Old 03-04-2014, 05:43 PM #17
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Default Re: Need help identifying tubes

Ok, guys. Thanks for all the info, they are very helpful.
One last thing I'm wondering is about the voltage supply. I've checked Sam's Laser Faq and some design on the forum. So I suppose a Variac and neon sign transformer plus rectifier should work for the tube. Still I'm wondering if I shall add ballast resistor to it? How are you guys designing the supply?
Thanks again for the suggestions.


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Old 03-04-2014, 07:10 PM #18
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Default Re: Need help identifying tubes

If you use a NST without a diode and capacitor and run it off the AC you won't need to ballast as NSTs are internally ballasted. If you rectify and filter it there will be no current limitation so you'd need to ballast it to counter the negative resistance curve.


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Old 03-06-2014, 12:57 AM #19
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Default Re: Need help identifying tubes

I'm also wondering if anyone tried to power the laser directly with those high voltage power supply (like the old Fluke 410B, which goes to 10kV with 12mA)? In this case can I just attach a ballast resistor in series with the tube and connect it to the output of the supply?
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Old 03-08-2014, 11:35 PM #20
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Default Re: Need help identifying tubes

Hi Guys
I'm encountering a big problem here: Since I don't have the spec sheet for the tube, I have no idea which side is the anode and which side is the cathode! Is there a way to figure this out by examine the tube? Or it just doesn't matter? (I guess since they usually mark out the anode then the polarity must matters somehow...)
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Old 03-09-2014, 12:54 AM #21
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Default Re: Need help identifying tubes

If you're running it on AC, then the polarity doesn't matter. On a DC tube, the cathode will always be the
larger one, but if it's an AC tube then they will both be the same size. A DC tube will work fine on AC.
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Old 03-09-2014, 04:39 AM #22
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Default Re: Need help identifying tubes

Yep, also if you aren't sure which is larger or not, the cathode is typically the output side on DC tubes.
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Old 03-09-2014, 05:25 AM #23
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Default Re: Need help identifying tubes

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Yep, also if you aren't sure which is larger or not, the cathode is typically the output side on DC tubes.
Thank you two for the reply. Indeed for most of the information I found (like many of the Chinese tubes) they use output side as cathode, but still there are some documents indicating anode on that side so I'm a bit uncertain. I'll try to check the size, but by 'larger' do you mean the metal is thicker?
Also, what would happen if I connect it in reverse with a DC supply?
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Old 03-09-2014, 05:28 AM #24
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Default Re: Need help identifying tubes

If you reverse bias a laser tube it will still fire, but the internal electrode may overheat. I'd just go with a variac and a NST and run it off of AC, no worry about polarity and it will have an inherently lower power dissipation than running smoothed DC.
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Old 03-09-2014, 05:42 AM #25
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Default Re: Need help identifying tubes

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Originally Posted by styojm View Post
I'll try to check the size, but by 'larger' do you mean the metal is thicker?
Yeah, the metal piece in there would be larger. Play safe and be careful not to get stung by
the wood bees.
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Old 03-09-2014, 06:54 PM #26
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Default Re: Need help identifying tubes

The end where the light exits the laser is the negative and the fully reflective mirror is the positive when using DC to drive these tubes .
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