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Old 09-22-2010, 10:29 PM #1
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Default Powering a CO2 tube with a NST?

Hey, I've been considering buying a small 40W CO2 tube, mainly just to cut thin acrylic sheet.

The ratings for the tube are 15kV @ 18mA run, 22kV trigger.

I have a 15kV 25mA NST here not doing much, and it'd make the perfect PSU for this laser, however I have some questions.

Do CO2 tubes need DC or AC? Not really a big issue as I can make a 20kV fullwave rectifier for about $20.

What about the extra 7mA of current from the NST? At 15kV, thats about 105 Watts, which seems like it could be a bit too much for the CO2 tube. If so, how would I go about dropping the extra 7mA, or even making it adjustable?

I know gas lasers are pretty much a direct short to power supplies, and all NST's are current limited anyway, but that extra 7mA is a lot of power at 15kV.

Also, not to do with powering the tube, but if I wanted to use it to cut materials, would I need special lenses to give me a focal point etc?

BTW: NST = Neon sign transformer.

Cheers,
Dan


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Old 09-23-2010, 02:11 AM #2
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Default Re: Powering a CO2 tube with a NST?

Hey Things --- They will run on AC but you will need a ballast resistor -- likely 50 KOhms depending on your PS.. ALWAYS have cooling water flowing and a Variac to limit the run current.
There's a guy on here building CO2 lasers using what you need. I sent him a 15 Watt tube and he is running it on AC -- no problems.

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Old 09-23-2010, 05:27 PM #3
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Default Re: Powering a CO2 tube with a NST?

AC/unfilteredDC is more of a wear on the electrodes since the peak current is higher for the same DC current. It works fine, but it will shorten the life of the tube a bit. Also, the arc is extinguished every time the voltage crosses zero and must be re-ignited 100/120 times a second. The output intensity will follow the current wave-form of the input. That doesn't matter for proof-of-concept or general burning, but it may be a problem for precision cutting.

The sealed tubes are designed so that the gas circulates because of the current moving the gas in one direction. You might get reduced output with AC because of the CO that is created during the lasing process not having a chance to recombine to CO2 because the voltage can't move it out of the excitation path.

So in short, AC will work; but if you can, try to run it on relatively-smooth DC.
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Old 09-23-2010, 05:47 PM #4
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Default Re: Powering a CO2 tube with a NST?

As H_M said you will need a ballest resistor that should take care of the 7ma or 105 watts of power but of course you will have to get a resistor that can handel 200 watts of power, maby a couple 100 watt light bulbs will work here naw I dont think they have enough resistance.
you realy should make a full wave bridge rectifier for it though so your tube life won't suffer and then theres the cutting with ac that you realy don't want to deal with.
just my $.02

BTW be carfull a NST will knock your butt across the room
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Old 09-23-2010, 10:44 PM #5
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Default Re: Powering a CO2 tube with a NST?

Haha yeah I know, I don't go near the NST while it's running

Building a fullwave rectifier is fine, theres like 4x 20kV diodes on eBay for around $20. I have a few HV caps too.

Anyone dealt with these before? NEW 40W CO2 LASER TUBE CUTTING ENGRAVING WATER-COOLED - eBay (item 230490524231 end time Oct-19-10 09:26:17 PDT)
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Old 09-23-2010, 11:58 PM #6
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Default Re: Powering a CO2 tube with a NST?

There you go !!! Microwave ovens have HV components, diodes inside - Ususally just one but the junk yards are full of them likely for free. Be careful with NST's -- many have a grounded center tap or something to screw up the works.
I agree, DC is best for these but AC in a pinch works to get excited about GAS!!

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Old 09-24-2010, 12:10 AM #7
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Default Re: Powering a CO2 tube with a NST?

If only powering argons were that easy, still haven't managed to get the switchmode PSU I bought from timelablasers working yet
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Old 09-25-2010, 09:42 PM #8
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Default Re: Powering a CO2 tube with a NST?

Rectify the NST and put it on a variac, and use a small ballast resistor.
You only need a few tens of nanofarads to smooth out the 15 Kv,

The NST has a air gap in its core that will limit its current by forcing the core into saturation. By placing it on a variac you can find the "sweet spot" where the impedance of the output coil acts as a ballast, yet where the core is not saturated.

I don't know about NSTs in the UK, but in the US the secondary is center tapped and tied to case, so watch out for that. Its a safety feature, because when the tube is mounted in the metal sign, it means you only take half the hit if there is a problem.

About 40 1n4007s will make you a decent rectifier, but there is a trick to it. each diode in the string used to make the bridge gets a resistor or small cap across it to equalize the voltage across the diodes, forcing them to share the voltage, 470K to 4.7 Meg is typical, with higher values being more common. The other trick is to avoid running said homemade rectifier unloaded, this reduces the peak across it to the tube's forward fall, which will be say 5 KV when lit.

Some doorknob caps for filtering, you don't need many:

MEC, 09000/022, 209/04, 2500pF 20KV, doorknob capacitor - eBay (item 150183621531 end time Sep-30-10 13:50:15 PDT)

Be careful!

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Old 10-01-2010, 02:11 AM #9
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Default Re: Powering a CO2 tube with a NST?

Quote:
Originally Posted by Things View Post
If only powering argons were that easy, still haven't managed to get the switchmode PSU I bought from timelablasers working yet

Powering argons really isn't that hard. Most of the old power supplies were just linear regulators anyway. Rectify and filter the AC, then feed it through a bank of transistors set up as a series pass regulator and burn up the excess in heat. I've heard of people using heating elements to ballast small argon tubes, at least for testing purposes. You do need to deal with the igniter circuit as well.
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Old 10-02-2010, 06:49 PM #10
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Default Re: Powering a CO2 tube with a NST?

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Originally Posted by james1095 View Post
Powering argons really isn't that hard. Most of the old power supplies were just linear regulators anyway. Rectify and filter the AC, then feed it through a bank of transistors set up as a series pass regulator and burn up the excess in heat. I've heard of people using heating elements to ballast small argon tubes, at least for testing purposes. You do need to deal with the igniter circuit as well.
The problem with the resistor is the tube tends to oscillate. You need both a resistor and a pass transistor if you go that route. 3-5 ohms and 1700 watts makes for a odd sized resistor.

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Old 10-03-2010, 12:44 AM #11
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Default Re: Powering a CO2 tube with a NST?

The problem here is our mains is 240V, the tube only needs 100V or so, so I need a large stepdown transformer. Closest I can find to that locally is 2 500VA toroidal transformers, but they are $100 each. I do have all the components for the pass bank, as I was intending on going that route before I bought the switchmode PSU, but it's seeming like a huge task to find it's fault. Wish I could just send it to someone to have a look at



timelablasers has tried getting the circuit diagram for it, but as with most companies, no luck.

I had a feeling the rectifier could be dead, but it's burried under 2 PCB's, and those insulating PCB covers seem to be glued on with silicone, so I haven't been able to get them off. At least, not without breaking them

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