this is Sams laserfaq

Leslie

Shaun

Sat Feb 16 2008, 05:27AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Thanks for the replies guys,

So apparently forced air cooling is a no-no, but thats no big deal; adding a pump and bottle is pretty trivial at this point.

I have a 0-30mA panel meter which I think I'll just put in series with the tube; that should do for getting it set up.

And plazmatron, would I be correct in thinking that as long as I have enough voltage to ionize the tube and the current is where it should be I will not have to worry about the exact tube voltage?

I know that gases have negative temp. coefficients; after all that's why NSTs are current limited.
Sam's laser faq says this on the subject:

" * The value of the negative resistance of the CO2 discharge is between -200K and -600K ohms. (The corresponding value for HeNe tubes is about -50K ohms). The effective ballast resistor value is selected to be 30 to 50% higher than this to assure stability.

For example, the dissipation in a 300K ballast resistor at 10 mA, would be 30 W. Depending on your actual needs, this may still be acceptable since it should simplify the power supply design not to have to deal with the negative resistance in the current regulator feedback loop itself. However, at the high end of the range where an 800K ballast is required at 20 mA of operating current, the corresponding power dissipation would be - ready? - 320 W! This is probably a bit more than is desirable.

"

So can I calculate the "negative resistance" in the same way a regular, just V/A? If so, then 9.3kV/21mA=443kOhm. If I take his advice then my resistor would be about 600K. Passing 21mA this dissipates 26W. That's do-able, but I wouldn't like it.

However, he also goes on to say this:

"The good news is that if you were to design an inverter type of power supply with an oversize or multiple flyback transformers, I think you would find that it inherently had a high effective series resistance - possibly enough so only a minimal external ballast would be needed."

Now theres a statement I like. I'll see how the flyback by itself works (closely watching tube current, of course) before I break the bank buying a high-voltage 30 or 50W resistor.

Or maybe I can pick up a 20mA NST on ebay...after all its meant for powering a gaseous tube...

plazmatron

Sat Feb 16 2008, 05:26PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Yep, so long as the voltage is about right its ok, like I say, current is the critical factor.
Ballast resistors need not be expensive. When building He-Ne laser power supplies, I have always used a string of common 1 Watt resistors, tapped ad different places for a range of tube currents.
I have a commercial He-Ne on the bench that uses several ordinary resistors as ballast.

Leslie

sparky99

Sun Feb 17 2008, 04:43AM

Registered Member #1159 Joined: Fri Dec 07 2007, 02:10AM
Location: Hudson Valley of NY State
Posts: 84

Hi Shaun,
I've been watching those for a while. That fellow has a bunch of 'em. I have a 40 Watt tube & power supply I got off ePay.
One more thing deserves mention here: SAFETY GLASSES. For about $28 you can get a pair of OD 6 (optical density) safety glasses. The good news is that 10.6um won't make it to the eyes retina.........but it will FRY your cornea. Even a reflected beam can do severe damage. Don't depend on hardware store goggles to protect your eyes! I, too, read that the plastic in the goggles will stop 10.6um....
but I was not willing to bet my eyesight on that. 'Nuff said!
Be safe & have fun cutting / burning things. Try a charcoal brickett.

Bob
Sparky 99

Shaun

Tue Feb 19 2008, 12:01AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

The tube came in today, and appears to be working great. I have it powered off a full-wave rectified 15/30 NST powered thru a variac. I would use my flyback driver to save on weight, but the FETs are dead and I'm all out of the IRFP250s.

I have not set up the water cooling yet, so I have limited my run time to 3 seconds at most. Tomorrow it's off to Ace for some rubber tubing and a pump.

I already want to get some optics and focus it down to a few microns, but I'll wait until I get it set up properly to worry about that.

That's about it so far, sorry for the lack of pics but I'm still waiting for my Canon 400D to come in (Yay Xmas bonus!).

Proud Mary

Tue Feb 19 2008, 12:31AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Shaun wrote ...
I already want to get some optics and focus it down to a few microns, but I'll wait until I get it set up properly to worry about that.

The wavelength of a CO2 laser is 10 microns, so even with theoretically perfect optics, that is the smallest spot diameter that can be achieved by focusing. Given that neither the laser beam, nor the focusing lens, will be perfect, you would be doing very well indeed to achieve a spot diameter of 100 microns.

Shaun

Tue Feb 19 2008, 02:47AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

I knew the smallest theoretical spot size was a function of the wavelength, and by "a few microns" I guess I may as well have said "wayyy thinner than it is now" since as-is its about half a centimeter wide and still cuts and burns easily!

EDIT: I just realized that no matter how well I focus it I'll never be able to tell the difference between 10um and 100um since I have no way of measuring something that small!

plazmatron

Wed Feb 27 2008, 11:22PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Shaun wrote ...

EDIT: I just realized that no matter how well I focus it I'll never be able to tell the difference between 10um and 100um since I have no way of measuring something that small!

You can always measure the diameter of the burn mark with even an inexpensive microscope. 100Âµ is quite large (the average packaging thickness for potato chips for example is about 30Âµ)

As Harry said, optics arent perfect, and with a single focussing lens, you wont get a diffraction limited spot (ie 10Âµ), you get what is called a beam `waist`, as the light is never really focused to a point. See image below.

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