How to Test and Set a Chinese (China) Made CO2 Laser Power Supply to Get Long Life From Your CO2 Laser Tube for Your Laser Engraving and Laser Cutting Machine System

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Introduction: How to Test and Set a Chinese (China) Made CO2 Laser Power Supply to Get Long Life From Your CO2 Laser Tube for Your Laser Engraving and Laser Cutting Machine System

Be safe when performing this project.

SAFETY IS MOST IMPORTANT.  IF YOU DO IT WRONG THEN YOU CAN BE ELECTROCUTED.  THE HIGH VOLTAGE AND POWER COMBINATION CAN BE LETHAL.  FOLLOW THE INSTRUCTIONS BELOW. DO NOT DEVIATE FROM THE INSTRUCTIONS REGARDING SAFETY.

The high voltage laser power supply settings can greatly affect the lifetime of the CO2 glass laser tube.  A setting above 30 mA will, in almost all cases, greatly diminish the operational life (changing an expected life of several years to months).  Even though the laser tubes are ranked in months, such as the 60 watt tubes, I have had customers use them for years before they finally loose power.

More recently, the higher power CO2 glass laser tubes, above 80 watts, and certainly 100, 130, 150, and 180 watts have been more inclined to have shortened lives because the power supply is improperly set.  For example, I had an unscrupulous seller who sold me a 150 watt laser with a power supply set at 34 mA.  My Mahoney laser power probe measured 145 watts.  Then, I contacted the seller because my multimeter read 34 mA.  His engineer said the power supply should be set to 30 mA for full warranty on the tube.  After setting to 30 mA, I measured the laser output power and got 129 watts.  Then, I contacted the company and returned the laser because the seller had actually sold me a 130 watt laser with a 150 watt price tag.

Whatever your reason for checking the power supply current, the steps are provided to demonstrate a safe manner to take a reading then take action to remedy.

Step 1: Do Not Touch the Red Wire.

1. Power off laser machine
2. Wait ten minutes. this is the time for the power supply to drain power from its electrical capacitors. 
3. Setup as shown in photos below.  You will need to purchase and use two sets of wires.  We recommend a black wire with aligator clips on each end and a red wire with aligator clips on each end.  These wires should be available at radio shack.

          A. See the power supply wire connection.  Do not touch the red wire.  It has extremely high voltage that is lethal.

          B.  See the thin wire.  It is colored blue, or black, or green as shown.

Step 2: Insert Meter Wires to Measure Current (mA)

C. See the meter below as an example meter.  Some meters are slightly different, so adjust instructions as needed.  Set meter to read current at mA.  Set meter to read DC current.  Insert red cable of meter into meter connection for current (labeled mA, or micro A, or A).

Step 3: The Meter Reading Should Show the Proper Units (mA) and a Zero Value

Step 4: D. Disconnect Thin Wire (colored Blue or Black or Green) From the Laser.

Step 5: E. Connect Your Wire to the Laser (aligator Clip Is Useful on Return Wire).

Step 6: F. Connect Your New Wire to Meter Red Wire. Use an Electrical Insulator, Like the Wood Block Shown, to Support the Wire at the Connection.

Step 7: See the Solid Electrical Connection. See Its Location, in the Center of the Wood Insulator Plank.

Step 8: G. Connect the Thin Wire From the Power Supply (colored Blue/black/green) to the Black Wire of the Meter Using Your Wire (alligator Clips Are Useful).

Step 9: More Cable Connections Located in the Center of the Insulating Board

Step 10: Confirm Electrical Connections. Take Reading.

Check and confirm all connections and review these instructions.  After you confirm that all connections have been completed properly, then power on the laser machine. 

Using the touchpad, select Menu and set the laser power to 5% and the time to ms.  Power on the the laser from the touchpad and read the meter after the number settles.  To confirm that you have no problems, there will be no sparks or electrical arcing.  If sparks or electrical arcing then stop all work, power off the machine.

If the meter provides a reading and no sparks or arcing, then proceed by setting the laser power to 100%.

The current measurement at maximum output power should be 28 mA for a long life setting for a standard Chinese CO2 glass laser tube.  If the value is higher (such as more than 30 mA) then the internal gases will be consumed much more quickly during operation and the glass laser tube will need to be replaced more often.

Step 11: Adjusting the Potentiometer to Reduce Current

To adjust the current downward, consult your power supply manual.  DO NOT open the laser power supply enclosure and insert your fingers.  Using a grounding wire, ground everything when working on a power supply.  I was shocked six hours after power off, by touching the underside of the circuit card inside the laser power supply.  Always use a wire connected to ground to remove any lingering voltages stored in the capacitors inside the laser.  In many cases, the enclosure does not need to be opened and the potentiometer can be adjusted.

