Mag XM-L Mod

[LilKevin715]

Justin and moderator007:

Please see my Vf vs Temp thread I created a while back(now updated), it should answer some of your questions.

I don't know if a XM-L could or even would benefit that much from a burn-in compared to a SST-50/90. Its one thing if the Vf was in the 3.7v+ range, but the Vf of a XM-L is pretty low already.

 

[Techjunkie]

Well, you should:twothumbs.
I am pretty sure I have seen you build some 4 or 5 amps mags with a XML. You didn't happen to check the Vf after a few cycles did you.

Not that I recall, no. XML are so low Vf to begin with, I've never had desire or need to pay as much attention to that detail for them as with the SST emitters. If anything, I've had the opposite problem with XML, having to add resistance to keep regulators from overheating. Generally speaking, all LED have lower Vf when they're hot than when cool, but burn-in of CREE LEDs (lower Vf even at cool temps after they've been broken in), is not nearly as drastic as with other LEDs (possibly even non-existent).

 

[Justin Case]

Justin and moderator007:

Please see my Vf vs Temp thread I created a while back(now updated), it should answer some of your questions.

I don't know if a XM-L could or even would benefit that much from a burn-in compared to a SST-50/90. Its one thing if the Vf was in the 3.7v+ range, but the Vf of a XM-L is pretty low already.

It is well-known that LEDs are negative temperature coefficient devices. The coefficient is given in the Cree datasheets. For an XM-L, it is -3mV/C. However, I don't think that the question was Vf vs temp. It is Vf vs If, with If at the upper end such as at 3A (or even more).

 

[LilKevin715]

Perhaps I should have been more clear with the linked thread. It was mentioned in reguards to regulated runtime.

There is no doubt mags do have resistance, but 0.1 ohms seems a bit much in a setup such as mine. CKOD measured the resistance of a stock tailspring (#1 and biggest offender) and obtained a value of 0.027 ohms. That seems more realistic as my regulated runtime was well over a hour. A 3 X C or D nimh setup with a AMC7135 driver is definately doable and fesible for a XM-L. I can't say the same for something like a SST-50 @ 5A though.

 

[Justin Case]

Perhaps I should have been more clear with the linked thread. It was mentioned in reguards to regulated runtime.

There is no doubt mags do have resistance, but 0.1 ohms seems a bit much in a setup such as mine. CKOD measured the resistance of a stock tailspring (#1 and biggest offender) and obtained a value of 0.027 ohms. That seems more realistic as my regulated runtime was well over a hour. A 3 X C or D nimh setup with a AMC7135 driver is definately doable and fesible for a XM-L. I can't say the same for something like a SST-50 @ 5A though.

The tail spring isn't the only source of resistance. Other sources include the cell-cell contact resistance, cell to switch contact resistance, switch contact resistance, and grub screw ground resistance. Also CKOD's measurement is the resistance of the wire itself. He did not measure the contact resistances between the spring and the tail cap, nor between the spring and the bottom cell. Of course his conductor measurement will show low resistance. He's checking the resistance of a fairly thick gauge wire. Contact resistance is very different.

See this link to get an idea of how bad parasitic resistance can be.