How high a certain LED emitter can be driven?

[akula88]

My background in electronics is rather limited, so what is the formula or how to determined how far you can drive a certain emitter?

Some forum discussions are about high drain lights which use batteries that are capable of certain current(amperes) of continuous draw. How is this determined?

Specific Examples are :
1. Surefire Fury DFT is said to require at least an 18650 that is capable of supporting a continuous draw of 10A, for the light to work efficiently.

2. On my Sofirn SD-05 with XHP.50, 'High' mode can only be fully achieved if I am using an unprotected Samsung 30Q 18650 3000mAh. If a protected KeepPower 18650-3500mAh (P1835J) is used, I can only put out a 'Med' output, even if settings is placed at 'High'.

3. When designing for a mod, how far can a certain LED be driven, example -- Cree XPL-HI or Luminus SST-20 or Samsung LD351? I have read of them being driven to as much as 2800mA. How much battery or which 18650 is needed (as far as continuous draw is concerned).

4. As far as Surefire (smaller) platforms are concerned, how far can the current 16650-sized batteries types can be push to drive an emitter? Does battery protection matter?

 

[WalkIntoTheLight]

Those emitters can be driven a lot harder than 2800mA. Cree might only publish specs as high as (for example) 3000mA, but the LED can be pushed a lot more than that. Most lights using a FET driver and a high-drain cell will push emitters to at least 5000mA, sometimes as high as 8000mA. It likely comes at a cost to the lifespan of the LED. But, if you only get 10,000 hours instead of 50,000 hours, do you really care?

Anyway, some flashlights (usually more expensive ones like Zebralight) will use a boost driver to determine the output. They don't require a high drain cell to achieve full output. A high-drain cell would still be preferred, but just for the health and lifespan of the battery. It's easier on the battery to draw 8 amps, if it's a high-drain cell.

Cheap flashlights often use something like a FET driver, which depends completely on the battery to determine output. A low-drain battery will give lower output.

Finally, I don't really like protected batteries, especially for high-output lights. Their protection circuit can trip at high current, completely cutting power to the flashlight.

 

[akula88]

Anyway, some flashlights (usually more expensive ones like Zebralight) will use a boost driver to determine the output. They don't require a high drain cell to achieve full output. A high-drain cell would still be preferred, but just for the health and lifespan of the battery. It's easier on the battery to draw 8 amps, if it's a high-drain cell.
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Finally, I don't really like protected batteries, especially for high-output lights. Their protection circuit can trip at high current, completely cutting power to the flashlight.

How far can a protected 16650 2500mAh be driven on a certain circuit before it trips itself?

 

[archimedes]

How far can a protected 16650 2500mAh be driven on a certain circuit before it trips itself?

It depends on the circuit, this is determined by the specific design.

 

[WalkIntoTheLight]

Usually they trip somewhere between 5A and 10A, but they could make it anything. If you don't know, I'd probably assume around 5 amps, or maybe even lower for something smaller than an 18650.

3 amps gives you about 1000 lumens from a typical Cree emitter like an XP-L (not high CRI), assuming the flashlight driver is also reasonably efficient. 5 amps might be around 1300 or 1400 lumens, because the emitter is being over-driven at that point. The emitter burns out somewhere around 8 amps, and you're really not getting much more output out of the LED past 5 or 6 amps.

So, for most cases, protected cells work fine. But, they do add extra resistance in the circuit, and when LVP kicks in you're suddenly left in the dark. And, IMO, the "protection strip" adds potential safety issues of its own.

 

[RetroTechie]

There's no single formula. Essentially it depends on which LED, how long a LED is run at which current, how it's mounted / how the light it's in is built, and how much you're willing to shorten the LED's lifespan. One way or another, it's usually HEAT that does the killing... :laughing:

Longer answer:
Voltage across the LED x current through it = power (watts). Some % of that gets radiated in the form of light or infrared, remainder (watts = heat) will somehow HAVE to be shed by means of conduction. Heat conduction is modelled using a concept called "thermal resistance".

In this context, you'd have thermal resistance from LED die -> its case (see: datasheet), LED case -> board the LED is mounted on (depends on mounting method, alu, copper or other material board, etc), LED board -> flashlight body (unknown), flashlight body -> ambient air. All these add up, the heat flow from the LED causes a temperature difference across each step in the "thermal path". Result is LED gets raised an X number of degrees above ambient temperature. Running at higher temperature simply makes it ehm... DEGRADE faster.

Calculation of this isn't rocket science, IF you know all the variables involved. But normally you don't. Or some variables you can estimate, but would be difficult to put an exact number on it. Thermal mass, texture / smoothness of contact surfaces, what cooling paste was used (if any), etc etc also comes into play. Read: what temperature your LED ends up at, is also unknown. What you DO know for sure: a ballpark figure for the LED's power requires a certain construction for the LED not to overheat. For example a LED that hangs free in air from a wire, doesn't sweat when you push 1 mA through it at a few volts. Push a couple of amps through it in same conditions, surely it will overheat & fail quickly. Or say you manage cooling a LED in liquid nitrogen, and only put millisecond current pulses through it, you may be able to push incredible # of amps (I suspect bond wires to the LED die would limit things there).

So it's kind of a black art. Much like overclocking the CPU in a computer. For a complete light, all this also applies to driver & batteries used.

Personally I see no harm in driving a LED a bit outside of spec, if you know what you're doing. Otherwise: just pick a bigger / more powerful LED & suitable mounting method.