# Vf vs Temp



## LilKevin715 (Nov 14, 2010)

After a emitter has heated up from a cold start, does the Vf continue to drop or does it level-off? (assuming good heatsinking)

I am asking as I've built two identical SST-50 mag's (details here) and I have gotten two different results. I've made some Vf vs time graphs for the two mags. Voltage measurements were taken every minute.

Here is the first mag. The Vf levels off at just below 3.5v, and then starts to drop after 72 minutes due to cells no longer able to maintain the proper voltage.







Here is the second mag. There is no levelling off of Vf and it continues to drop. This particular mag obviously has a higher Vf than the first one. The Vf drops faster after 80 minutes due to the cells. 






Is the second mag in regulation at all? Even though the Vf isn't flat like first mag the Vf is declining at a steady rate from 34-80 minutes.


----------



## LilKevin715 (Mar 7, 2012)

Replies to this thread were lost due to the forum crash. However I feel it might be helpful to others by updating this thread with additional info and my conclusion.

Please see the linked thread in the first post for the build details.

The first mag ran in regulation (Vf of ~3.5v @ 2.8A) for around 72 minutes before dropping out of regulation as visually evident in the Vf vs time graph. The second mag (initially identical build to the first) had an unknown regulated runtime due to no "clear" indication in the Vf vs time graph. 

My initial thoughts of mag #2 was that it wasn't running in regulation at all. This could possibly be due to too much voltage sag from the batteries, a abnormally high Vf of the emitter, resistance of the host, a combination of factors, etc. This would be visually evident in the Vf vs time graph. Keeping the scientific method in mind... I decided to do a experiment to eliminate some of the unknown variables. I transfered the guts of the light to a 3D rebel mag and decided to run 4 X C cells. This would virtually eliminate the possibilites of a abnormally high Vf and voltage sag of the batteries.

I did a re-run of the second mag and the results obtained can be shown in the Vf vs time graph below.





With this new data I can determine that the Vf of the SST-50 in the second mag is ~3.55v @ 2.8A. I am puzzled though at the overall shapes of the graphs for mag #2 when compared. The first run of mag #2 2D (3 X C cell) setup was a smooth gradual curve. The second run of mag #2 still has that gradual curve down, but bottoms out at ~3.55v. The rise in Vf after that is a bit puzzling as well, thermal runaway? At the end of the run it is clearly evident when mag #2 runs out of regulation as there is a sharp drop in Vf due to the cells are basically empty at that point. It also makes sense at that time (~99 minutes) as well becuase for this particular setup running a driver based on AMC7135 linear regulators: Battery Capacity / Amp Draw = Runtime. So for my setup and cells 4500 mah / 2800mah = 1.6 hours or 96 minutes.

So to conclude mag #2 was running in regulation in the first 2D (3 X C cell) iteration. By changing the power source to 4 X C cells it only changed how long it ran in regulation (~80 minutes vs ~99 minutes).


----------



## Justin Case (Mar 7, 2012)

I don't have any hands-on with the SSTs, but the various Cree datasheets give a spec for the temperature coefficient of voltage. For the XM-L, it is -3 mV/C. And any informal measurement of a bare Cree will show that Vf indeed decreases with temp. That's why you want to drive LEDs with constant current, not constant voltage.


----------

