The K40 power settings is set with a potentiometer.
Stock setup is garbage and does often give up after a couple of months or starts to give strange results for your power settings.

How it works

The potentiometer is a variable resistor. In our case with the K40 machines there is a 0-5 V power reading on the IN connector on the PSU, this is the power setting for your laser tube output. Where 0 volts are no power and 5V is full power.
Your setting on the potentiometer limits the power coming in (5V) to a lower value sent back out from the wiper-pin to the IN-connector on the power supply.
If you turn your potentiometer half way you get roughly 2.5V back to the power supply – please note that this is more than half power as the PSU can deliver more than the laser tube can handle, so always use it together with a mA-meter. (milliAmpere meter)

How to upgrade it

Changing your stock potentiometer is a good upgrade, you can buy a replacement for a couple of dollars and it can easily be installed on your K40 laser machine.
Choose a high resolution potentiometer to get easier fine tuning on your power setting, 1, 5 or 10kohm often advertised as “multi-turn”. Here is a example on Aliexpress.
Higher resistance is equal to less load, but in our setup with the K40 machines this does not matter.
We have a article how to upgrade your potentiometer here: Upgrade your K40 potentiometer.

Extended upgrades

You can also find a kit with a volt-meter – easier to set and remember your power output here: Power potentiometer upgrade from Hawklaser.
Follow or guide to make your own potentiometer upgrade kit at home

Digital solutions?

If you like DIY stuff and making circuits with Arduino and similar solutions you can make your own digital setup.
There is modules ready to buy from Ebay and similar but making them work with the K40 from a user perspective is not recommended. It´s hard to use and your power settings will vary depending on what you use the machine for.