Hello! I got a new Small Sun zy-t13 flashlight 3 days ago, and it isn’t working after 3 days 
I wanted to put more heat conductive grease under the led star, so i desoldered it. I made it, turned it on. The led flashed up and turned off. I did that switch-on 2 more times, and then the led didn’t even turn on. I disassembled it, tested the led, and it may be wrong because before this ‘incident’ if i tried to measure resistance of the led on DMM, it slightly glowed. It is not glowing anymore. I dedomed the led, and unfortumately broke the wires (they might be burnt already) but if i gave voltage directly to the die it worked. So maybe the wires were wrong.
And now the main point: The driver is still working(??) i just could measure the voltage, it is about 7.8V on high, 6.5 on medium and 2.6v on low, modes also working, because i don’t have a spare led yet. My question is it possible to tell if the driver is still working properly? If i know well, it is a constant current circuit, and the load(led) has a balancing role in the circuity, so the empty voltage of the driver is not important, because it will cange when i put a led on it.
Is it possible to test it without maybe sacrificing another led?
I’ve been told that in general, it’s not good to test the driver without an emitter attached. On some drivers, doing that can damage the driver, but you can’t tell for sure which drivers need an emitter vs. don’t need an emitter.
Your best bet would be to get an emitter on a star, hook that up to your driver, and test it, I think.
Tough to really say. You haven’t modded the driver yet, I presume? The original came with 2A constant current driver.
I haven’t tried this, but technically you can put a 1ohm resistor as load (try to get 10W or at least 5W ones) and the voltage across the resistor should show 2V. That means the driver’s working properly regulating the current.
UNLESS… the driver has limited regulation range… then you won’t get 2V, but whatever you get should give you a clue whether it’s working or not. For example, the voltage show near 0V, that’s showing the driver’s burnt and cannot supply current under load. But let’s say you see (example):
3V: Probably driver works, just cannot regulate down to 2 (1ohm lower load than XML)
1V: Same as above, though reverse, cannot regulate UP (1ohm higher load than XML)
As I said, we don’t know what load resistance properly simulates an XML, but going with 1ohm is just an easy method to convert current to voltage. Having measured some voltage across the 1ohm gives you good chance that your driver still works.
And be quick on the measurement, the resistor will heat up quickly.
No, it is completely stock. The wires were of crap quality, so the insulation might be melted, and then the wire contacted the aluminum board’s side.
So the 1 ohm will act as a load instead of the led?
So U=r*i ; and 2V=1 ohm * 2A
I will try it just tell me it can’t get worse than now.
What if i use different resistor? Because i have some high perf. resistors, but they are much bigger in resistance. maybe kOhms.
Update: I have some kOhm 10w resistor, but managed to find a “0.1OhmK” resistor shows 0.7 on my DMM, but it is only maybe 1-2W. Should i try it?
Resistor in kOhm wouldn’t work, the driver is buck so the max voltage is your 2cells = 2x4.2V = 8.4V. You can only go max 8.4V / 2A = 4.8ohm, bigger than that your driver will not give 2A anymore and the result won’t tell you much of anything.
And 0.7Ohm will work?
r*i=u
0.7x2=1.4V should I measure?
Hmm… I don’t think any regular multimeter can be trusted to measure low resistors… unless it’s a REALLY expensive MM. The internal resistance + lead resistance interferes with the result too much.
Especially if the resistor is rated 0.1 and you measure 0.7, that’s way too different, I’d trust the rating more than what the MM says.
Nothing wrong to try this anyway… so you can expect get between 0.1x2=0.2V and 0.7x2=1.4V anyway… within that range your driver seem to be doing the job.
I’ve looked at its datasheet, and it is actually 0.68Ohm, so the DMM might be quite accurate. I calibrated it from usb 5V.But i’ll try to get a 1ohm today.
Update: I bought a 10w 1 Ohm(1.4on my dmm) resistor. As you mentioned, the voltage drops exactly 2.00V, rarely 2.01V.
So it is working. Thanks for the help.
Great! Now you can put this is in:
http://intl-outdoor.com/noctigon-xm20-mcpcb-cree-xml2-u2-1a-led-p-749.html
Dedomed XML2 on copper + regulated 4A + 62mm reflector = jaw dropping fun! 
You might even be glad your old emitter burnt.
i’ve actually bought an xml u2 for 3.7$ on sale, but i think it will do. I tried to dedome the dead xml in gas, but i ripped the wires when i tried to remove a piece of silicone near the wires. Next time i’ll leave that piece and hopefully it will work. Another problem is that the reflector’s flat base is about 18mm in diam., so the solder blobs interfere with the reflector if the board is too small, so I will reflow the new led onto the stock board.