Looking for a new laser tube for your K40 laser machine? Check out Cloudray 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 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.
k40 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.
k40 ma meter installed

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 attach 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!

ma meter rear

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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Getting full scale deflection and yes its in the -ve side so faulty meter
But tips for others – to remove hot melt glue rather than risk heat on delicate components try a little isopropyl alcohol and the glue cones off.
First attempt at cutting the case I covered the psu with bags but a tiny shard of metal had found its way through and when I powered on – bang and had to buy a new psu
Be extra careful guys and if in doubt – dont


Updated to a 50mA ammeter, and it is pegged all the way to right after I fired a test shot. Does that mean the power supply is bad or try a different ammeter?


I connected the black negative wire from the laser to the ma-meter neg post then from PSU to the plus post on the ma-meter.


I attempted to do this to my k40 and the meter needle doesn’t move at all


Hello and thank you for this awesome tweak!

I would like to implement it but for some reason (probably to prevent me from implementing it) all the connectors in my K40 are stuck in hot glue…

I have identified the blue wire at the end of the the tube. Would it be ok if I cut it in the middle and “insert” the meter along the line, without having to disconnect it from the board?

Also, and more importantly, could it be ok to use wire connectors (as in the picture) to make the wire longer, if the blue wire turn out to be a little short to reach my meter?


Thanks, you made me think that I could use my soldering iron to get the hot glue to melt. It seems like I would risk making less damage since I could apply the heat more precisely. I also have the feeling that it would be best to do this with a very low heat setting in order to avoid damaging the soldering on the board.

Kåre Rasmussen

Hi All

Great guide thank you

I cut the wire from the Anode of my K40 laser (Front end) and put my amp-meter in series. The amp meter is 0-30mA. but when firing the laser, the amp meter maxes out already at a setting of 5%. Am I missing something, or is my Amp-meter faulty?

Kåre Rasmussen

No not on the red wire :-), on the Black wire that is crimped with a blue that goes to the L- on the power supply. I tested with a known 20mA current I also maxed out, so I guess it’s broken

Kåre Rasmussen

Hi again, I got the new one today, installed it, and it works as expected. But thank you 🙂

A followup, what is the Max mA I can trow at it (standart 40W K40), and what is the max water temp before I need to stop it for cooldown?

Kåre Rasmussen

Great thank you. Right now I have 10L of water at room temp, so I gues thats just small runs for me 🙂

Jeff Scroggins

Can you advise of a link on Amazon or Ebay for what type of wire to use? When I search Wire, 1,5 to 2,5mm2 im not coming up with anything.


Sorry if this question has been answered elsewhere, but if you upgrade to a 50w do you also need upgrade to a 50w power supply?

Kyle Yochum

Hey HP,

This site has proven invaluable to me in my learning process. Thank you.

Regarding your note where if I am to mount the mA Meter on the control panel cover I understand I should take care to insulate – could I ask you to elaborate a bit?

My question is should I be insulating the connections/terminals on the backside of the meter? Or should I insulate the mounting screws with like rubber washers?

Thank you! Also, would you be open to me sharing photos if I follow any of your guides? I don’t need anything in return, I’d just like to give back. I’m on your site every day reading and re-reading. Amazing work.

thomas wallace

Your instructions are very clear. I thought i followed all and double checked the wiring all secured. However when I turned on the k40 the digital display keeps fluctuation from 0 through to 90 over and over without stopping.
i tried a laser test and the analog meter went to 20.
what can i do to correct this.


Great guide. Works fine! Thanks a lot!


Do you a shunt with your amp meter? Or don’t you need one.


Hello is it possible to install 50ma miliamperometertok40 laser?

Thank you.


Mus tit be an analog meter or can it have a digital readout?

David Sherman

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?


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?

Joel Pereira

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?

Bradley Garms

My new K40 is wired like the guy above L-1 to ground block on back wall of power supply compartment and the tube iis wired to the FG terminal. So that being said am I actually wiring between the L-1 terminal and the FG terminal or are you saying to rewire the laser machine to your old specs? This K40 has the all digital display with buttons to adjust power no potiometer like originals. I just want to be sure what you are saying before I wire something wrong and blow up my machine.

Thank you for this web site it has given me a lot of good info as I am brand new to the Laser arena.

Bradley Garms

I read the grounding guide, thank you again. I will remove the paint to make sure the case has a proper ground and I will swap the L-1 and FG wires before installing the ma meter per your advise.
I need to ask you another question concerning the wattage of my machine because from what I have seen on everything I am watching and reading I believe my laser is a 30 or 35 watt machine. The Laser tube is 635mm long not 700mm long and is 50mm dia. Am I correct in what I have told you? And as I am sure the seller will probably fight me on this so my next question is where do I find replacement 635mm laser tubes? I have looked on line for hours and found no one that carries them. But there is a removable plate on the end of the machine in the laser tube compartment I believe this is to allow for me to upgrade to a full 40watt 700mm x 50mm laser tube if so will my existing power supply work with the bigger tube?

In my opinion you should have been a Priest or some kind of clergy because I have read your site backwards and forwards including the questions and your answers and my friend you have the patience of Job and you are also a great instructor because your explanations can be understood by the novice or expert. I hope you can shed some light on my new questions so I can either kill myself or proceed. Thank you.

Bradley Garms

Ok Ma meter is hooked up. Question, will digital push button controld adjust the ma up and down? Question, I am the one with the 635mm tube, what ma would you recomment me to run on that so I dont burn out tube, you said 16ma above but isn’t that pushing it? I haven,t recieved my new submersible pump yet so is the one that comes with the laser unit a submersible pump that I can use for a few days? Air assist is hooked up. Also waiting for new inline exhaust fan so will use the one with machine to start. I just want to get lens cleaned and aligned and see if this thing works. Thanks again

Bradley Garms

This unit has a temp probe hot glued to the power supply, do you have do not exceed temp for that , I have looked but haven’t seen it any where, figure i have it might as well monitor that also. And how much anti freeze to how much distiller water? I will be using a 5 gallon bucket to start but will be shifting to a small ice chest as it will keep the water cooler longer and keep ice bottles longer I think.

Guy Snyder

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


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?


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.

Steve Tagg

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.


Steve Tagg

Great! Thanks for that.


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.


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?


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


My ma meter didn’t have – marked on it. What would happen if its wired the wrong way round

Kay Ceynowa

Danke ,,, ganz tolle Anleitung !!!
Hat sofort funktioniert.
Wer sich an Ihre Dokumentationen und Regeln hält, macht nichts verkehrt und hat viel SPAAASSS mit seinem Laser.

Gruss aus Lübeck …. K.C.