If one KL4 draws more current than another ..

on the same set of batteries, that one has a higher Vf right?

If:
1) The KL4 circuit is constant current regulated, then: The LED with the higher Vf will cause it to draw more current from the batteries.

2) The KL4 circuit is voltage boost only, then the LED with the lower Vf will cause it to draw more current.

I think 1) is the answer.

Thanks
why would it draw more with a low vf in a boost only, I'm trying to reason this out

Because from two LEDs the one with the lower Vf draws more current at the same voltage.

The other way to look at this would be to use actual numbers as an example:

LED#1 - Vf = 6.5v
LED#2 - Vf = 6.8v

If the voltage being applied is 6.8v, LED#2 will draw "normal" or close to its rated current, while LED#1 will be overdriven, which in turn will cause it to draw a higher current than LED#2.

Now, must of these converters (at least the good ones) should control the current, which makes them more inmune to the actual Vf of the LED. Remember that a relatively small increment in the voltage past the Vf will result in an abnormally higher current consumption, so a constant current LED driver would be a better way to go.

I have no idea what SF uses on the KL4 circuit - others here might know.

Will

Surefire uses a constant current boost converter in the KL4. They also strive to use the brightest luxeons that they can get, but they do not bother with things like color and Vf, as long as the performance is there.

As stated in a couple of posts above, the light with the higher Vf luxeon will draw more current, assuming that the batteries in both of the lights have the same amount of voltage. The total voltage of the batteries in a light will also affect the current draw. The current draw will increase as the voltage drops.

And the reason why a constant current circuit will draw more current with a High Vf led vs a Low Vf led, is because to boost the voltage, the circuit has to draw current. So if it has to boost the voltage a lot higher for a high Vf led, then it must draw more current to do so.

So, with batteries at the same measured voltage, which vf led would draw more current, low vf, or hi vf?

Bill

Bill: in the case of the KL4 it would be the led with the higher Vf.

Thanks