Install a mA-meter on your K40 laser

Install a mA-meter on your K40 laser

Adding a mA-meter to your machine is one of the most important modifications you can do on this machine.

The mA-meter will read the power trough your laser tube allowing you to set the power to a safe value to prevent the laser tube from over powering and thus saving the life span of the laser tube.
Read more here about the most common reasons why your laser tube dies.

Tools needed

– Wire stripper, cutter and crimp tool.
– Pliers or small wrench for nuts on the meter
– Drill bit and jigsaw to cut the hole in your panel.

Parts needed

– DC mA-meter, 0-30mA – you can get one in my store here | Or trough Aliexpress here
– Wire, 1,5 to 2,5mm2
– Wire crimps (ring type)

How to do it

This is how the power wire from your power supply is routed on your machine without a mA-meter.

The negative cable coming from the left part of your tube (standing in front of the machine) and is connected to the L- on the power supply.

This is what we want to achieve

The negative wire comes from the tube to the input on the meter, and output from the meter going back to the L- on the power supply.

The easiest way to do this is to mark where you want the meter in your lid, take measurement of the size of hole needed (different for all meters) and drill/cut the hole, or make a external holder for the mA-meter on your lid.

The important steps
Before doing anything, remove all power cords and let the machine sit at least 30min before even thinking about touching something close to the power supply.

Step 1: Remove the L- wire from the power supply and route it up in the lid where your mA-meter will be located. Cut the wire so you have enough to crimp a connector on it without putting too much tension on the wire.

Step 2: Crimp a ring crimp connector on the wire, and attatch it to the + marked input on the back of the mA-meter. Make sure to tighten the nut, add a dab of hot glue/super glue to prevent it from vibrating loose. Caution! If this wires comes loose it may create sparks, arcing or even fire due to the increased resistance if the connector isnt properly fastened, so take care to make sure both are properly crimped and fastened!

Step 3: Now check if the piece of wire you cut off earlier is enough from the mA-meter down back to the L- on the power supply. If not, cut a new wire with the length you need.
Add a crimp connector on the end and fasten it to the mA-meter.

Step 4: Install the wire coming from the mA-meter back on the L- pin on the power supply.

Step 5: Please, go over all your connections once more. Make sure the crimps are good and everything is fastened properly. Also check that the terminals on the mA-meter is well insulated if you have the meter above the lid. This is no joke, touching this will kill a grown man straight up!.
I suggest you add some electric tape on top of the crimp connectors afterwards, or seal them in hot glue to make sure nothing can touch them in your machine and causing a short.

You are now done!

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Comments (19)

  • Venomouse Reply

    Followed instructions and didn’t die. Would recommend. Great tutorial

    2018-06-25 at 05:43
  • Roland Reply

    Have a question. Your drawing shows the wire from the tube going right to the PS. On my machine the wire from the tube goes to this tube/coil on the side of the machine then to the PS. Is this some kind or resistor?

    2018-08-12 at 18:48
    • HP Persson Reply

      Yes, some older types of PSU´s have a extra green cap mounted inside the machine, if it´s routed to this first, the meter should go between the tube and the cap/coil thing.

      2018-08-12 at 18:55
  • Dave Reply

    Question, would it be beneficial at all to attach a ground wire to one of the mounting screws?
    I am waiting for my meter to be delivered and I will be attaching it to the lid so all connections will be internal.

    2018-09-13 at 17:41
    • HP Persson Reply

      No, run a solid wire from output on the PSU to the mA-meter, and the other one to the tube. They are grounded inside the PSU but i think there is a sense on it, if you ground it somewhere else the PSu might stop firing the tube.

      2018-09-13 at 19:33
  • Steve Tagg Reply

    I intend to fit a milliampere meter as you suggest – it makes sense. I have ordered the meter from you and was wondering wondering about the type of cable to use if the cut off cable is not long enough. I presume it must be insulated to withstand more than 20kV?

    Thanks for your very informative site.


    2018-09-16 at 17:48
    • HP Persson Reply

      Negative lead does not have much voltage, it´s “consumed” by the tube. A 1.5mm2 cable is good enough. Your machine probably has one already in that size.

      2018-09-16 at 18:20
      • Steve Tagg Reply

        Great! Thanks for that.

        2018-09-17 at 15:42
  • carloss2305 Reply

    I have followed your instructions and installed the mA meter. It works fine. Good to see I have not been overpowering my laser!
    Next mod. is smoke assist (I already have air assist).
    Thank you for your very helpful and informative site.

    Steve Tagg.

    2018-09-28 at 20:11
  • Klaus Reply

    Great Idea! Newbie question: But how do I control to not over power my tube? Do I need a potentiometer or a knob or something like that, to deal with the power shown on the mA-Meter?

    2019-01-19 at 20:02
    • HP Persson Reply

      Most lasers has a potentiometer or a digital panel controlling the power settings. If you don´t have one, you need one to set the maximum output to not over power the tube.

      2019-01-20 at 12:48
  • Guy Snyder Reply

    Is the process any different when installing the meter on a k40 with digital controls?

    2019-03-07 at 15:49
    • HP Persson Reply

      It´s the same

      2019-03-07 at 20:44
  • Joel Pereira Reply

    Your drawing shows the wire from the tube going to L- pin
    On my machine, the wire from the tube goes to FG pin; The L- pin is connected to the ‘ground wire joint’
    Are the L- pin and FG pin connected?
    And when installing the meter, should I install the meter between the negative end of the tube and the L- pin?

    2019-07-23 at 19:16
    • HP Persson Reply

      I would suggest you are changing it to how it´s supposed to be connected. FG and L- are connected internally but they should not be connected like that.
      Install the meter between negative cable from the tube and L-, then you know everything is up to code. Some PSU´s has a controller on the L- to measure some stuff – so its always best to go with the standard, even if the FG works technically.

      2019-07-24 at 08:07
  • charles Reply

    i have a K40 with digital readout that shows the useless % instead of ma. I want to do this mod, but what happens to the digital % readout once i switch to this. this is left out in the instructions. how do the ma meter and the digital interact with each other?

    2019-07-29 at 21:56
    • HP Persson Reply

      They dont interact, but you can still use the digital readout for quick power setting. Example if you cut 3mm arylic at 25% at the digital readout and the power output on the mA-meter is within safe levels (not over 15-16mA) you can use that as a guide to set the power easier.
      Use the mA-meter as a safety check, so you never go above 15-16mA to spare the life of your laser tube. You can go 18-20mA for a short short run but it´s not advised to do so.
      The mA-meter just reads the real power output of your tube – the digital panel sets it.

      2019-07-30 at 09:45
  • David Sherman Reply

    It looks like my ps has 5 wires running in and out of the back one main pos to the tube the ground for the tube comes straight from the wall power jack then there is a green block labled as follows FG:AC:AC:L- I thought I would hook the amp meters from the L negative to the ant meter then back to the El negative block this did nothing I also tried to put a passive magnet type milliamp reader are the negative lead from the power block to the main power and that did nothing. I don’t want to cut the wire coming from the power block that is going to the tube just to install a power meter any suggestions?

    2019-08-24 at 21:54
    • HP Persson Reply

      You need to put the meter between the power supply and the negative cable going to the laser tube, it doesn´t matter how you do it as long as its in between.

      2019-08-25 at 20:17

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