Upgrading a K40 Laser Cutter, Part 2: Power Limiting and Precision Upgrades

Want to increase the life of your K40’s laser tube? How about more precise power control? Start here!

While the K40 laser cutter (aka an eBay/Alibaba laser cutter) is branded as a 40 watts, it really isn’t. For reasons I won’t go into in this article, it is closer to a 30W laser in practice. Unfortunately for us, pushing it to full power will quickly degrade the gas mixture in the tube and shorten the life of the laser tube. If you’ve got a large number of people using your laser (such as at a makerspace) or if you just want to increase the precision of your power knob, this article is for you.

This fun little hack (for once, this is actually a hack, heheh, none of that “life-hack” nonsense) has the added benefit of increasing how precisely you can set your power with an analog potentiometer by allowing you to use the full range instead of just a small portion. The cost and time for this upgrade is about $0.30USD and about 20 minutes.

To follow this tutorial, you will need the following.

Here is a list of everything you will need or might need. You don’t have to use my links if you don’t want to, but it would help me out and give me incentive to make more content 😉 I would never recommend something I don’t use myself.

Required:

• Resistors
  • The values are different between every machine. I’d recommend getting an assorted kit (like this one) if you don’t have one.
• Solder
  • Leaded or lead free, doesn’t matter.
• A soldering iron
  • I use a Hakko (not sponsored, just love em)

Recommended:

• Heat Shrink

The first problem:
In a nutshell, the laser power supply that comes with a K40 laser cutter will push out roughly 30-35mA which will force more power through the tube than it is rated for. The sad result is that the gases inside the laser tube will start to degrade if you push too much power through the tube. So, unless we want to buy a new tube every few months of heavy use, we’ll want to make sure to avoid max power. What we want to do instead is cap our power to roughly 16mA (ask the internet why).

The second problem:
We have an EXTREMELY limited range on the laser knob in which to set our laser power. Assuming we don’t exceed 16mA, the furthest I can turn my machine’s power knob (yours may vary a bit) is 40 degrees from the lowest setting (the left-most position). Conversely, the lowest I can actually turn the knob and the laser still fire is roughly 15 degrees from the left-most position. This means that I can only use a span of 25 degrees to set my laser power. Given the knob rotates around 270 degrees, this means that I am wasting 90% of the laser dial’s rotational range and losing a lot of precision.

Let’s talk about how the power adjustment circuit works and then go into how we are going to change it. If you open up the electronics bay of your laser and take a look at the power knob, you’ll notice it is basically just a potentiometer. Mine was labeled as “1k, 2W” meaning that the potentiometer had a total resistance of 1000 Ohms and that it could handle a total of 2 watts of power dissipation (power isn’t super important here…). There are three terminals that have wires tied into them – a blue, a red, and a green. What this tells me right off the bat is that the potentiometer is being used as an adjustable voltage divider. In other words, when you rotate the knob, you change the voltage of the red (center) wire. The laser power supply then reads this voltage and adjusts the amount of current it pushes through the laser tube, thus changing the output power of the laser.

Laser Power Pot

In short, the laser power supply uses a voltage control signal which is great news for us – manipulating voltage signals is extremely easy and only requires a few resistors to make it happen.

Okay, so what now? Let’s take some measurements. To start with, close up your laser and rotate the power knob all the way to the left (counter clockwise) until you hit the potentiometer’s internal stop. Hit the laser firing button and watch the ammeter. Chances are, it won’t move. What you need to do now is rotate the knob a degree at a time until tapping the laser fire button just barely fires the laser. On my system, this was around 2mA. Now open the electronics bay and measure the voltage between 0V and the center pin of the potentiometer. To get a 0V reference, you can either try to figure out which of the three potentiometer terminals is connected to 0V, OR, you can just touch the black lead of your multimeter to the chassis of the laser power supply; I had to make a small scratch to get a good connection. Measuring that center terminal of the pot, I got a voltage of about 0.5V (yours may differ). Write down your value.

