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CO2 Laser Power Supply

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LAN_403

Aussienitro

Sun Jan 10 2010, 04:18PM

Registered Member #2399 Joined: Mon Sept 28 2009, 09:29AM
Location:
Posts: 27

Hello,

I'm thinking of buying a "cheap" laser tube off ebay to play with, however I'm unsure how to power it (short of buying a dedicated laser power supply).

Lets take the example of a 40W tube with a triggering voltage of 22kW and operating voltage of 15kW, does this mean I can feed it with 15kW 50Hz pulsed DC from a half wave CW multiplier, then have a push switch to "inject" 22kW (from an additional stack on the CW) to fire the laser up? If not what is actually required?

Thankyou.

Dr. Dark Current

Sun Jan 10 2010, 06:03PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I think the laser is usually powered from a current limited PSU which has its open circuit voltage high enough to strike the tube, and when the tube strikes, the PSU powers it with current rather than voltage (correct me if I'm wrong).

For 40W@15kV, you could use a normal TV high voltage transformer with a suitable driver.

...

Sun Jan 10 2010, 07:06PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

CO2 tubes are 'only' about 10% efficient, so you need about 500w of drive power, which is a bit much for a tv flyback.

In any case, the proper method to drive a CO2 tube is with a constant current source, which will 'float' up to 25kv or so to trigger the tube, but limit the current output to the proper tube current. The easiest way to do this is with a NST that is full wave rectified (although for the higher voltage tubes it becomes hard to find a NST with a high enough output voltage).

You can run these tubes in a pulsed mode, in fact some tubes work best with a ~95% duty cycle on a ~1ms peroid, for reasons that escape me.

As to the push-to-start button, keep in mind that if the tube is running pulsed you will need to start it on each pulse.

LutzH

Mon Jan 11 2010, 03:15AM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hello:

The tube wants DC, so a simple NST with a FWB rectifier will work fine. Keep in mind 50-60Hz NST are rated at RMS not peak voltage. So a 15KV NST will give you about 22KV rectified and filtered. You can run this tube with a 12-15KV, 30ma NST. Just do not overdrive it and keep it cooled, I have a nice 10KV 80ma transformer from an old medical laser, which I converted to a bench top HV PS.

The commercial power supplies use an HF switch mode power supply with a voltage multiplier circuit. The VM is a 2 part design. First is the VM which is rated at the normal operating voltage of the tube. Next the VM is extended with a few extra stages, designed only for HV with smaller caps. and not for full current.

Both VM outputs are then connected to the laser tube. Initially it requires the full voltage of both sections of the VM, but then as the tube ignites, the current all flows from the fist VM section, automatically switching out the additional section by cutting its power, due to it now flowing through the laser tube via the lower stage of the multiplier.

Hope this helps.......

Erlend^SE

Mon Jan 11 2010, 03:42PM

Registered Member #1565 Joined: Wed Jun 25 2008, 09:08PM
Location: Norway
Posts: 159

I am not sure about this at all, but I think a xenon bulb ballast may do as laser driver.
It starts the bulb with high voltage, then use power-control or something afterward to control it.

Probably not a cheap option, unless you can get a old ballast.

Electroholic

Mon Jan 11 2010, 08:38PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

Or, just make design the VM so that the operating curve will hit ignition voltage at low current and then drop to optimal operating point.

BTW if you limit the current after ignition to a few% of the rated full current, you can keep the tube ionized and it will not lase. Kind of like a standby mode.

LutzH

Mon Jan 11 2010, 09:14PM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hello:

Yes a xenon power supply will ignite the tube, but then it will fail to sustain the discharge, because xenon lamps run at a DC voltage of 50-150 volts, the HV is for ignition only.

There are so many designs on the net for CO2 power supplies, just do a search and you will find many variations of the above discussed basic concepts. The easy route is using a 12-15KV NST with a rectifier and a filter capacitor. Use the old fashioned magnetic transformer 50-60Hz kind, not the new HF ones. If the rectified / filtered 22KV from a 15KV NST is not enough, then just add a single stage VM to boost the voltage to get ignition. To do this you:

Add a FW rectifier to your NST, with a filter cap. Connect this output to your tube. If it does not ignite, then add a single voltage doubler stage, just for ignition. Do a search under CO2 laser power supply, and you will find several examples, with schematics etc. So many folks have done this, so why try to re-invent the wheel, its already done for you on the net.

Yes these NST's are heavy, but its so simple, and since the transformer is current limited, no exotic protection circuits are needed, just use a simple power resistor, to limit the filter capacitor discharge current spike. The parts are easy to find. If you need capacitors I have some new 35KV .003mfd capacitors which I can offer you for $6.00 each with free shipping.

Take a look on line to better decide on which drive method you want to use, then just order the parts on ebay. If you decide on the NST approach, you can get these locally from a neon supply store to save on the shipping. Remember you want the old style just like the ones you use for tesla coils. A CO2 laser tube is just like a neon sign in an electrical sense, except that the ignition voltage is a little higher for a given tube length, and it used DC.

It will even lase on AC, but the output will be pulsed at 60Hz, and I am told that it is not good for the tube electrodes to run it on AC, some folks who use AC may disagree on this point.

Happy Lasing.....Lutz

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