I had been waiting on this driver from FT. It was supposed to be 500mA on a single NiMH and 800mA on two. I put it together in the mag-mod I’m building my mom for moms day and measured the current and it’s pulling 2.2A at the tail. I took it apart and tested emitter current and on the bench on a the same two [brand new] alkalines it pulls the same 2.2A while only outputting 230mA to the emitter.
The goal of this light was long runtime but this has to be the worst driver I’ve ever seen. Could something be wrong with it? Do I have any other choices for a ~.8-1A driver that actually only pulls ~1A from the two cells? I’ve noticed on this driver the LED- pad has continuity to ground so the positive side is where the “regulation” occurs? (Verified LED+ is NOT a direct path to BATT+)
The driver is supposed to deliver 500 mA to the led. To do this it must boost the voltage from 1.2 volts to whatever the led needs which is probably 3 + volts. Hence the high reading at the tailcap. This will not be technically correct what I've said and others will chime in but may give you an idea whats going on. It is a boost driver.
Hi,
at the moment I am examining those drivers.
They are cheap and have usually a effiency of 78 - 85% which is not really good nor bad. If yours draws 2.2 Amps and just gives the emitter 230mA something is wrong.
Actually a quiet good led driver with a max current of 950mA (depends on the sense resistor, here 150m which means 633mA for the Led). Also the driver is the typacal applications circuit from the datasheet:
2.2A sounds fairly accurate for that driver. I have tested the one with modes: http://lygte-info.dk/review/DriverTest%201-2%20AA-AAA%205-Mode%20800mA%20Linear%20Booster%20%28YJP%29%20UK.html
NiMH is 1,2V, LED emitter ~3,5V so to deliver 500mA to the emitter with 75% efficiency (which actually is prety decent for boost circuit) you need 2,3A from the battery. Very close to your result. Output current appears to be pretty low though.
Perpetuum mobile will do. With 1A from 2 cells don’t expect more than 500-600mA to the emitter.
If you figure out how to get more output than input, patent it and you can hire Bill Gates & Carlos Slim to shine your shoes and fetch your newspaper every morning.
I should clarify since I used both terms NiMH and alkaline. I built this light specifically for (and tested it with) brand new duracell alkaline’s measuring 1.49v. The meter is my ExTech and had always been pretty accurate, I just now measured a few other known draw lights I have around and the readings are right where they should be.
On the bench, from 2 duracells I’m getting 2.2A draw with only 243mA going to the emitter.
I now understand what your saying about the ~2.2A draw being correct for a host circuit but isn’t the output kind’a low? At 1.5v it’s advertised as doing 500mA, with 2cells it’s advertised as doing 800mA, for me this particular one isn’t even doing half of the 500mA at 3v.
It's two separate things going on here. First, it's normal for a boost driver to take a low voltage/high current input and turn it into a lower current/higher voltage output. That's just how they work, if they didn't, they wouldn't be boost drivers. Second, that specific sample you have there is broken in some way. It should be drawing less on the input side, and doing more on the output side. Sounds like it's not a problem with that driver model, just the one piece you happened to get.
On small parts like this I always order more than one, not because I expect it to be broken junk, but when something like this pops up it's nice to have another one to compare. Hope for the best, plan for the worst, etcetera and so on.
My favorite single mode most efficient driver for 2AA is still the Sandwich Shoppe Nexgen:
…yes, pricey, but for those special projects where nothing less will quite do…
If you want to use it for Alkaline, best to use 400-500mA (or lower and put your own sense resistor on). Alkaline capacity really drops fast at the higher current.
