I have several cylindrical LED flashlights that have a large number of LEDs in them (the one I'm looking at has 9, which I think is typical.)
They take 3 AAA batteries, wired in series, to give around 4.5 volts (More, on fresh alkaline batteries, but voltage probably drops to 4.5 or less under load)
I've had a couple fail and taken them apart. I don't see any current limiting resistors or any components. The little PC board the LEDs are wired into is crowded so it's hard to tell what's going on, but I think they are wired in series.
I gather that if you wire your LEDs in series and can get the total voltage drop across your LEDs to equal the supply voltage you can omit a current limiting resistor? But I don't see how you get a drop of exactly 4.5 volts out of 9 LEDs. With LEDs with a 1.2 volt drop, 5 LEDS would equal 6 volts, so that would work.
Actually, I just looked at the PC board and the LEDs appear to be wired in parallel. Are there ultra-bright LEDs that you can drive directly off of 4.5 volts without a current limiting resistor? Or are there special purpose ultra bright white LEDs made for 4.5 volt supply that have internal current limiting resistors?
Follow-up question: Does anybody know if the 12 volt LED bulbs that are in landscape lights have a voltage regulator in them? I'm interested in using them as room lights for a photography setup, where I can quench the lights as the camera shutter opens. If they are simply LEDs and resistors, the'll quench well within the approx. 50 MS before the camera shutter opens. If there's a regulator in the system, they probably won't.
EDIT: I looked more closely at the PCB, and it's wired with all the LEDs in parallel. The flashlights that have failed have had a failure in the switch/wiring, not in the PCB assembly. The one I photographed got messed up by leaking alkaline batteries. I was able to clean it off and repurpose it as a subject light for my photography. It's "grotty" appearance is the residue of the battery electrolyte paste, and the wires are wires I soldered on in order to power it directly from a trio of AA batteries. (The original flashlight uses 3 AAA cells wired in series.
Here is the back of the PCB, showing the traces:
And here is the front, showing the LED lenses:
There is nothing else to the flashlight aside from the metal barrel, the battery holder, and a switch. I guess it's possible there is a current limiting resistor in the battery holder that I missed, but I doubt it. Plus I've driven the LED assembly for 15 minutes or more from 3 AA batteries, so if it was going to heat up and fry, I would think it it would have happened already. My guess is that these are ultra bright white LEDs that have a 4.5 forward voltage drop. Is there such a thing as LED packages with built-in current limiters?