Thanks guys
Max drive current should be 5.84A x 3 = 17.52Amps
I was also using djozz’s great graph and it seemed to measure out correctly for me when I verified voltage across the emitters at 6Amps.
I was using ~7v as a minimum target voltage to feed to the drivers while still maintaining regulation (think I verified 6.9v at the leds @ 6Amps). So I have faith that the basic calculations are near enough correct.
I know I drop around 0.6v by the time the cable plugs into the back of the light including connector losses and I was using a 0.2v drop across the 7135s. What I didn’t get round to measuring was how much I drop across my twisty contact interface going into the driver contact board. That’s the last place where I would imagine larger losses are to be found, unfortunately I didn’t have access to the resistor array dummy load and 1000w psu that I used to test for resistance losses within the rest of the system.
So, starting with an 8.25v battery voltage (under full load) I should have a vF overhead of 8.25 - 0.6 - 0.2 - 6.9vF = 0.55v
Here’s the same graph with pack voltage included. It’s still well above 8V at the end when output has collapsed down to 50klux. And again, turning the light off for a bit then on again the pack voltage will return to that same level but output will be up near 70klux again. Can’t explain why that would happen if the pack voltage was inherently too low to maintain that level of output right?
Urgh I dunno, I’d be rather surprised if the voltage was limiting output this drastically but I may still have some extra resistance losses eating into my overhead.
I’ll need to get some accurate current measurements to figure out what is going on. I did do a test with my crappy harness but that was dropping 0.36v before it even left the battery pack and ammeter so that’s a bad test setup. Max drive current I saw in that test was still 16.7A though and it held it up for 1.5minutes fairly steady before again starting to drop off more quickly.
At the end of the day it’s a bit of a catch 22 with these linear drivers. Especially with a battery pack like this which doesn’t really sag.
If I improve resistance losses and get a higher voltage to the drivers
- Theoretical longer time in regulation
- More energy for the 7135s to burn off
- Possibly getting 7135 overheat, throttling and flicker earlier negating the extra regulation time.