so… i have a little palm sized 3xAAA coleman light. I kinda like it, its decently built solid… its little… nice for what it is.
the package claims “150 lumens”. whatever… ad-copy…
emitter LOOKS an awful lot like a cree xp-e…
little plastic pill, tin top for th emitter. it pops open, and it appears to be direct driven by a single resistor on the hot line.
my DMM says “6.5 ohms” on the resistor…
ledcalc.com has the LED calculator… i assum it would be as simple as changinf that resistor, to affect chages in the current?
maybe a little trim resistor (0 to 10 ohm?) to vary it as i wish?
snce resistors add in series, i was thinking a 1-ohm or 2-ohm resistor in series with the trim-pot so as to never be able to blow it…
or, is that calculator only valid for th noral typ of LEDs i see on panel indicators, not valid for power LEDs like we use?
also… do i enter 3.0 volts like i want to drive it from the datasheet?? or… should i use the differce between the 3xAAA battery holder voltage wth “no load” MINUS the measured voltage at the emitter pad when running, to get th “voltag drop” fro the calculator, assuming thats the right sort of calculator to use…
i know, newb questions… but still, we all started somwhere…
well, yeah… switching in a real driver, or just a 7135 WOULD be “doing it right”…
but…
its not like direct drive doesnt have its “place”… i want to play with direct drive on this little crap light, so if i ever do it on a big home fabricated light, i have more confidence what i am doing…
plus? it is “budget” forums… all i have to DO is hook up another suitable resistor in parallel with it…
so…
1) can i use the LED calculator
2) is voltage drop the theoretical 3 to 3.5 volts from the datasheet?
3) or, should i subtract voltage reading at the pill pads, from the voltage supply to get drop?
For LEDs current varies greatly with small changes in voltage. You can certainly do it your way but output will vary with cell voltage and led temperature and you won’t be able to calculate a particular drive current. That said, if you’re just ball parking it then yes just use the data sheets or measure the voltage across the led at different R values and go from there. With the trim pot you can always diddle it.
yeah, i realize i am ballparking things… not much in electronics goes from “design” to “working” without slight changes in values throughout the circuit, lol… (otherwise? small trim pots would not exist, lol)
cool to know i am more or less followign the “plot line” well enough… in another thread? guy was talking about fabricating a direct drive wow factor light, for his mtg2 emitter? figured i would “play” with lights layign around first.
yeah, thats ALMOST the light i th picture? Thats th 115 lumen one… mine was almost the same light, head design is slightly different (150 lumen version) body style similar though.
(mine has “2 cutout lines” for very “one cutout line” on the HEAD in the photo… but, very similar.)
i’m not in LOVE with this little coleman light or anything? its just fairly cool, for what it is… small hand size, little bit of throw on the reflector. STURDY little sucker though, body wise. i like th “hand feel” too.
its one of my “going out to get a bucket of coal” lights… not my “scan the woods for deer while i get coal” lights, lol…
shrugs a second resistor in parallel with th existig hot-line resistor, makes a little more light, a littl less run time? it would be cool… i SHOULD hve some small value, 1 or 2 watt resistors up in the collection… somewhere, lol… i’m a packrat, ha ha.
