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Argon Laser Tube cathode voltage

T

timelablasers

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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!
 





Cyparagon

Cyparagon

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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.
 
GreenLaser

GreenLaser

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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.
cheers
 
Last edited:
T

timelablasers

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Ah, that's better. thanks for fixing up the words. Thanks for your input too. That makes sense.
 
GreenLaser

GreenLaser

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yeah sorry my arm is broken kinda hard to type i just forgot to proof read. woops.
cheers
 
MarioMaster

MarioMaster

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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.
 
diachi

diachi

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Cyparagon said:
Why does it sag at low voltages?
Click to expand...

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.
 
MarioMaster

MarioMaster

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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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