How to Connect an mA Meter

You are here:

For more information, read our documentation on PWM Control, which explains why having an ammeter is important.

This article will focus on how to connect an mA meter to a K40 or similar laser. There are other tutorials available online which cover the electronics compartment lid modification in depth, so we will not cover that in detail in this document.


  • Wire stripper/cutter
  • Soldering iron or crimper
  • Small wrench or pliers for meter nuts
  • Wire protection (heat shrink, electrical tape)
  • Wire (>= the gauge on the cathode)
  • Drill, Dremel, and/or Saw

IMPORTANT: Power down the machine and allow the laser power supply unit (LPSU) to discharge for at least a few hours or overnight before proceeding to avoid electric shock.


Some people drill and cut out a spot on the control panel lid, while others choose to 3D print a mount to house the meter. Once the mount location is determined and implemented, insert the mA meter and make the connections. Refer to our LPSU Guide for common terminal connectors on the laser power supply.

Make note of the orientation of the positive and negative terminals on the back of the ammeter. Secure the connections with crimps or solder. It is important to ensure that the mA meter is insulated – protect the connections with heat shrink, electrical tape, or hot glue to avoid anything making contact with the wires, creating an electrical hazard.

The original cathode wire will be used to connect to the ammeter. An additional wire will be needed to connect from the ammeter to the LPSU. Measure the gauge of the existing wire from the cathode terminal for compatibility, but typically 16-18 AWG (0.75 – 1.5 mm²) is recommended.

Most Common Wiring


The left terminal (-) on the mA meter connects to the L- (Laser) pin on the LPSU (not to be confused with the L that is on the rightmost connector with the DC voltages).

If you have also have a ground wire or short connected to the low voltage laser return line, you may also need to disconnect it from L- to prevent issues with the ammeter current. The power supply should be grounded to the chassis.

High Power LPSU Connection

The right terminal (+) on the mA meter should connect to the ground wire going to the output mirror end of the laser tube, on the cathode. Ensure you are not connecting to the opposite end, as the red wire is high voltage and can be lethal. Do not touch this wire.

The left terminal (-) on the mA meter should connect to the FG (Frame Ground) pin on the LPSU. The power supply should also be grounded to the chassis.