Is this drive too direct?

nikon · Jun 16, 2004

I bought a China-made light with four LED's and a clickie tailcap at Ace Hardware. It uses three AA batteries. There is nothing between the batteries and the LED's, not even a resistor. Is this thing likely to burn out soon?

RussH · Jun 17, 2004

My guess is that you won't have problems with NiMH batteries and probably won't have any problem with alkalines. I have an old CCrane trek lite that is similar. It does, however, have a 10 ohm resistor between the 3 batteries and the LEDs. This obviously doesn't limit current, just drops the voltage a little. This light came with 2 LEDs, I added 2 more (existing holes in board). One LED did burn out after extensive use (several sets of lithium AAs). I haven't gotten around to replacing it yet. I'm using NiMH batteries for about 3.7v most of the time. the light is brighter with AA lithiums, but that is what burned out the one LED that failed. Note that the newer CCrane lights have a circuit in them. Apparently they thought it was worthwhile.

The_LED_Museum · Jun 17, 2004

[ QUOTE ]
nikon said:
I bought a China-made light with four LED's and a clickie tailcap at Ace Hardware. It uses three AA batteries. There is nothing between the batteries and the LED's, not even a resistor. Is this thing likely to burn out soon?

[/ QUOTE ]
As long as you don't use lithium AA cells (like Energizer L91 cells), you shouldn't have problems with the LEDs blowing up. Notice I didn't say you won't, but you shouldn't. At least, not until you've run a number of sets of batteries through it.

nikon · Jun 17, 2004

Thanks, gentlemen. I guess this would be a good light for using up old batteries.

3rd_shift · Jun 17, 2004

I have had single leds get hot when direct driven by a nimh cordless phone 3x battpack.
The older nicad battpacks didn't get the single led enything beyond slightly warm.
My vote would be to use some 1000mah radio shack nicad aa batteries with that light for that "showroom bright" look without being overdriven.

gwbaltzell · Jun 17, 2004

[ QUOTE ]
RussH said:
... It does, however, have a 10 ohm resistor between the 3 batteries and the LEDs. This obviously doesn't limit current, just drops the voltage a little. ...

[/ QUOTE ]

Of course this limits the current! Plug in the math. Into Google™ put:((4.5 V - 3.6 V)/10 ohms)/2 =
which gives: (((4.5 V) - (3.6 V)) / (10 Ohms)) / 2 = 0.045 amperes
45 mA each is just under the absolute limit of the typical white 5 mm LED. By adding two more you are right down in the rated range of 20 - 30 mA.