CO2 laser power supply issue

[RDZombie]

this is my 4th co2 laser and have never had this problem with my other 3, brand new 15-18 watt range sealed tube. i made the power supply out of an oil furnace igniter transformer (10kv @35ma), homemade bridge rectifier, and a ballast resistor on the anode. without a filtering capacitor she works well, however i was trying to install a filtering cap across the dc HV output for a more stable output. caps are 2200pf @20kv and i have 11 of them to toy with. now the issue come when i try to install the cap(s). i get erratic plasma discharge. looks similar to when you dump low current AC into a xenon flashtube. pulsed (appearing) plasma discharge within the tube that snakes instead of the nice straight discharge you would expect. any other co2 makers out there with an idea? i was thinking up the ballast but i dont have any more high watt ballast resistors on hand.

 

[CHP]

Where did you get your tube?

The amount of total capacitance isn't that much for a 60 Hz system. Try lowering the ballast resistance. Try putting the caps on the output of the bridge rectifier - it sounded like you had them at the terminals of the laser.

 

[RDZombie]

CHP, The tube was an ebay special but it looks pretty well made . I have the 2200pf cap mounted right off the bridge rectifier (on the bridge's terminals actually). What size cap would you suggest, as i think youre right and i need a smaller cap.

 

[Corona]

Problem is likely the transformer cannot output enough current to actually keep the tube ionized AND charge the filter caps. With the caps in the circuit, once the tube fires, the capacitor(s) discharge and the voltage falls off big time. The capacitor impedance goes to near zero (essentially a short circuit) and the transformer output collapses.

Yes the transformer is rated for 350W output but it may have too much DC resistance in the secondary to do the job. Or the core is undersized, being designed to make an arc, which quenches just like a gas tube...

With AC or rectified DC, the tube fires at the peaks of the AC waveform and conducts until near zero or zero crossing (and repeats for the opposite half cycle). The energy in the waveform between zero and peak is wasted since the tube does not fire at lower voltages...

BTW, you want a larger capacitor at low frequency (120Hz), not a smaller one. And it is 120Hz because you are full-wave rectifying a 60Hz AC source.

What's the spec on the tube and how does it compare to the transformer output specs?

 

[CHP]

I was saying that the capacitance you had is small for filtering 60 Hz. The current limited transformer has a "weird" V vs I curve. It sounds like you are on the fringe with the transformer. Can you try a variac to increase the input voltage to the transformer?

 

[RDZombie]

thanks guys for your help. these are the specs for the tube:

Length: 450 mm
Power Output: 14~16W but tube says 15-18w
Triggering Voltage: 12 ~ 15KV
Operating Voltage: 7.8 ~ 9.3 KV
Operating Current: 12 ~ 15 mA
Outer Diameter: 34 (mm)
Power Stability: < 5%
Operating Mode: TEM00

I had tried upping the capacitor and it didnt make too much of a differance, slowed the "pulse" rate. currently i have it on a variac with a max output of 140V.
I have a few NST in the voltage range but with 10ma more current. i will give one a go.

 

[CHP]

Do you what material the output coupler and resonator are? Are your optics hygroscopic?

 

[RDZombie]

Do you what material the output coupler and resonator are? Are your optics hygroscopic?

i dont know but the HC and OC are sealed to the tube and are in direct contact with the cooling water so they couldnt be hygroscopic in nature as they wouldnt last very long.

Im building the tube into a "gasket printer" so i can cut my own engine/case gaskets for my bikes. Because this is only a ~16 watt tube and the beam will be pulsed (via an AOM) id like the output to be a s stable as possible. beam will be focused prior to delivery.

 

[Corona]

Good idea to use a NST instead, as these are designed to deliver the rated power at 100% duty cycle. There's a good reason the NST is much larger than the furnace ignition XFMR.

Use as many of those capacitors as you can possibly manage; the better your filtering, the more constant your output will be. I suggest placing a low value resistor in the cathode lead and using it as a current monitoring device - if you use a 1k ohm resistor, it will measure 1V per milliamp of tube current, and is MUCH safer than trying to measure the HV across the ballast resistor. A 1/2 watt resistor is adequate (225mW at 15mA).

Then you can set the ballast resistor to the correct value for the output of your DC source without risking life, limb, and the laser tube (human muscles can break almost anything when 15kV is applied).

Also keep in mind that the NSTs >10kV are often center tapped on the secondary, so you can use a 2-diode rectifier arrangement to advantage and have a good, solid ground reference to keep things safer (with a full wave bridge and isolated secondary, you can get in trouble very easily as neither potential is safe to contact). This is one reason the nastier NSTs have that CT secondary...

A 30kV 15mA NST, center tapped, with a 2-diode bridge should get you in the range you need to power this beast.

Good luck

 

[CHP]

RDZombie,

How is the performance of the laser tube. I was thinking about getting one of these.

 

[RDZombie]

Corona: Godd idea on the CT NST with the half bridge. never even crossed my mind and i so happen to have a rather beefy 25KV NST laying around. i will be sure to give that a go.


CHP; Its actually really not bad at all. At $150 BIN on the fleabay for a brand new tube i wasn't expecting much but the quality is quite good. Ill try to get a couple short vids of it in action and post em up. With just the rectifier and no filtering cap. or regulation its able to burn through both sides of a coke bottle in under 2 seconds at 12 inches and at 11 feet in under 10 seconds ( of coarse as with most of these types of gas lasers the beam divergence is pretty substantial)