nFlex question

I have been running an nFlex in a Dorcy Aluminum 4AA light for about a year. The 4 NiMh AA batteries draw about 750ma at the brightest setting with the nFlex set to 1A. It is very bright but not driving the TWOJ 3W at the full 1A. If I assume 90% efficiency then the LED is drawing about 675ma. (I have not measured draw at the LED) I've accepted this because even with the reverse polarity protection shorted, the nFlex has about a 1v drop and with the TWOJ vf of about 3.4v the 4 AA NiMh batteries are right at the limit. Today in preparation of making the same light but with 8AA batteries I dropped an 8AA battery holder with fully charged NiMh batteries into the same light and measured the draw. (Had to hold meter in place without tailcap). To my surprise it too drew about the same 750ma and not the 1.1A that I had expected. Can anyone explain what I may check or what I am missing?

Sflate,

I have also built a Mag 1D a while ago with nflex with a Lux III, and remembered the same dilemma. It was resolved after I re-read the manual from George.

Did you get a chance to configure the nflex to the full 8 levels, since the default is less and it doesn't go up to 1A?

8AA nimhs would be about 9.6V and your J bin is probably around 3.7V at 1A.

You say you measured 750mA at the tailcap (and you expected around 1.1A - why?).

3.7V x 1A = 3.7W at the Luxeon.

Now, if we assume 85% efficiency then we have 3.7/0.85 = 4.35W at the input.

So, with 9.6V at the input, less drop in the meter and and other resistive paths in the light and battery holder and the ~1V drop in the nFlex protection diode etc, then we have about 8V (give or take) on the input to nFlex's switcher. If you measured 750mA at the tailcap, that means about 8v x 0.75 = 6W at the input.

That still would be high, since we have a 6W - 4.3W discrepancy. Maybe you have more loss in the battery holder, contacts and/or meter. It would be useful to measure the input voltage right on the nFlex input pins to see what the voltage is there.

One way to get a good estimate of current to the LED without putting the meter in series with the LED is to measure the voltage across R5 (green resistor with R10 written on it). R5 is 0.1 ohms, so whatever voltage you measure across it X 10 is the current (in amps) going to the LED(s).

george.

AuroraLite - Yes it was configured correctly for the 8 levels at 1A. I think George is right that the meter and contacts are adding a lot of resistance. I hadn't even considered that. Unfortunately I cannot get to the LED or nFlex very easily anymore so I will just have to measure it next time I build one. FYI, I was expecting 1.1A because I figured that 1.1A x 90% (efficiency) would be about 1A to the LED. I guess you can't figure it that way as I can now see from George's calculations. My next 4AA will hopefully be a bbFlex so I won't have to worry about how close the voltage drops to the vf of LED. Can use the bbFlex though with 8AA so I will still need at least one of those soon.