I`m planning on modding one of the really old type LED lights with the nasty white/purple 5mm leds and dropping in 9 of these new Warm white 5mm leds in there instead.
now I know these leds are 3v and the 3xAAA series battery holder with primary cells makes about 4.5v (enough to damage the leds) so it relies on voltage sag and the batteries own internal resistance to not blow the 3v parallel wired leds.
I`d like to run this light on an 18500 Li-ion cell when I`m done, is this likely to ruin my work given that the battery will allow 15A draw?
Definitely, you will need something to limit (passive resistor, e.g 1/2W 6-10 Ohm to limit to 20mA assuming equal sharing) or regulate (active control loop with current sense feedback) the current.
i have a bunch of those 9x5mm lights and bought a batch of high CRI 5mm from Rangwan in Thailand to swap out such as you are planning.
How are you planning to hold the 18650, are you making a new body tube? i really like your idea of getting rid of the AAA so i may copy it.
I have a load of spare tubes from 21700 lights so they can use 18650s, just sitting in a tin doing nothing, so I was going to cut one down to 18500 length and use that (18500`s are the right length for this).
There`s not much room in there to add anything elaborate and a resistor would be too big to handle the power drop, I did think about soldering 3x 1n4001 diodes in parallel between 2 coins (laying flat) and putting that between the spring and the battery to give me a .7v drop capable of carrying 3A.
there`s no Pil or driver board, there`s just a single fiberglass PCB, so space it really tight!
if you are lucky your led boards already have a resistor.
a few of the better built ones i kept here typically have a 2.2 ohm.
with even more luck you can use 18650.
but 18500 with a rolled up strip of cereal box just drops in.
i have some high cri leds that tolerate overdriving well that work fine without resistor on 18650 and 18500.
it comes down to the vf of yours and how they respond to overdriving.
With brandy-new alkaleaks, all 9 will draw maybe 250mA total, so about 30mA per, but will drop quickly as alkaleaks suck.
Ideally, you don’t want more’n 20mA per, so 180mA total. So you can’t even really throw a single 7135 at it.
That means pretty much a single resistor to limit current, but it’ll be the same, too much current at first, then drop sloooooooooowly over the total SOC ’til it hits the knee voltage of the LEDs.
Simple stoopit way to do it is to cut a piece of plastic like from the side of a milk carton, and fold over a small resistor, one pigtail to hit the negative end of the cell and the other to hit the spring. Tape the body of the resistor in place.
Want to limit them to 20mA each? Say their knee voltage is 3.6V, so 4.2V - 3.6V = 0.6V.
Then 0.6V / 180mA = 3.3Ω.
Wanna go less worst-case, use 2.7Ω or even 2.2Ω, up to you.
well if I go the resistor route, it Might be possible to modify the pcb a little, it`s basically an outer circle for the neg side of the leds and a whole circle of copper with the spring in the middle and the Pos side of the leds soldered in.
if I cut a circle around the outer part of the inner circle thus isolating the Pos side of the leds, and bridge that cut with 4x 10 ohm resistors making 2.5 ohm but at a highter wattage.
That should still be flat enough to fit even with 1206 resistors.
I used the 7125 regulator, was able to fit it right on the LED board…. can’t quite remember the details, but clipping the middle supply pin on the chip, straddle the output and positive pins across the LED feed traces, then isolate the spring from the outer negative rail, add a wire from the spring to the center negative supply/heat sink on the 7135…
Arghh sounds more complicated than it is to implement. I would have to pull one apart and photograph to simplify that jumble of explanation. If you have the parts in hand though, it will be obvious how it could work.
They work great with these wonderful 5mm LEDs, a nice flood light. I use them with 18500 cells as well as 3 AAA.
…… as I recall now, some of these lights did not use a spring for the negative feed, but a small wire that contacted the body across the edge of the LED board. Same basic mod though, the 7135 still fit directly on the board.
However you do it - please document it and take pictures.
I ‘direct drive’ my 9 and 12 LED lights since there’s not much space (and so am limited to 3xAAA), but have constant current regulators laying around since a year or so. Just need an idea how to cut up traces and place the chip safely.
The LEDs I have from rngwn have no problem being driven to 40ma. Check the test data in the thread reference above.
If you are not using them…I don’t know, maybe order some. The 2300K are a delight as well for night light, candles, etc. I’ve used over 100 already. A handy item for the toolbox.
