On Monday, 16 April 2018 01:25:26 UTC+1, Mike S wrote:
On 4/15/2018 5:01 PM, mike wrote:
On 4/15/2018 4:39 PM, Mike S wrote:
I was given a broken LED light (on a headband) that takes 3 AA
batteries, can't find the lumen spec as there is no company name or part
number on the ct. board. If I just run the 3 batteries directly to the
LED will the batteries drain too fast or will I damage the LED? I don't
know if I have to limit the current for either case.

It depends...
What components are on the circuit board?
Does it look like a voltage or current regulator?

A typical cheapo light might use crap "Heavy Duty"
batteries and expect the series resistance of the battery
to limit the current.Â* Putting Alkaline cells, which
have lower voltage, can overheat the cheapest ones.

Others have sophisticated control circuitry.

There's a lot you can do to figger it out, but the nature
of your question suggests that you don't have the tools
to do that...or you'd have done it already.

I put an 18650 lithium battery into a 9-LED
Harborfreight 3X-AA cell light.
4.2V is less than 4.5, right?
Well, it was bright...but the LED's burned out
in short order.Â* I put 1.5 ohms in series in the next
one because that's what I had.Â* Has been working fine.

You can actually calculate the best resistor, but
since you don't know the specs of the led, there's nothing
to put into the equation.Â* And the range of voltage over
the life of the battery
can exceed the drop across the resistor at full charge.

Safest thing is to put something like 10 ohms in series with
a full battery and drop the value until it gets as bright
as you dare.

Sometimes, you can use google images to find lights that look
like yours and go searching down those paths for representative
specs.

Thanks, I looked through a lot of google images and ebay head lights and
couldn't find this model. There was a component of some sort on the ct.
board that had a large dollop of very tough epoxy completely covering
it, I would have destroyed it getting the cement off, after about 10
minutes of trying to carefully shave/chip it away I gave up.
There were 3 resistors and a transistor on the m/b in addition to the
mystery component, and since something was malfunctioning and reflowing
the solder joints didn't help I couldn't measure the voltage at the LED.
There were 4 smaller LEDs on the board as well, complicating the ct.
enough that without knowing the mystery component I didn't know how to
sort it out. I like your approach, starting high, try decreasing values
of series R until I have decent brightness, then see how the batteries
and LED hold up. Thanks for your thoughts.

If you had uncovered the chip your light would be dead for sure.
4.5v is far too high to run an LED direct - at least with most LEDs.
If it's a 20mA LED, which I can't assume, a new battery of 4.65v would need the R to drop about 1.15v at 20mA = 57 ohms. A near flat battery at 3.5v would want a resistor of 0 ohms. And there's the problem with running an LED off a battery & resistor. What's really wanted is a constant current circuit. There are simple ways to do nearly that, but it's not as simple as a resistor.


NT