It’s possible, based on your results, that the sense voltage listed in the datasheet is no longer accurate. Maybe it’s using the same sense voltage used by QX5241, approximately 205mV. Or it could be 180-290mV, who knows. Really it’s just as likely that somewhere in this mess of a driver there’s something messing up the normal sense voltage setup.
I’d dig up an R100 and put that in parallel with the R200 (for 0.0666666666666 Ohms). That should give you approximately the current you want. (maybe 2.7 - 2.9A based on what we’ve seen so far, maybe 3.75A based on what the datasheet claims (250mV sense)) If current is too high, discontinue use until you’ve increased resistance again ;-).
Pushing this current too high will generate problems related to the SS34 diode, the inductor, the no-doubt-crappy FET, etc.
EDIT: yes, you are correct about R300 and about R200 and R300 in parallel.
Nothing, the “R” represents a decimal point. R10000000 would also be the same thing, they are just trailing zeros.
It looks like a 1206, but I’d measure it if I wasn’t sure.
I’d also still consider an order from Mouser. For <$10 you should be able to get out of there with a decent assortment of precision sense resistors. They’ll get to you faster and if you choose the right values you’ll be in a good position to tune your output exactly as you want.
OTOH I always feel a lot of pressure to pile everything I can into a Mouser order so I don’t have to order again right away. With FT you just buy what you want, one item at a time.
BTW, for the record, I’ve added the emitter leads and tested the other board (the one with the blob) and it’s performing the same way as the first one did before I added the R300 resistor in parallel with the original R200 resistor.
So my paranoia about the blob of solder was unfounded :)…
I’ve ordered some of the R100 and R200 from Digikey, just in case, so I’ll have a pile of them :)… Shipping actually wasn’t bad… they apparently ship via USPS first class now, so it was about $3+ for the shipping.
If you’re game, let’s go ahead and stack R200, R200, and R300 on one driver for 0.075 ohms. That will be a good test to see how the sense voltage scales at close to 3A before you place any orders.
The lower we take the resistance, the higher Vcsn seems to be! (2.8 * 0.075 = 0.21)
R100 and R200 in parallel should put you at 3.15A+
~6.4v Vf + ~0.21v current sense = regulated to maybe 6.61v? At that point your batteries are dead anyway. That sounds pretty decent to me. Assuming the driver survives of course!
I wrote a fairly long post about this, but lost it after the forum went down for that unexpected maintenance.
I don’t have much time now to re-write it, but here we go while it’s still semi-fresh in my mind. The PWM / Modes chip does not seem to be the common CX2829, who’s pinouts appear to be:
As we can see from ohaya’s pictures, pins 3 and 4 appear to be “not connected” on the unmarked SOT23-6 package. If that’s correct it can’t be CX2829.
The pinout we appear to have on this driver is :
Vin or PWM (probably Vin.)
GND (sort of…)
NC
NC
offtime cap?
PWM or Vin (probalby PWM)
Assuming any of that is correct, pins 3 and 4 probably need to be pulled to GND or Vin in order to change mode groups, if it’s possible at all. If ohaya will set the driver on “medium” and probe those pins for voltage with a DMM we can get a better idea what’s going on and then recommend an action.
So, I guess I have a stupid question (sorry, being a little self-deprecating today :laughing:… Why NOT short the resistor? Would current go to infinity or something?
Or is the concern that current would go high such that it might overheat the driver (which kind of seems like a good thing :laughing:?
I’m not ignoring you all - I’ll try to get those physical measurements you all have asked for, today, after work.
Jim
EDIT: Also, BTW, I only have one working driver (I KNEW I should’ve ordered more of them). I used a heatgun to pull the R200 off last night, and a bunch of the components came off at the same time. I’m planning to try to re-construct the driver when the new resistors come in, but not now since it won’t work anyway - that was part of the reason I asked about shorting the resistor…
Also killed one XM-L2 so far on this “project”… kind of amazing that it’s not more :)…
Tough luck on the components that came off. Putting them back on would be good practice for you…
The buck circuit rapidly switches a FET which puts a current through an inductor. The inductor smooths out the voltage to an intermediate voltage between the full “on” voltage and the 0v off state. The duty cycle of the FET is determined by the voltage drop across the sense resistor. Duty cycle means what percentage of the time it’s turned on. Shorting the resistor will produce negligible voltage drop and push the circuit to 100% duty cycle. At 100% duty cycle Vin = Vout and Iin = Iout (more or less). In other words the buck circuit will stop stepping the voltage down and go into DD (direct drive).
Since the purpose of using buck drivers in flashlights is generally to run a much higher input voltage than output voltage (MT-G2 on 3s/4s, XP- or XM- on 2s or 3s), DD is catastrophic for the LED.
Taking those measurements isn’t directly useful to me either, as I don’t plan to run this driver for anything, but I think it would be a nice thing to do for other people. Maybe bridging some of those pins enables even more blinky modes ;-).
Don’t worry about it! Despite the time we find to chat online, I know most of us have limited time for experiments. It sounded like a joke with a grain of truth and I took it as such, but if you never find time to do the test I couldn’t blame you either.
Did you lose any of the stuff for the driver that fell apart? I was at least half serious about putting it back together. It’s definitely doable if you want to, there are orientation marks for all the components with critical orientation. I feel a little bad about that one since I pushed you to steal the resistor!
