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Old 04-25-2010, 02:04 PM #1
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Default Argon Laser Tube cathode voltage

Hi All,
I have a question about the voltage supplied to the cathode of an argon laser tube. Say I have a tube and measure the voltage supplied to the cathode by the PSU as being 2.55vDC. Would the cathode blow if it was connected to a different PSU supplying 3.5v? How sensitive are the cathodes to voltage variations? Is there a reason why some PSUs might supply DC and others AC?
Thanks in advance!


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Old 04-25-2010, 04:57 PM #2
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Default Re: Argon Laser Tube cathode voltage

It would almost certainly not blow, since they are only hot enough to reach thermionic emission under normal voltage. You may shorten the life though since more material evaporates at higher temperatures.
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Old 05-10-2010, 09:20 AM #3
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Default Re: Argon Laser Tube cathode voltage

i dont think it will blow. the reason some use AC and some use Dc is because Ac is more relible current than Dc wich can chop and churn and run low.
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Old 05-10-2010, 09:39 AM #4
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Default Re: Argon Laser Tube cathode voltage

Hmmmm, I'm not sure I understand that last reply.
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Old 05-10-2010, 10:27 AM #5
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Default Re: Argon Laser Tube cathode voltage

read again
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Old 05-10-2010, 10:56 AM #6
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Default Re: Argon Laser Tube cathode voltage

Ah, that's better. thanks for fixing up the words. Thanks for your input too. That makes sense.
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Old 05-10-2010, 10:57 AM #7
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Default Re: Argon Laser Tube cathode voltage

yeah sorry my arm is broken kinda hard to type i just forgot to proof read. woops.
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Old 05-10-2010, 10:03 PM #8
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Default Re: Argon Laser Tube cathode voltage

Filament temperature does need to be precise - too low and the cathode may sag or deform. Too high and lifetime will be shortened. I would try to match the original supply as closely as possible.
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Old 05-11-2010, 05:03 PM #9
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Default Re: Argon Laser Tube cathode voltage

Why does it sag at low voltages?
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Old 05-11-2010, 06:11 PM #10
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Default Re: Argon Laser Tube cathode voltage

Quote:
Originally Posted by Cyparagon View Post
Why does it sag at low voltages?
Tungsten becomes soft at 700-800*C, and then hardens again after this at the cathodes operating temperature. If the voltage is too low, the tungsten cools down too much and softens, causing it to sag under it's own weight.

Also, it needs to be AC with a center tapped secondary to give you 2.55-2.6VAC at up to 25-30A. the centre tap usually goes to the last transistor on the passbank ... can't remember if it goes to the emitter or the collector. At least that's what it does on any schematics I've got.
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Old 05-11-2010, 06:11 PM #11
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Default Re: Argon Laser Tube cathode voltage

Taken from Sam's Laser FAQ - "The more correct thing to try is undercurrent the cathode and invert it. Pure Tungsten has a known sagging temperature region where it is butter soft at 600 to 700 C. However, the cathode is not pure tungsten - it has rhenium, barium, aluminum and calcium - so the exact softening point is known only to the cathode manufacturers. Normally, the cathode runs at 1,080 to 1,120 C by design. At 800 at C it actually starts getting tougher in shear and you're not going to do that well because of the additives. You'll end up cooking and outgassing if not actually melting the nickel or molyB section that joins the tungsten to the kovar in the lead throughs."
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Omnichrome 643 air cooled Ar/Kr ion 10mW 676.4nm 647.1nm 568.2nm 488nm 476.2nm (high hours)
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