Tea and coffee looked shit so I got myself a Costa on the way home
Just put the Christmas tree up. Unfortunately bought some new lights and the sodding things were battery powered. 200 LEDs battery powered Fortunately it has a 3 cell AA holder thus 4.5V. Pulled 100mA so I dug out an old Nokia charger and a USB cable and did some surgery and added 4.7 ohms in series to drop half a volt. Problem solved and it didn’t cost a penny to fix!
Just had a very similar adventure with some little $4 desk lamps from the Dollarama; in fact, my grocery shopping an hour or so ago included some 3.3 ohm/3W resistors for ballasts. We bought the little desk lamps cuz they were cheap, but convenient as they are bred like rabbits and we quickly ran out of NiMH AAs for them. 😝 So I soldered a spare USB cable inside one, figuring "this thing has a commodity LED driver IC with touch sense with 3-level PWM output control; surely it has current limiting..."
Yeah, it has current limiting alright... The IR of the batteries themselves. 🙄 Plug it into a 1A Apple phone cube, not some random cheapie, and it lights up great; nice & bright. But after about 5 minutes I start to smell warm ABS and the power cube is hotter than a $2 pistol. Worse yet, I can feel warmth in the middle portion of the upright stalk, a place where I know there's NOTHING with angry pixies in it except a couple 26 gauge-ish wires.
Some minutes of Fluke-vestigation later, I discover it's pushing 1.3A on HIGH compared to 700mA with batteries; dropping 2.2V across those 26 ga wires.
So I scrounge around in my stuff and find a 3.3 ohm 1/2W power resistor from a junk PCB; it's perfect, dropping current to 800mA and everything is running cold as a stone except the resistor itself which is "sizzle-your-finger" hot. A little head-scratching later I realize that since all my resistor assortment is 1/4W, there's no really feasible way to make that up for 5 or 6 of these little lamps while dissipating a total of approx 1.5W per unit.
Now at this time I've lost my sense of humor about Mr Ohm and just want the damned thing buttoned up & off my bench, so I scrounge up a little scrap of aluminum strip, fold it over the resistor, then squeeze some thermal grease inside to make a 1/2W resistor handle almost 2. 30 min later the mass is just warm to the touch, so I button things up & call it a win.
mnem
Except for the other 4 or 5 similar lamps calling my name from other rooms in the house...