Looks like the linear driver is up and running. See posts starting w/ #51 over here: [retired] [WIP] 20mm single sided & 17mm double-sided ?-amp linear driver - surprisingly good!
I did some figuring. There is a lot to account for when estimating how much power will be dissipated by the FET. Under lighter loads the battery voltage will not sag as badly and resistances in the light will not result in such large voltage drops. Going from 7A to 3A may recover 0.5v in resistive losses alone. Then you’ve got another 0.1v per cell from the reduced sag.
I did some crappy math, really it was not good: I didn’t write anything down and I did not do exactly what I intended…
- 7s1p settles on around 7A DD. Dropping that down to 3A should result in around 1.8v (0.7 from Vf, 0.7 from reduced battery sag, and 0.4 from reduced resistive losses) for the FET to burn off. At 3A that’s 5.4W, which I think is fine as long as a TIM cube or potting is used.
- 8s1p really has the cells taking a beating in DD. When we regulate to 3A though, it’s the FET that takes a beating, dissipating ~9-10W with fresh cells. The LED is getting ~19W at that point, so you’re running at a pretty low efficiency. Once you get down to 1A you’re only dissipating 4-5W in the FET… but for ~6W output, so the efficiency figures still look very bad with fresh cells. With depleted cells things look much more rosy of course.
Generally speaking 7xAA is the most powerful configuration most folks should go with. Depending on how well you minimize resistive losses in the carrier & etc you should be able to keep a regulated 5A or 6A throughout most of the discharge.
EDIT: It’s worth noting that these efficiency figures are not entirely specific to this driver. Linear drivers which achieve more or less constant current without much PWM will be affected. 7135’s should not be expected to behave any better I think.