There are times when you might want an odd-value resistor. Rather than run out to the store to buy a 3,140 Ω resistor, you can get there with a good ohmmeter and a willingness to solder things in series and parallel. But when you want a precise resistor value, and you want many of them, Frankensteining many resistors together over and over is a poor solution.

Something like an 8-bit R-2R resistor-ladder DAC, for instance, requires seventeen resistors of two values in better than 0.4% precision. That’s just not something I have on hand, and the series/parallel approach will get tiresome fast.

Ages ago, I had read about trimming resistors by hand, but had assumed that it was the domain of the madman. On the other hand, this is Hackaday; I had some time and a file. Could I trim and match resistors to within half a percent? Read on to find out.

Metal-Film, Through-Hole Resistors

Your run-of-the-mill through-hole resistor is a metal-film resistor, made by depositing a thin layer of metal onto a non-conductive ceramic cylinder. The metal film is cut into a helix, and the length, width, and thickness of the resulting metal coil determine the resistance. Since the deposited metal is so thin, between 50 nm and 250 nm, you might think that trimming this down by hand is going to be a bit finicky.

Jumping straight to the punchline, when I was trying to change the resistance by small amounts, maybe less than 5% or so, it was trivially easy to land spot on the exact desired value. I had bags of 1 kΩ and 2 kΩ 1% resistors, and I figured I would make a whole bunch of mistakes while learning.

The reality is that I went over the target once out of seventeen attempts, and that only by one ohm. The rest of the resistors are trimmed as well as I can measure — down to the single ohm. (My meter and probes have a 0.3 Ω offset, but there’s nothing I can do about that.) I pitched the “bad” one, made one more, and had a perfect set in short order.

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Here’s the whole procedure. I put the resistor into some insulated clamps, and clipped my ohmmeter to either end. I used a small round file, and just went at it. The first few strokes get you through the relatively thick coating, but once you see metal, or notice a blip on the ohmmeter, a very light touch with the file is the rule. Maybe blow some of the metal dust off between strokes when you’re getting close, but I didn’t notice that it made much difference. Seven or eight light strokes with the tiny little file brought the resistors to a ten-point landing.

Indeed, because it’s easy to go too far at first, I found that ideal candidate resistors to file were the 1,990 Ω ones. Many of my 1 kΩ resistors came in at 999 Ω, which makes it hard to get through the casing without overshooting the mark. I probably could have just left them. The good news is that most 1% resistors will be off by more than a few ohms in either direction, otherwise they’d be sold as 0.1% resistors. And of course, you need to pick source resistors with a lower resistance than the target — you’re not adding metal with the file.

So you only need to have one value of resistor in your kit, right? Absolutely not. Creating a 1.2 kΩ resistor from a 1 kΩ original is asking for trouble. I got it to work a few times, again down to the single ohm, by restarting the filing process in a different place rather than simply going deeper in one hole, but I don’t recommend it, and I can’t think of when you’d need to. Just add a 200 Ω resistor in series and trim that. Remember that you’re thinning down a metal spiral that’s only 100 nm thick to begin with. Easy does it.

Surface-Mount?

Filing down through-hole resistors to exact values was so much easier than I had anticipated that I decided to take on something harder. I tacked a 2.1 kΩ resistor onto some stripboard. Wouldn’t you know it, it read out exactly 2,100 Ω, so 2,105 Ω became the target. That didn’t go well at all; I ended up with a 2,722 Ω resistor faster than I had expected.

The second started out at 2,103 Ω, and I just went at it without a goal in mind. By going very carefully, I got it’s resistance down to 2,009 Ω before it jumped to 2,600 Ω and beyond. Lowering the resistance doesn’t make sense at all. Maybe I was dragging some solder into the gap and effectively thickening the metal layer? I went looking for information, but didn’t get any further into the construction than Vishay’s datasheet: “metal glaze on high-quality ceramic” which doesn’t enlighten much.

After two more attempts, I couldn’t get the SMT resistors in trim at all; the layer of deposited metal is just too thin. And anyway, I’m not sure how useful it would be — the thought of soldering and de-soldering seventeen of these isn’t very appealing.

Conclusion

Trimming through-hole resistors is awesome. I made a complete set of matched better-than-0.05% (!) resistors for an 8-bit DAC in half an hour with nothing more than a file and an ohmmeter. And on my first try. You could easily make a 10-bit DAC this way. The result was an order of magnitude better than I had hoped, and it wasn’t hard at all. Amazing. And nothing says cool like a hand-made, artisanal DAC. (For odd values of cool.)

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