Guess at current-limiting resistor for LEDs?

[patc]

Hi guys.

A month ago I found a 24-LED camping light for $4, runs off 4 AA's. I grabbed two!

The 24 LEDS are setup in parallel in a ring, each getting the full 6V from the batteries. There is no resistor to limit current. I want to cannibalize the LEDs and the small boards they are on for bike lights. Problem is - battery life, as you might expect, sucks. Light output down sharply in about 20 minutes.

My electronics knowledge is extremely rusty, but I know there should be a resistor in there. Without knowing specs of the LEDS, how do I determine what resistance I need? Will a current-limiting resistor also help the battery-life issue?

Thanks.

[Coreyjon]

Any resister will limit current assuming you are running it in series. If I remember right I used to use 222 ohm resisters in series with each LED, and that should help out a little bit. Hope that helps.

[patc]

Any resister will limit current assuming you are running it in series. If I remember right I used to use 222 ohm resisters in series with each LED, and that should help out a little bit. Hope that helps.

Well, I expected *any* resistor would help, but no point tossing 10k-ohm in there and wasting power. One in series with each LED? Why not a single resistor at the source?

Yikes, I used to know this stuff.

[n4zou]

Here. http://ledcalc.com/

I don't know if I would bother. 3mm or 5mm LED's are so old school. Not much light for battery drain. New high power LED's from Cree and SSC are very bright, driver circuits are cheap, and efficient optics get the light where you need it. Here is the link to Martins page where you will find everything you need to build truly bright LED bike lights.
http://pilom.com/BicycleElectronics/BicycleElectronics.htm

Here are links where you buy your parts.
http://www.dealextreme.com/products.dx/category.917

Exceptionally good optics.
http://www.luxeonstar.com/category.php?id=121&link_str=121

[Stax]

I have been playing around with these $10 surface mount led modules. The high intensity LED array is extremely efficient. I get about five hours on a 9v battery and they can be daisy chained. They are blinding.

http://www.ledwholesalers.com/store/index.php?act=viewCat&catId=66

I haven't used them for a bike application yet, but I plan to.

http://www.ledwholesalers.com/store/images/uploads/2016.jpg

http://www.ledwholesalers.com/store/images/uploads/2016RD.jpg

http://www.ledwholesalers.com/store/images/uploads/2016WH.jpg

[Pig_Chaser]

Hi guys.

A month ago I found a 24-LED camping light for $4, runs off 4 AA's. I grabbed two!

The 24 LEDS are setup in parallel in a ring, each getting the full 6V from the batteries. There is no resistor to limit current. I want to cannibalize the LEDs and the small boards they are on for bike lights. Problem is - battery life, as you might expect, sucks. Light output down sharply in about 20 minutes.

My electronics knowledge is extremely rusty, but I know there should be a resistor in there. Without knowing specs of the LEDS, how do I determine what resistance I need? Will a current-limiting resistor also help the battery-life issue?

Thanks.

The formula is: R = (Vbat - Vf) / I
Where R=Resitance, I = desired current, Vbat = Battery Voltage and Vf = Forward voltage of the LED

Now your situation is a little tricky as you don't know the specs for the LEDs... personally i like to just put in a variable resistor in the ballpark (as previously suggested, 222Ohms is probably in the ball park), and tweak it until the brightness/runtime is achieved. Then measure your variable resistor and swap it out for a standard value. I'm guessing your going to want between 75 and 350ohms.

I would string your leds in parallel with a resistor for each for simplicity. Although you could come up with an endless combination of series/parrallel arragements.

Just as a side note i believe there probably is a resistor network in the lamp.. possibly covered by a dot of black epoxy? It could be quite small and not resemble a resistor in the least. Did you actually measure 6V across the leds?

n4zou is also quite correct about power leds, a single cree/soul will probably be brighter than all 48 leds maxed out. But you could make a great "be seen" light out of those led's or some cool accent lighting or something. 'cides tinkering is FUN, good luck.

[patc]

Now your situation is a little tricky as you don't know the specs for the LEDs... personally i like to just put in a variable resistor in the ballpark (as previously suggested, 222Ohms is probably in the ball park), and tweak it until the brightness/runtime is achieved. Then measure your variable resistor and swap it out for a standard value. I'm guessing your going to want between 75 and 350ohms.

A site with "typical" values for LEDs suggested around 80 Ohms, so I will look for a variable resistor of, say, 1 to 300 or so.

I would string your leds in parallel with a resistor for each for simplicity. Although you could come up with an endless combination of series/parrallel arragements.

I actually just cut up the existing circuit board for simplicity. While there is a lot to be said for doing it right, sometimes the barbarian approach is fun too! This puts all 24 LEDs in parallel (but actually spread across several fixtures), but means the only simple way to add a resistor is at the battery.

It could be quite small and not resemble a resistor in the least. Did you actually measure 6V across the leds?

Not as far as I can see - nothing at all. I will break apart the switch from the original fixture, in case something was built in that.

But you could make a great "be seen" light out of those led's or some cool accent lighting or something. 'cides tinkering is FUN, good luck.

It works great as a "be seen" system! Got a lot of attention in my test runs. Not really visible in bright sun, but visible when dark overcast and extremely visible at night. All MacGyvered from a cheap camping light and spare parts.