Some of them do have resistors, it all depends on the LED that they are
using and the battery configuration. The Luxeon and Cree LED's have
forward voltages of around 4-4.5V and up. The cheapest of these that
you can buy in stores run off of 3 AAA batteries, but as the batteries
wear they can drop below the voltage required by the LED. You can't get
a good LED flashlight with just 2 AA, C or D batteries because the
voltage isn't high enough, unless there is a boost circuit, and that
makes the light quite a bit more expensive. The new Maglite LED
flashlights have a boost circuit, but they don't have a good means of
dissipating heat so they are not as good as they could be. The small
police-type flashlights that use a pair of 3V Lithium batteries
sometimes have resistors, or sometimes they just drive the crap out of
the LED to get a little more brightness, but again the good ones like
Surefire have circuitry to maintain the voltage right where the LED
wants it.
> Led flashlights get away without an external current limiting resistor
> because they've got a big honking current limiting one in the battery.
> They also tend to use higher power drawing diodes. But better flashlights
> have driving circuits that provide both current and voltage regulation.
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