Rewire your K40 laser cutter machine

We have all seen it – strange cables going from anywhere to nowhere and back again.
The K40 is a potent little machine and with this guide we show you how to rewire the machine from the ground up with every detail you need.

Parts and tools you need

– 0.75mm2 cable for the small 12 and 5V connections
– 1,5mm2 cable for the high voltage wiring. Make sure to choose something rated for at least 250V.
– Wire strippers, cable crimpers and screw drivers.
– Crimps, spade and ring terminals.
– Cable sock or cable protector from nylon or similar material.
– Soldering iron – for better connection on the crimps.
– A cold beer or other drink
– EMI-filter – highly recommended!


Take out a pen and a piece of paper. Make notes of your current setup what cables goes where. Not all K40 laser cutters are the same and you can have one of them with strange wiring inside.
Only use your notes as reference as this guide will tell you how to rewire your machine properly and in a safe manner.

Take some photos of the cables and save it, if you do something wrong it´s good to have if you need to revert back to your old setup.

The K40 laser cutter machine has two types of PSU´s
We will describe both types in this guide as they are very similar.

psu type 2 enhanced potpsu type 1 enhanced pot

First step – laser tube and high voltage

Before doing anything – make sure the power cable has been pulled out from the machine at least a few hours.
Laser tube check: Follow the negative cable from the laser tube to the PSU – it should be located on the L- connector. If not – move it!

In the rear of your machine you have a bolt going through the metal with the ground wire connected from the C14 inlet connector.
c14k40 ground bolt martin samuels
Remove the ground bolt and remove the cables from the C14 connector.
Clean the metal around the ground bolt and mount it back without any plastic washers, you need good proper connection between the bolt and the metal case.
k40 ground bolt fix

High voltage wires

Here is my example of a high voltage rewire kit with a EMI-filter.
The lose ring terminal goes to the ground bolt shown above.
k40 replacement wire loom
The length of the cables depends on your machine and your PSU location. Take a piece of cable and measure how long pieces you need.
In this example the length needed from the rear C14 connector to the lid power switch is 40cm, and from the lid to the PSU is 30cm.

Types of crimp terminals used

fer 8 12c 8 awg wire ferrules insulated red reverse
Wire ferrule crimp connector
cfr4na187f red fully insulated female disconnect 3 16 inch
Female “disconnect” type terminal (suggest a better word in the comments please 🙂 )

Wires needed

2x of 40cm wire (black and red, or other colors just make sure you know which is which)
3x of 30cm wire (black and red)
1x of 70cm wire (yellow/green for ground)

Crimps needed

2x 30cm wires needs to have the female disconnect crimp in one end and ferrule crimps in the other end.
2x 40cm wires needs female disconnect crimp in one end and the other end connected to the EMI-filter live pins.
1x 70cm ground wire with the ferrule crimp in one end and the other end connected to the EMI-filter ground pin.

Chart over the cable kit we just made

Installing the cables.

The two 40cm wires and the 70cm ground wire is connected to the EMI-filter (which is connected to the C14 outlet connector).
Route them together with the 70cm ground wire inside a cable protector/sleeve cut to length. The ground wire should stick out at least 30cm from the other cables.

Take the two 30cm wires and route them inside a cable protector/sleeve cut to length.
Put the two sleeved cables next to each other and turn the cables so you have 4 disconnect cables in one direction.
Pull the 70cm ground wire back through the sleeve with the 30cm wires so you get 3 wires with ferrule crimps in the other end.


The 4 disconnect wires goes to your 4-pin power switch (all machines have one from delivery).
The three ferrule crimps goes to your laser PSU.
Ground to FG
Blue to AC-L
Brown AC-N

Controller rewiring

With default laser PSU 24V
Cable from your controller boards could be re-used. Inspect them for damages or bad crimps.
In this picture the cables are numbered 11, 12, 13 and 14
11: Connected to 24V+
12: Connected to 24V-
13: Connected to 5V input
14: Connected to L or LO (depending on controller version)

With external 24V PSU
Remove the cables 11 and 12 from your LaserPSU (blue)
Connect your external 24V PSU to your 24V+ and negative on your controller board.
Take a extra ground cable from your 24V PSU to the ground bolt in the rear, or one of the GND pins on your LaserPSU, number 12 as a example.

Power control wiring

Number 8, 9 and 10 controls your power.
8: Ground to your potentiometer
9: IN – to the “wiper” on your potentiometer
10: 5V to the input on your potentiometer
If you pot goes backwards and lowers power turning it to the right, swap places on IN and 5V on the potentiometer and try again.

Security wiring

If you have a flow sensor, laser enable button, lid switch or other security solutions you want to connect them here.
Number 5 and 6 is the protection. Without a closed loop between these the laser will not fire.
Connect your switch between these pins.
If you have multiple switches, run one cable from number 5 to one pin on the first switch, and from the same switch to the next switch and back to number 6 after the last one. The connection should be made in series.
Cable ferrule crimps are suggested for proper connection.

External power

If you want to install lights, fans or other products to your laser machine always use external power for this. Never connect anything extra to the LaserPSU as this one is on the limit to what it can handle already.
When installing external power, always route a extra ground cable back to the LaserPSU or back to the ground bolt in the rear, you want them to share the same ground.

If you have suggestions or recommendations on other wiring guides – let us know in the comments.

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