Possible problem with Cree XM-L U2 -1B

LED Flashlights – General Info 18650 Flashlights
1

I recently built a Convoy S4 for a friend, standard Nanjg 105c, anyhow I get a call saying the light won’t go on high?
So I go have a look and the light is actually changing modes but High is very low, low is ultra low and low is a perfect moon mode, strobe also low.

First thought flat battery , swap for new battery , same problem , measure tail current and I’m getting next to nothing on the meter, checked that the pill has still got a decent negative path.
Replace driver with a new Qlite 3Amp from IO, luckily the drivers arrived yesterday so I had spares.
Anyhow same problem after driver swap, Unfortunately I’ve no other XML in my stock.
Anyone else concur I’ve got a semi blown XML Led?

2

What is the resistance through the tailcap? Same issues when removing tailcap and completing the circuit with a "Insert name of nearest metal object that can short batt neg and flashlight tube, here" ?

3

Thanks, but read above, I measured tail cap current , very low on high, same with shorting out as described.
If I’m up to it I might take an emitter out of a Roche to test today….

4

Can you post some pictures please. Maybe a part isn't soldered in the perfect way or has a connection, where no connection should be.

5

Thanks , My connections are fine as all the modes work fine just at a reduced rate,and the light has worked ok for sometime , I suspect my friend left the light on high for too long.

I’ve also removed the emitter and direct connected to a 3.7v source and the led shines very low
My guess it’s kaput?

6

If you are positive that the LED was subjected to 3.7V, then yes, it’s toast.

Perhaps he did leave it on for too long?

If you have a good eye or a loupe, check if the bond wires (3 thin wires to the phosphor layer) are still intact.

7

LEDs don’t normally die unless something goes really wrong (accidental direct drive) or its really overdriven, right?

8

That’s correct, it’s very hard to blow an LED with a single 18650 and an AMC7135 linear driver.

If they overheat, I heard that they turn blue… maybe the friend could have noticed that?

I’m sure there was adequate heatsinking… perhaps the reflector didn’t push down on the PCB hard enough?

9

To answer all of the above, the led was tight against the pill, I built the light myself.

Led is pristine like new no discoloration at all and through a 50x magnification I can’t see any legs missing or broken.

Last I fired up the bench power supply and applied 3.6v to 4.8v and it’s easy to look into the light so just above moonlight mode, current draw the needle doesn’t even move.
Light does get brighter as I increase Volts.

Me thinks it’s Kaput, Thanks For all the replies above.

10

Try to reflow the base first before throwing it away. Could be the connection from emitter to base has some problem.

11

Hmm, it’s a solid idea
I’ll have to learn to do this….

12

You can reflow on a stove top.

Place a piece of metal on the stove & put the led star on top. Med to med-high heat. You can poke the solder with an unbent paper clip to check when its reflowed. It also gets shinny.
Remove from heat with pliers.

The metal can be cut from a soda can even.

13

+1, Has happened more than once for me. Reflowing fixed the problem.

14

It sounds like your LED is ~toast, but it’s possible there’s some fault on the mount. Reflowing would fix the heatsink joint, but the LED base itself could be weak. I’m assuming you resoldered the LED leads on both ends… I had one with an old-fashioned “cold solder joint”, which took me aback. (EDIT: I just re-read the OP and noticed you’ve already changed the Driver ends. Sorry…)

If you have a DMM, you could try to check Continuity through the LED itself. That should get you a little bit of light. Just touch the solder pads on top of the LED, not the heatsink. If it doesn’t light up, switch ‘+ ’ for ‘- ’ and retry. If that works, retest on the heatsink solder pads. Different? Reflow. Same, Re-place! If you’re really still, you might be able to measure R between the heatsink ‘- ’ and the LED ‘- ’ and vice-versa. A non-zero R shows your Root Cause.

Also, did you direct-connect to the LED solder pads or to the heatsink ones? Either way, why not try that again with a DMM in Current mode inserted in one lead? This gives you Iled, which is all the XM-L cares about (keep your Source under 5v). That’s a useful troubleshooting data point.

I’m scratching for outliers here, and would “+1” the idea of just ordering a new XM-L for three bucks…

15

When you do, don’t you have to be very careful to hold it w/o wiggling or tipping? This “trick” would be the only way I’d have to reflow anything, so I’m curious. Also, do you cool it in any significant way after it gets “shinny”?

I was thinking it might be useful to have a cool, wet sponge or paper towel handy…?

Good tip, BTW, and thank you!

16

If you’re just looking for quick & easy reflow, you can always hold the base with a plier and run flame straight underneath.

This is my $3 reflow oven:
http://www.fasttech.com/product/1196000

It’s very hot, so it’s easy to overbake (I’ve burnt two aluminium bases when learning)… so be alert and remove flame right after the solder melt.

17

I haven’t had any trouble when moving it off the heat. The surface tension of solder provides some resistance to movement. Just let it cool on its own after removing from heat.

I’ve actually used a food thermometer to keep an eye on the temp and prevent overheating.
Spring tweezers are much better then pliers only $1.20 on ebay. The edges aren’t rounded so they aren’t the most comfortable, a little filing fixed it.

18

Ok applied 4 volts direct to led same thing low power, I then re flowed the led off using a butane torch and basically fried the sink pad , too much heat …haha
But led came off and now is totally kaput.
No I know how not to reflow ….hehe

Got some new XM-L2 in the post so no worries , light will be up and running again with a brighter emitter than before and a Qlite 3A driver too.