Is LightBurn only giving you 10% of your laser’s power?
Same wood, same file, 100% power, one pass. One piece cut clean through. The other barely got scratched. The difference was one number: LightBurn’s S-value max. If it doesn’t match your controller, 100% in software isn’t 100% at the laser.
Applies to
- Process
- Diode & CO₂ lasers on GRBL controllers, run from LightBurn
- Settings
- Edit → Machine Settings ($30) · Edit → Device Settings (S-value max)
- Symptom
- Weak cuts or faint engraves at 100%, often on a brand-new machine
Why it matters
GRBL sets laser power with an S-value, scaled against a maximum. The controller keeps its max in $30, labelled max spindle speed. LightBurn keeps its own S-value max. LightBurn sends 100% as its number, and the controller reads that against its own.
The Crawlspace Craftsman’s Fox Alien 20 W diode had $30 at 10000. LightBurn’s S-value max sat at 1000. So ‘100%’ sent S1000, which the laser read as 10%.
At 150 mm/min, 100%, one pass, it barely marked 1/4 in plywood. He added one zero to LightBurn’s S-value max, and the same settings cut the 1/4 in ply clean through in one pass.
Ten times the power. One zero.
What good looks like
- LightBurn’s S-value max and the controller’s $30 are the same number
- 100% in software visibly cuts harder than before
- Power scales evenly — 50% looks like half, not the same as 100%
How to apply it
- In LightBurn, open Edit → Machine Settings and find $30 (max spindle speed). Note the number.
- Open Edit → Device Settings and find S-value max.
- Set S-value max to match $30. The Crawlspace Craftsman left the controller alone and changed LightBurn — the simpler fix.
- Re-run a small cut at the same settings and compare.
- Re-dial your saved material settings, since every percentage now means something different.
Exceptions & tradeoffs
The mismatch can run the other way. If LightBurn’s max is higher than $30, the controller treats every value at or above its max as full power, so the top of your power slider all does the same thing. Matching fixes both.
Non-GRBL controllers handle power scaling differently — Ruida CO₂ controllers don’t use $30 at all. Check your controller’s own settings.
Mind you, more power isn’t always better. Past clean cut-through you just add char. Get the full range, then tune with a test grid.
Common mistakes
- Blaming the laser module when a new machine cuts weakCheck S-value max against $30 first
- Fixing the mismatch, then reusing your old settingsRe-test; your old 100% was really 10%
- Editing firmware $-values to chase more powerMatch LightBurn to the controller instead
Frequently asked questions
- What should S-value max be in LightBurn?The same as your controller’s max spindle speed, $30 in GRBL. Common values are 1000 and 10000; check yours under Edit → Machine Settings.
- Why is my laser not cutting at 100% power?A common cause is an S-value max mismatch. If LightBurn’s max is 1000 and the controller’s $30 is 10000, 100% in LightBurn only drives the laser at 10%.
- Where is S-value max in LightBurn?Edit → Device Settings. The controller’s $30 value is under Edit → Machine Settings.
- Do I need to redo my material settings after changing S-value max?Yes. Every percentage now maps to a different output, so re-run your test grids.
Where $30 and S-value max live in LightBurn, and how a 10000 vs 1000 mismatch gives 10% power — Get the MOST POWER out of your Laser by doing THIS in Lightburn! ↗ at 01:08.
Matching S-value max to 10000 and the before/after cut on 1/4 in plywood — Get the MOST POWER out of your Laser by doing THIS in Lightburn! ↗ at 04:10.
The demo, the Fox Alien numbers and the 1/4 in plywood result are The Crawlspace Craftsman’s. He made this exact mistake on his own new laser, which is why the video exists. The reverse-mismatch note and the re-test-your-settings reminder are mine.