resistors in led torches

[grahamhunt]

I managed to destroy my work torch except for the led cluster on examining the the remains i am unable to find a load resister it has 19
white leds and 3AAA batterys What is preventing the leds overcurrenting
????????

[holtzman]

I was surprized to find the on/off button in my torch to have some Ohms (built in resistor?). There is some resistance in batteries, too.

[Klaas Robers]

Three batteries in series gives 4.5 volts. That is more or less the voltage that white (= blue) LEDs need to illuminate at their full brightness. So I fear there is no resistor.

With 19 LEDs in parallel the nominal current (20 mA per LED) will be about 0.4 A. At that current the voltage of the small AAA batteries will drop due to the internal resistance. So the internal resistance of the batteries is used to limit the current. If the batteries get more and more exhausted the light output will dim.

In the older flashlights a 3.8 V incandescent lamp was used to run on a 4.5 V battery. I always was puzzeled where the remaining 0.7 volt was gone. Well, in the internal resistance of the batteries.

[grahamhunt]

Thanks for that.I had assumed it must have something to do with the batteries limiting the current and you have explained it perfectly