Now, we do the same thing for the 16mA portion. Adjust the knob bit by bit until you find the knob location that gives you 16mA during a test fire. Open up the machine and take a measurement. I got about 2.2V on my system. Write down your value.

We also need to figure out the max voltage that pot can put out. This is pretty easy – Measure the voltage between the laser power supply chassis and the two outer pins. One should read 0V and the other around 5V (our max). Write down which wire is which.

For the last bit, we need to find the *actual* resistance of that potentiometer. Unplug the machine and unsolder the outer two wires from the potentiometer terminals (write down or label which wire goes where for when we resolder them with resistors later). Use your multimeter to accurately measure the total resistance and write this down as well. While it might say something like “1k” or “10k” on the pot itself, chances are the actual value is going to be 10-20% off from this and we need an accurate resistance for our maths in a moment. In my case, I read 1100 ohms, or 1.1kOhm.

Okay, so we have our measurements but we need to do something with them. To get started, let’s restate the problem in terms of the voltage signal coming off the pot. Instead of that pot going from 0V to 5V as it does now, we need the pot to go from 0.5V (our minimum firing voltage) to 2.2V (the 16mA spot). This means that we need to tweak two voltages – the lower voltage needs to come UP slightly and the upper voltage needs to come DOWN quite a lot.

More good news is that we can do both of these things by extending the voltage divider concept the pot already uses. By placing a resistor between the pot and the 0V wire, we can raise the lower voltage. By placing a resistor between the pot and the 5V wire, we can lower the upper voltage. We just need to figure out how large these resistors need to be.

To start with, let’s draw our diagram. Let’s call the first resistor (the one between 0V and the pot) “R1”, the total resistance of the pot “R2”, and the last resistor (the one between the pot and 5V) “R3”.

We can now use the basic voltage divider equation to get our values. This equation is: Vout = Vin*(Rx/Rtot). Vin in this case is our 5V and Rtot=R1+R2+R3. Our  equation now becomes Vout=5*(Rx/(R1+R2+R3)). From here, we start plugging in values and make two equations we have to use to find our two values, R1 and R3.

The first equation looks at the lower bound (Vmin): Vmin=Vin*(R1/(R1+R2+R3)). The second equation looks at the upper bound (Vmax): Vmax=Vin – Vin*(R3/(R1+R2+R3)). If plug in some variables, solve a system of equations, and do some hand wavy stuff to keep the blog post simple, we get our R1 and R3: R1 = (Vmin*R2)/(Vmax-Vmin) and R3 = R2*(Vin-Vmax)/(Vmax-Vmin).

Now, we just plug in values. Vmin=0.5, Vmax=2.2, and R2=1100. This gave me R1=(0.5*1100)/(2.2-.05)=323.5 Ohms and R2 = 1100*(5-2.2)/(2.2-.05)=1811.8 Ohms for my machine. Great! From here we have two options: we can either use the standard resistors values that are closest (in my case those would be 330 Ohm and 1.8 kOhm) or try to stack resistors together to be as precise as possible. Because my values were so close to the standard resistor values, I chose to go with the stock values. Your mileage might vary 😀

Solder your R1 (mine was 330 ohm) between the 0V wire (mine is green) and the terminal that wire was originally connected to and cover with heat shrink. Do the same for R3 (my 1.8 kOhm) and the 5V wire (mine was blue), covering with heat shrink. I’d recommend putting a piece of cardboard or something over the laser power supply to prevent solder from accidentally falling in and causing a short.

Close everything up, plug in the laser, and start testing! If you’ve done everything right, a test fire with the power knob all the way to the left should just barely fire the laser. Likewise, with the power knob all the way to the right, you should get 16mA.

Tada! You are done! You’ve limited the power of the laser to increase the longevity of the tube and given yourself more precision in the process! Happy lasering 😉