I am interested in a mod to direct drive a CREE Q5 with 3 [800mAh] Eneloop aaa using little or no discrete circuit resistance. From battery charts I can expect to get 500mA for 1hr at 3.6v min from the series batteries. The forward drop of the Q5 is about this. As a EE I understand the lack of regulation of this approach and the part-to-part variability of both batteries and LED. I could determine and insert a proper resistance to trim back current if necessary, providing it is more than sufficient to begin with.
Has anyone tested this set-up? How much initial current with fresh batteries might I expect? Would I be better off starting with a lower Vf LED bin?
I appreciate any advice here.
If you want to try direct drive or a simple resistor, you will need to find a fairly low Vf CREE due to the low battery voltage available. If you can find a star mounted Rebel 100 type LED emitter, this would also be a great option because the Rebel's tend to have a bit lower Vf than the typical CREE.
Direct drive may well work, and if not your plan to use a very low ohm resistor will definitely work, but I like lights that have nice flat regulated output followed by a nice long mood mode. To get this I think your best bet would be to find a fairly low Vf Cree (around 3.3 volts at 400mA to 500mA) and then use it with one of the 7135 linear reglator boards that DX or Kaidomain sells.
Here's a
simple 350mA board (this is for qty 20 but they should be willing to quote them in lower quantities if you contact them by email)
The 7135 IC is a 350mA constant current regulator chip, and there are also boards available that have 2, 3 or 4 of these chips in parallel for proportionally higher current, as well as some that have CPU's which give some additional in between current levels and special flashing modes that are created by using a PWM chopping circuit which switches the constant current on and off.
1 Amp Multi-Mode 7135 board
What makes this chip so attractive is that the dropout voltage is only 120mV, so useing the above low Vf 3.3 volt CREE emitter as an example, a 7135 based driver board will have a
fully regulated ruler flat output down to just a little over 3.4 volts (or about 1.15 volts per cell), at which point the NiMH cells should be 70% to 80% discharged, then the output will gracefully tail off in a nice long moon mode that will run for another hour or more as the light level gradually drops off.
If you only have higher Vf voltage CREE's available then even direct drive may be disapointing with 3 x NiMH cells, but in that case you can try this flexible boost driver.
Multi-Mode flexible adjustable boost driver.
Because this is
boost driver, it will run into problems and tend to go into un-regulated direct drive if the battery voltage is more than a few tenths of a volt higher than the Vf of the emitter, but if your CREE emitter drops more than 3.5 volts when it is driven at 500mA, then this driver is the only way you will be able to keep the output well regulated as the batteries discharge. The board is multi-mode, but there are instructions here on CPF to easily bypass the CPU control and run it as a fixed output driver. One thing that is nice about this board is that even if you run it with fixed output it's still fully adjustable via a small built in trimmer control potentiometer which is included on the board.
The adjustable output of this boost type driver board is nice, but there seem to be confilcting reports on how efficient it is when driven around 3.5 volts, so if possible I would avoid the boost type driver entirely and try to stay with a lower Vf CREE and one of the above linear 7135 drivers (or just a simple resistor if you don't care so much about regulation).
