I had a customer tonight who bought three MN 21/23 12 volt cells. Nothing odd about that, those are the batteries most commonly found in the small keychain car-alarm transmitters. About five minutes later, I happened to see said customer waving around his small LED flashlight, looking quite happy.
Not good. Uh-oh.
I went over and politely asked if he still had his dead batteries. He did, and as I had suspected, the old ones were 1.5 volt “n” cells.
I freaked out a little bit, and asked him if he could please remove his new batteries so I might check them. And those little things were HOT when he took them out.
So anyways, did the 36 volts this 4.5 volt light had to take come close to killing it’s LEDs, or would it have just toasted itself, sparing the light sources?
Hmmm, strange; I’m surprised that the voltage didn’t instantly pop the LEDs (literally pop them - I’ve seen LEDs physically explode when connected to inappropriate supplies), but I can think of a couple of reasons why this didn’t happen:
-Perhaps the Flashlight contained some kind of driver circuitry - it is not terribly uncommon for LED flashlights to include inductor circuits that drvie them with pulses of high current - because (as I understand it) the circuit works by creating a magnetic field, then driving the LED off the induced current from the collapsing field, or something like that, connecting to an inappropriate source just makes the thing oscillate differently, without oversupplying the LED.
Or
-Perhaps the 12v cells simply weren’t capable of supplying the kind of current that the LED wanted to draw.
Certainly supplying 45v to a 3 to 5v LED without limiting the current should be expected to cause spectacular failure.
The forward voltage of white LEDs is AFAIK in the order of 3-3.5 V (don’t have a data sheet handy) which means that. if the 36 V batteries had zero internal resistance, they’d driven circa 20-35 times the regular current though the LED(s). But in that case the LED(s) would have died in a fraction of a second - it seems that the internal resistance of the 3 x 12 V cells was the main factor limiting the current. Do you have specs on internal resistance or short circuit current of the cells?