Measuring tailclip current?

The emitter is probably seeing 2.8A. Each battery is supplying 1.6A so 3.2A and the driver is probably about 85% efficient so that brings it a little less than 2.8A.

I think that is a reasonable explanation.

The short answer is no, but you can make an estimate.

Calculate the power used from the batteries, i.e. voltage * current, with two LiIon batteries and 1.6 ampere it is: 2*3.7*1.6 -> 11.84 watt

A led has about 3.6 volts over it, this makes the led current power/voltage: 11.84/3.6 -> 3.29 ampere, but this result is only valid in a ideal world, the driver will have some looses. For a buck driver it might transfer about 90% of the power, i.e. the real current is 3.29*0.9 -> 2.96 ampere.

The above estimates are not very precise:

The battery voltage (As seen by the light) depends on the actual charge condition of the batteries, the current draw and the loss in the ammeter.

The led voltage depends on the led type.

The efficiency of the converter varies with design and type, the best buck converters is above 95% and a boost converter can be below 70%.



It comes out to be .33/2.7 ~=.122, which is pretty much what I expected. This still potentially slightly off since 2.7 isn't the current, and rather the point. Backcalc assuming the driver is constant power (the somewhat uncommon driver in the 3800 3xml light, which I picked specifically for this) and reasonable droop for cells (fresh pana cells, assume 3.8v) it's actually approximately (3.8-(2.7*.122))*2.7/(3.8 ) = 2.585A, or .33/2.58 ~= .127ohm.

However, this feels somewhat off to me. For a more common application, ~3A for xm-l in DD, if a cell measured ~3A on DMM, .127*3 = 0.381v. On cree's specs, .38v is literally the difference between driving at 1A and 3A, and the curve is even steeper after 3A. So unless we're to believe 3A measured (which many members do on similar cheapo meters) actually means 5A+, which kills emitters, or chinese circuits even in DD drain significant power.

gcbryan, HKJ - Thanks to both of you for showing me the math that shows my light is most likely pushing the XML emitter to its limit for a single emitter setup.

I think i got a little disappointed with it when my Sky Ray 3800 arrived with its crazy output.

Agenthex, actually nowadays I rely more on my reflected ceiling/walls for "lumens". This is the bottomline, where the beef is. Sealed And of coz "throw lux". Sometimes i bring them out for walks...late at night when there are few people. LOL!

Yes, I'll eventually get around to doing the Everyman-Lightbox which any member can make and test their lights with reasonable consistency. Did some prototyping w/ it, but might have to get some more milk cartons to test if internal reflectivity is consistent.

Thanks HKJ! In your guide i couldnt find a poignant note why the user MUST NOT use the mA/uA-setting of the DMM when measuring tailcap amperages e.g. in Low-mode or Moonlight-mode. You did mention the burden voltage on BLF and this is the exact reason as i learned (once the concept of shunt resistor, reduced accuracy and burden voltage was explained to me by Dave's webpage and video) but maybe you could include the warning more explicitly in your guide.