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Bad LED Flashlight Design
Photos of Flashlight
This flashlight contains:
  • 4x 1.5V AA Batteries [wired in series] (with a capacity of 2000mAh [Battery Capacity reference]) for a total of 6V
  • 50 Leds wired in parallel
  • A 1 ohm (Ω) 5% resistor to limit current
I have decided that this iss a bad design for a flashlight is because of it's unrealistic power requirements, which would cause the flashlight to go dead after being on for maybe 30 minutes.

Also, the flashlight top, that contains the leds and current limiting resistor, had some wires that came disconnected from the main body.
I have repaired the flashlight (reattached the top part), after this blog was written


Let's take a look at how Ohm's law can be used here

V=6V, R=1(Ω)
I=V/R and P=EI

I=6V/1(Ω)=6A
P=6V(6A)=36W

Which is quite a beefy power requirement for a flashlight...
If we divided I and P, by the # of leds (which is 50)
then the average values per led are:
Current/Led=6A/50=0.12A=120mA/Led
Power/Led=36W/50=0.72W=720mW/Led
** If any led fails it will cause the power (720mW) to be dissipated by the current limiing resistor
if enough leds fail, you could exceed the power rating of the current limiter.
the power rating, of the resistor used, is probably somewhere between 1 and 2 watts.
A resistor that dissipates, too, much heat could cause a fire.


Most common-use leds will operate between 20-30mA of nominal current (the ones most commonly found at Radioshack)
It serves to read the datasheets for the led and/or read the documenation that comes with the led.

If the flashlight is used as is, then The flashlight will last for approx. 1/3 of an hour or about 20 minutes.
remember the current capacity of an alkaline AA battery is approximately 2000mAh or 2Ah.
2Ah/6A=0.3333hours x 60min/h=20minutes

I can't speak for anyone but myself, however, in a power outage or other situtation I'd like a flashlight that last longer than 20-30 minutes continously on.
If 4x D batteries were used (@12000mAh)
then it would last for 2 hours (12Ah/6A=2 hours)
however, the flashlight's physical dimensions would be much larger.

A more appropriate design would be, as follows:
  • 4x 1.5V AA batteries wired in parallel [instead of series] (8000mAh)
  • A resistor of 1(Ω) (to limit current) [unchanged] -- you probably should use a higher resistance to reduce strain on the batteries
  • 50 leds [unchanged]-- less leds would help to reduce strain on the batteries
Ohm's law for the new design:
V=1.5V,R=1(Ω)
I=1.5/1=1.5A -> 1.5/50=30mA/led
P=1.5(1.5)=2.25W -> 2.25/50=45mW/led
8Ah/1.5A=~5 hours 20 minutes runtime
It should be noted that the led's Vf (forward voltage) drop would need to be taken into consideration, it would need to be aboit 1 to 1.2V.

In conclusion: Adding more leds to a flashlight does not help, it only increases the power requirements and shortens
the runtime of the flashlight, requireing physically larger batteries.
The best designs use as few leds as possible (this is the sole factor that determines the size and number of
batteries as well as how long you can use it for)