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co2 laser power supply

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HDF49

Mon May 27 2013, 08:18PM

Registered Member #977 Joined: Thu Aug 30 2007, 06:57PM
Location: England
Posts: 74

I'm planning on building a small co2 laser as part of my A levels, I have pretty much most things clear in my head other than the power supply, and I can't find a neon sign transformer at a price I can pay anywhere. The arc length will be roughly 400mm, and for the power supply I'm aiming for about 10-15kv DC at 500 or so watts. The problem is that the laser tube has negative resistance once the arc has struck, so I either need a current limited PSU, or a big ballast resistor, from what I've heard. As it is I have one very large AC flyback, a few medium sized DC flybacks and an 8 amp 230 volt variac, I know that the ZVS driver is more than capable of 500 watts, but is there a way of limiting the current output easily? Does the air gap i the transformer do this? And if i go down the ZVS route what value components will I need? I had my large AC flyback running at 500 watts for a bit then my driver broke, and I don't know why. I'd rather not use a ballast resistor; I don't want to waste so much power. I've seen quite a few half bridge flyback drivers based on a tl494 chip for pulse width modulation, which controls power, would that be sufficient for current limiting? Or would I need some form of feedback? Would rectifying the output from my large AC flyback cause any problems?

ANother option I've been thinking about it a microwave transformer, as a large one has plenty power and some have magnetic shunts which act as a limiting, from what I've heard, or I could use another as ballast, however I'd need to step up the voltage with about 4 stages, and I want to minimise high voltage components as that's when it gets expensive, and that could also end up with lots of current ripple. So I'm not sure about the MOT route, but I'd certainly get the power.

I basically need to produce about 15kv DC at around 500 watts with a way of stopping a runaway current across the laser tube, any help really appreciated.

HDF49

Wed May 29 2013, 08:54AM

Registered Member #977 Joined: Thu Aug 30 2007, 06:57PM
Location: England
Posts: 74

Anyone?

Daedronus

Wed May 29 2013, 10:13AM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

don't forget about the ignition pulse, that is far greater then the working voltage.
You need a constant current source, a ZVS will not do that.

Dr. Dark Current

Wed May 29 2013, 10:45AM

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

wind a large ferrite transformer and full wave rectify it (do not use any filtering capacitor). Wind it as a high-leakage transformer (primary and secondary on the opposite core legs), or use an inductor in series with the primary, or both. Then you can drive it with a transistor bridge and use the PWM to set output current, or simply use a variac on the mains side.

Shrad

Wed May 29 2013, 01:28PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

a TV flyback can power such tubes as a 10W CO2 laser

you have to use a ballast resistor and read sam's laser FAQ

the open circuit voltage should be enough to start the tube and as voltage drops with current being drawn, you'll fall right in the correct region

you just have to calculate this ballast value and starting voltage versus max current

HDF49

Wed May 29 2013, 01:58PM

Registered Member #977 Joined: Thu Aug 30 2007, 06:57PM
Location: England
Posts: 74

For the ignition pulse I plan to use my variac and then throttle it back to maintain the arc. Or an scr pulsing a capactor discharge through an ignition coil.

OK, thanks, will it being high leakage stop a runaway current? And what will the purpose of the inductors be? To provide a magnetic reluctance and slow the change of current? This is my transformer core, it's pretty hefty and I'm happy to experiment with winding secondaries, reckon it can provide the power?
35n3h8n

This is the driver I've been looking at https://sites.google.com/site/uzzors2k/ET_OfflineFlybackDriver.GIF, and put the mains through my variac to control the output as well as with the pot for duty cycle, is it a reliable circuit?

I've been trawling through Sam's laser page, it's an absolute gold mine! I just left a few questions unanswered. I'm aiming for between 10 and 20 watts, and taking in to account the efficiency of a co2 laser I thought 500 watts in seemed apt. I would like to keep the ballast to a minimum, as I don't want to end up wasting a lot of m power as heat, but It does look like the simplest option.

Thanks for the help

EDIT:
I've been looking at some bridge drivers, this is what I've come across
http://www.stevehv.4hv.org/FBD/FBschematic.JPG
If I were to omit the 2n3055 regulator and put the bridge straight of a rectified variac that should give more than enough oomph? Does the 555 provide good PWM too?

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