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ObjectiveTo construct a powerful CO2 laser using commonly available / ebay equipment. | ||
StatusDone, upgrades still possible. | ||
How They WorkA CO2 laser is a tube (in this case sealed) with a mirror on both ends, one of which is semi-transparent. Inside the tube is CO2 gas and mixture of other gases to lengthen the tube's life. This low-pressure gas is excited by high DC voltage between the two mirrors, and the beam escapes through the semi-transparent mirror. Around the central gas tube, there is another tube "jacket" where coolant (water) flows to remove the heat from the tube. This keeps the tube from breaking and also boosts efficiency. Another method of cooling for non-sealed tubes is a constant recirculation of the CO2 gas.Light output is 10,600nm (nanometers), which equates to 28,282.3 GHz, or 28.2823 THz (terahertz). This is in the far-infrared area of the electromagnetic spectrum. The IR beam is invisible to the eye, and will be absorbed by almost any material. If the energy absorbed is higher than the material is able to dissipate, that material will melt and burn.   | ||
Power Supply Basics & DesignProducing high voltage DC isn't terribly hard or simple. The basic version would be to connect 120VAC wall voltage to a step-up transformer to increase voltage, and then into a rectifier to make it DC.Step-up For my implementation of this, I use a variac to vary the 120VAC from 0-120%, for a maximum output of 144VAC. This is a heavy-duty beast of a variac, rated at 20A. The output is connected to the primary of a step-up transformer (from a large neon sign) that has a turns ratio of 1:125 (rated 120V in, 15,000V out). When the variac is set to 120%, the secondary (output) of the transformer will be 18,000VAC:  Rectification Now that we have a high AC voltage, we can rectify it to produce DC. A basic "half-wave" rectifier will simply eliminate the negative half-cycle of the AC voltage waveform. The graphic below shows this (notice it only has positive cycles of a sinusoidal wave; DC, but poor). A "full-wave" bridge rectifier inverts the negative half-cycles to get double the positive cycles of the first rectifier.  To quantify this DC output, we can geometrically find the average positive area. Doing so gives us the following formulas for determining the average DC output of any rectifier. (Note the the AC input voltage needs to be in peak form, so the formulas convert from standard RMS measurement to peak as well.)  Using those calculations, the 18,000VAC will be converted to 8,103VDC (average) after half-wave rectification. With a full-wave rectifier, the DC output will be 16,205VDC (average). The voltage drop of 0.7V across each diode is negligibly small. For some reason, I could not get the full-wave rectifier to work at all! Rectifier Construction There are completed rectifiers available for purchase, but I wanted to build it from scratch. For this I looked at a basic rectifying diode with relatively high ratings: the 1N4007 diode:  The current handling is well within our needs, but the 1N4007 diode can only handle 1000V across it. This is called the Peak Inverse Voltage or PIV. We tie many of them together in series in order to 'up' this rating. With my half-wave rectifier, I used 16 of the diodes to make sure I wouldn't exceed the rated voltage. Unforunately I don't have any way of measuring this voltage. | ||
Cooling SystemAs mentioned at the top of the page, there is a "tube around the tube" where water or coolant can flow to cool down the CO2 gas chamber. This is also encased into a 3rd tube where it is suspended. The pre-made tube has connections on both ends which allow for cooling system connection. For my laser, I bought an aquarium water circulating pump. That is stuck into a water basin and flexible clear tubing connects the output to one end of the laser tube. The outlet end comes back and drips back into the basin. This constant flow keeps the tube cool enough, but for more demanding applications ice could be added to the basin, or it could go through a fan/radiator instead of a basin. | ||
ResultsWell, it lights thing on fire, even a good 30 feet away. It instantly cuts through materials like styrofoam, and slowly cuts through wood. With a bit of focusing (discussed in the next section) the useable power will be much better. More details later. | ||
Upgrade: Focusing & OpticsThe beam leaves the laser tube with a diameter of about 6-7mm. Thats about 38.5 mm squared of area. This is the only way I've ran the laser so far, but I've ordered a focusing lens. This is a special lens made of Gallium Arsenide so that it will pass and not absorb the laser energy like most 'transparent' materials. Focusing should dramatically increase power density, so cutting will be much improved. Via the lens, I should be able to focus that 38mm squared beam to less than 1mm squared; thats an increase of about 40 times more power density! The only further problem will be removing burned material to allow the laser to keep penetrating further material in order to keep up cutting speed.Upgrade: Rectifier FilteringThe wavy outputs of the half-wave and full-wave rectifiers are considered poor quality DC. Even though we averaged the voltage to come up with an actual quantity, it would be beneficial to actually average the waveform out with a filter. All that is needed for a simple filter is a capacitor. First a half-wave filtered rectifier is shown, then the full-wave version. You can see the smoothing different:  | ||
Additional DetailsThe most in-depth wealth of CO2 and all laser knowledge can be found at SAM's LASER FAQ.1N4007 Rectifier Diode Characteristics   Electromagnetic Spectrum 
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