EDIT 12/21/2025 - Problem Solved!
I have subsequently tested several other small Convoy flashlights in my collection and have found all lights except S21D have excellent thermal regulation. This list includes mulitple M21E’s with different LEDs, multiple M21F’s, M21B, M21H, S21E, S21F, and T8’s. The curve below is typical of one such thermal check run.
As Tmax hit ~45°C, output immediately starts dropping to about 35°C. To test response, I applied cooling with an external fan, which results in further drop of temperature, and nicely, an immediate increase in output (lumen). All indicative of excellent and fully functional thermal regulation.
ORIGINAL POST: Thermal regulation of S21D has been a puzzle and topic of discussion for a long time. We know it’s there because output goes down within a few minutes, but… output then stays at a constant level of about 40% of max (and NOT lower) even temp remains high.
In nearly all other lights, including the newer Convoy light S21F for example, output would drop much lower than 40% and temp regulation clearly active as shown by a zig-zag runtime curve. As temp goes up, output goes down, as temp goes down output goes up, etc., resulting in the zig zag pattern. S21F for example would zig zag to keep temperature for example would stay in the 40’s centigrade. Not so S21D - you would see a straight line as illustrated below. (I hope I’m making sense here LOL.).
The bad thing is it could get really hot as owners have found out. The possibly “good” thing is that sustained output is higher and more stable than any other quad Nichia lights that I have.
