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GRBL: Low or No Power Output


Low or no power output with a GRBL device. The two most common causes of this behavior are:

Setting Speed Too High in Variable Power Mode

The most common cause of this issue is commanding your laser to run faster than its maximum allowable speed, leading to power downscaling. When Variable Power mode is enabled, the power level is 0 when the laser is stopped, and increases as the laser head accelerates toward the requested speed. This makes the resulting burn more consistent. However, if you ask the laser to go faster than it is able, it will never reach the requested speed and therefore never reach the requested power level either.


  • Set the speed within the allowed speed range of your machine. You can check your machine's maximum speeds in the Machine Settings window. The X and Y Maximum rates are the fastest you can command your machine to travel, in mm/minute.
  • Double-check the units of speed you are using. A given value in mm/second is much faster than the same value in mm/minute. Most diode manufacturers provide suggested settings in mm/minute. If you prefer to use mm/second, take care to properly convert the values. You can switch to mm/minute, or another unit of measurement, in the Settings window.
  • If you want the same behavior as LaserGRBL, which uses Constant Power mode (M3), enable the Constant Power toggle in the Cut Settings Editor.

S-Value Max Set Incorrectly

If you've already lowered your speed and are still seeing issues, the S-value max setting in LightBurn could be incorrect. This number tells LightBurn how to represent 100% power for your machine. Current versions of GRBL default to 1000 for this, while older versions use 255. The important thing is for the numbers to match. If your machine expects 1000 to mean full power and receives 255 from LightBurn, you'll get roughly ¼ of the expected output power.