This is correct. Now, the question is: when you measured the voltage drop of the battery, are you sure the current draw was 2A? If you weren’t using the DMM to measure the current at this time, the circuit resistance would be less, and the resulting current would be higher. So, maybe the driver was in regulation after all.
Another thing to note is that since this is a buck driver, the current from the battery won’t equal the current to the emitter in general.
Edit: my mistake, it is a linear driver, not a buck driver.
I said before the 2A were measured at the tailcap, but it wasn’t. My bad. I had the 2 DMM setup with one in series and one in parallel to measure both I & V simultaneously. So I just tried again, this time used some very short leads I made myself and found that the current dropped significantly when I connected one alligator clip to the threads of the flashlight body. So my former measurements were most likely skewed.
This time around, with the same battery, fully charged (4.22V) I got 2.9A and 3.85V at the battery without the alligator clip. Interestingly, with the alligator clip presumably producing some nasty contact resistance, the current dropped to 2.56A but the battery voltage remained at 3.85V. Does this make any sense? The driver is not supposed to produce more than 2.7A to begin with. LOL, I have no idea what I’m doing
I tried a different approach: ceiling test. So I measured the lux output pointing the light to the ceiling, noting the results every 5 minutes:
That does seem rather quick to drop out of regulation, but it may not be too out of the ordinary. For example, the XPL tested here has a VF of about 3.5V at 2.9A. Your cell, after about 15min of 3A draw is at 3.65V. Adding in the the ~0.15V drop of the linear regulator, the light would go out of regulation at this point. If your cell has gained a bit in IR, or if your LED has a bit higher-than-normal Vf, it would go out of regulation even sooner.
Using a cell with a lower IR, such as the samsung 30Q, and using a lower Vf LED, such as the XPL2, would dramatically increase regulation time.
Great stuff, thanks for the info and for the excuse to go back to tweaking my lights haha. As for the battery, and since these F13/F15 are 26650 lights the LiitoKala INR26650 seems like a good option for a low resistance cell (0.04 Ohms), right? Test of LiitoKala INR26650-50A 5000mAh (Cyan)
Yep, that liitolala cell is a top choice. To be more accurate, the IR as well as the shape of the discharge curve determine how long the light will stay in regulation. Look at the discharge curve in the test with time on the x-axis. With your setup, the time it takes for the cell to reach ~3.65V at 3A is the approximate regulation time. So for the liitokala cell that would be about 36 minutes.
So this would be the XP-L I’m looking for right? http://intl-outdoor.com/noctigon-xp20-cree-xpl-high-intensity-v3-3c-p-902.html
I’ve done some googling and some suggest it’s more of a thrower compared to XM-L2, some others say there’s no difference. I don’t want my bike lights to be too throwy. I’m gonna get a couple of them anyway to experiment a bit.
As for the batteries, the protection board adds to the IR, right? Would it make a significant difference if I removed it?
One more question please. The 0.15V drop of the linear regulator you mentioned, is that something specific of the LD-25? Trying to figure out from here where that 0.15V comes from
OK, so I tried the ceiling test again with a Panasonic NCR18650B. It dropped out of regulation after some 17 minutes. That’s exactly when it would be at 3.65V according to HKJ’s graphs!
So I’m going to conclude that there’s nothing wrong with the build itself. Apparently I’ll need to keep an eye on those Samsung 26F’s
No, the XPL2 is the one with the lower Vf. Like this one. It should give a similar beam to the XML2. The XPL HI would give a more narrow beam with more throw.
The protection circuit would add resistance, I’m not sure if it would be significant.
The 0.15V drop also exists in 7135-based linear drivers. Not sure exactly where it comes from, but in one of the graphs in HKJ’s review he has “voltage change in driver”.
One thing to consider with the XPL2 is that the driver will be dissipating more heat because of the lower LED voltage. In some cases the driver components could get too hot. I don’t know what would happen with your driver, but just so you know of the possibility.
Thanks again mate. I’ll use the F13 as a “test bench” for that XP-L2, but I’m in Europe and mtnelectronics charges $13 for shipping. Time to shop around
You might be able to switch to a better FET. Rds-on which is the internal resistance of the FET at a given gate voltage varies significantly and is the main criteria for choosing a particular FET. Cheap drivers use cheaper fets which start showing higher resistance sooner on the discharge curve. One of led4powers drivers with the sense resistor set to 2.5A should perform better.
It’s probably time for me to go read about drivers because that sounds like chinese to me LOL. Thanks for the tip, I’ll try to learn a thing or two about this stuff and make an informed decision, but for now I’m a bit reluctant to swap drivers. What I like the most about the LD-25 is that it has no PWM. All modes are regulated. I don’t want potentially distracting PWMs in my bike lights
Wow, I’m so impressed with this LD-3 driver. It has everything I’m looking for and more. The configurability is simply AWESOME!
I’ll be ordering the 3A version, so here’s another question please. The efficiency at 6A based on HKJ’s review is about 95, while the 1.2A medium mode starts at 70.
So at 3A, would you expect the initial efficiency to be closer to 95% or 70? The LD-25 starts at 80 (4.2V in) so this LD-3 is probably much more efficient, but how much?
Also, I take it it will work just fine with both XM-L2 & XP-L2, right? Will the lower Vf of the XP-L2 be an issue for efficiency when coupled with that LD-3, considering that it stays in regulation for much longer?
Oh, I think I finally managed to parse that. So you were essentially saying that replacing the mosfet for a better one, one with a lower “ON resistance” could improve the driver’s performance.
This is the mosfet in the LD-25 http://www.onsemi.com/pub/Collateral/NTD70N03R-D.PDF. What would be a good replacement?
OK guys, so I followed your lead and I’m very happy with both the LD-3 driver (really awesome) and the XP-L2 HD emitter (Vf @ 3A measured at 3.24V vs 3.55V for my XM-L2) in terms of performance
The only issue is that the XP-L2 HD is smaller and seems to be out of focus because it’s producing an ugly tint shift between the hotspot and the spill, where the spill is much warmer. In short, The hotspot looks good, the spill is noticeably warmer, and the flood (the outer portion of the beam) tends to cool white.
I’ve been googling for a solution but couldn’t find anything. I think getting the emitter a bit further down would improve focus, but I may lose output in the process. Are there XP specific reflectors with a smaller emitter hole?
