Weird runtime plot: thermal protection ???

Originally posted by c0t0d0s0 on 8-15-07 HERE.

This is a runtime plot of a light that's using Shark+Remora combo to drive 3 SSC emitters in series at ~850 mA and running on a single AW C-size li-ion cell:

The graph clearly shows slight brightness dropoff as emitters overheat, slight flickering as the converter drops out of regulation, all of which is to be expected... but what's that step in the graph at ~22 minutes of runtime that looks like thermal protection kicking in? I've never seen anything like that before with Shark boards. They're not even supposed to have thermal protection, aren't they? Is it the Remora? Not that i'm complaining, but I'm curious as hell what's going on there...

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LED Zeppelin08-16-2007 12:53 AM
Re: Weird runtime plot: thermal protection ???

I noticed something similar, but with my eyes, not a meter.

I built a 5 X Q4 Shark/pot light powered by 3 X 17670. I only had 2 cells on hand, so I ran the light with one dummy cell. The light would be fine for a short while, them it would step down in brightness as the Shark struggled to keep up the boost. After I received more cells and ran it off three, it has been fine.

I think the rule of thumb is to keep the Vin over half of the Vf of the emitter string when using a Shark. You're not quite there with your setup.

I'm not sure if this is related to your graph, but ever since I have been more careful of my cell solution with multi-emitter lights, and try to keep Vin as close as possible to the Vf.

c0t0d0s008-16-2007 08:07 AMRe: Weird runtime plot: thermal protection ???

Yes, this makes sense. However, while my setup is pushing the Shark to near its limits, it's not quite there yet. It doesn't seem like the converter is struggling too much... Before I added the Remora, I made sure to test this setup on a partially discharged cell, and it know that with the pot turned to the max, it was able to maintain ~980 mA to the emitters, so 850 mA with the Remora should be a bit easier on the converter. It definitely remains in regulation after the stepdown. Something in there makes a "conscious decision" to step down the current. The real struggle begins in the last ~7 minutes of runtime... again, not complaining, I'm actually surprised the setup works fine for so long until then! Just curious what mechanism triggers the stepdown... but whether it's temperature or Vout/Vin ratio, the converter is smarter than I thought!

dat2zip08-16-2007 01:02 PMRe: Weird runtime plot: thermal protection ???

Can you estimate what the voltage on the battery is when it steps down.

I'm thinking the Remora drops out at some point and the steady state happens to still produce a voltage that is near full regulation.

I wonder if the "flash" condition or non powered up Remora is the same as the step down and if you add the resistor to fix the flash if the light would turn off under these same conditions.

Let's see... the uP stops working around 1.8V. Regulator overhead is 0.1V and the two series diode is another 1.2V or so. That means the uP stops working around 1.8 + 0.1 + 1.2 = 3.1V. Would this be the same voltage where the step occurs?

Wayne

c0t0d0s008-17-2007 07:03 AMRe: Weird runtime plot: thermal protection ???

It's kinda hard to do since everything is already potted/assembled... All I can tell you is that the Remora appears to be working fine - the modes switch and everything - until the the last couple of minutes before the protection kicks in, at which point the light is definitely out of regulation and flickering. I believe the protection in AW cells is set to cut power at 3.0V.... this makes me think that the stepdown occurs at higher voltages, in the 3.5-3.7V area, but then again, I didn't measure it.