I'll start out with a disclaimer that this post is long and complicated, but I think I've read a lot of threads and web pages on the subject and can't seem to find a lot of consistency. When doing a sense resistor mod to increase the current of a buck driver, when is it a good idea to use a more robust toroid? These have to be one of the most complicated types of driver (except maybe buck-boost), and I always seem to find conflicting info, leading me to believe that it really depends on the individual driver. I found some higher amperage rated toroids on aliexpress here that made me wonder:
tons of resistance values to choose from
But, how can you tell what resistance value is right for the toroid on a particular driver, regardless of current? I know that a lot of drivers are limited by their diode rating as well, so most of the 3a diodes on common cheap drivers would have to be changed.
For example (assuming it could handle it), if I wanted to increase the current of the pretty common LD-4b driver by double, I could add another R100 over the existing R100 current sense resistor, and would probably have to change the 3a diode as well, right?
LD-4b found here :
Are there any other considerations besides those three components for a basic buck driver current increase? The sense R, the diode and the inductor toroid?
This might be a separate topic, so forgive me if off-track, but how does a driver determine how much overhead voltage it needs? here in post #51 modding to incread the drivers current was pretty easy and successful, but when I tried to go over 4a it was not successful.
I also read here that the LD-4B needs between 3-4v overhead if you want to push it past 3a (it supposedly is 2.0-2.4 stock). so that makes it obviously require 2 series cells for a single standard led.
But ,I asked Rmm about his mtnbuck driver for an xhp50.2 with two cells and (if I remember correctly) his response was that the over head require was around or under 1v.
Now, I know his driver is much, much higher quality than those el-cheapo buck drivers, and intended for higher current mods. But what exactly determines the necessary overhead Vin besides quality construction? I think I've read a lot of the discussions about buck drivers on here, but it seems like you have to be an expert to know what influences these things in each driver, so there may not be a concrete answer and it might just depend on the individual driver. But any help to make sense of these things would be greatly appreciated. Especially since a lot of leds now are such low vf like the sst-40, xp-l2, nichias, etc
I know the vf increase with increasing current at the led, so at what current can a single cell instead of series cell and a buck driver be used with a pretty low vf led like the sst-40?
As a side note, those ld-4b or ld-40's have their place if you want to to do a quick driver swap and install a red led or other really low vf led like that, but other than that I don't think they're worth the $5. I put the version that always starts on low (from kaidomain) in a T20 with a photo red Xp-e and love it. But, totally by accident and that's about the only use I can see for them. They lied about the two lowest modes, they're a lot lower than the specs state. On low, my multimeter reads 0.0 at the tail cap with a single cell into a red emitter, and the beam is barely visible 10m away in the dark, and you have to put your hand to the lens if the lights are on in the room. On medium, the tail reads .12 or something and the beam is much more usable, and high mode seems to be in the vicinity of 2a. However, since I use this as a hunting/shooting light with a pressure switch, it works out well because I can "ramp" up the output from really, really low to high based on the distance and skittishness of the target (feral hogs mostly). Plus the ld-40s seem to be flimsy. It was fun to break a handful of them and learn stuff though, but I won't be using them in other builds I don't think.
Anyway, thanks for reading and any answers to my questions. Probably much better off just spending the money on the mtnbuck if you want to go over 4 amps for any amount of time, but with all these low vf leds hitting the market, I thought I'd ask. Thanks!