driver for 6v lantern, 10mm white LED

[Daravon]

I have one of those generic flashlights that takes a 6v lantern battery. I also have some 10mm (I think) white LEDs I bought at Digi-key a long time ago. I was going to rig it up with the goal being an emergency light with infinite shelf life that lasts for many days or weeks on a battery. I don't know anything about the LEDS but I pulled 50ma out of thin air as a probable current for long life. I was going to buy a driver board from dealextreme, but I don't know if any go this low of current, and they all seem to be round format when they don't particularly have to be. I've also thought about using a resistor but I'm not sure how wasteful that would end up being.

 

[gillestugan]

I assume you want to have more than 1 led in your torch? 6V is not an ideal voltage. If you want to put several leds in parallel you will have to get one resistor per led to prevent imbalance. This is the cheap and ineffcient alternative.

An even cheaper is direct drive by putting two in series. This will get you maximum efficeincy but probably low output unless you are lucky to have leds with very low Vf. (its easy for you to test. If you have 100 leds this may be an alternative even if the leds are underdriven.

I would recommend you to use a step-up driver and wire the leds in strings. You can for example use the shark driver from sandwichshoppe to drive 12 to 120 leds at 50mA. Put 6 in series and then paralell 2-10 of the strings and adjust output current to number of strings.
Putting them is series will equalize the variations in Vf of the different leds, preventing one led from getting much higher current then another.

Sorry, but I don't know of any step up driver from DX suitable for your leds.

 

[Lynx_Arc]

using a 6v lantern battery a 50ma load would last longer than the usuable light you would get out of such a bulky light would be. using 3AAs and a resistor at 50ma would get you 50+hours and depending on the battery in the 6v lantern is could last you over 300 hours continuously. IMO unless you are in hurricane alley this may be a little too much runtime for the size and low output.
If you are good at soldering I would gut a PR based bulb and get a resistor of perhaps 60-100 ohms and make a bulb you can swap in and out so no modding is necessary.

 

[Daravon]

using a 6v lantern battery a 50ma load would last longer than the usuable light you would get out of such a bulky light would be.

I'm sorry I don't understand this. Are you thinking the LED will burn out before the battery goes dead?

It's broken, so modding is already necessary. Low output is not really a problem, really I think 1 of these 10mm LEDs will be fine. I was planning on using 1. No good way to use more and still make use of the reflector.

 

[PCC]

How is it broken? Bad switch? Blown bulb? Dropped it down a cliff and you've duct-taped it back together? Inquiring minds want to know!

A 10mm LED that looks like an oversized 5mm LED is going to give you very little light. I have one soldered on to a 9V battery connector with a resistor and use it around the house so that I don't cast a lot of light out there to disturb my family in the middle of the night for bathroom runs and stuff. It's not bright enough for much more than that.

Using a resistor calculator, you'll need a resistor of around 68 ohms (for 40ma draw, 3.3V forward voltage, and 6V battery). With a 13000MAh battery (the good Energizer one comes to mind) you should get a few weeks of light out of that little 10mm LED.

Realistically, though, you might want to look into replacing the incan bulb with a drop-in LED bulb that is readily available from places like Sears, Home Depot, REI, etc. Keep in mind that most lights that take 6V lantern batteries are reverse polarity so that may limit your options greatly. You probably already know this but LEDs do not work with reverse polarity.

 

[lctorana]

At a nominal 50mA with a (say) 56 ohm resistor, you could expect the following runtimes:

El-cheapo 4D lantern battery: <100 hours
4F General purpose (standard duty) eg Eveready 409: <140 hours
4F Heavy Duty (eg Eveready 509): 160 hours (1 full week)
4F Super-Heavy Duty (zinc chloride) (eg Eveready 1209) 220 hours
4F Alkaline (eg Energizer 529): 520 hours (yes Virginia, that's three weeks continuous)

 

[Lynx_Arc]

I'm sorry I don't understand this. Are you thinking the LED will burn out before the battery goes dead?

It's broken, so modding is already necessary. Low output is not really a problem, really I think 1 of these 10mm LEDs will be fine. I was planning on using 1. No good way to use more and still make use of the reflector.

what I am saying is the runtime on such huge of a light for a dim bulb is so long that you probably will never use the battery up if you turn it off when you don't need it. a battery that powerful can idle at 500 ma for days instead of 50ma for over a month .

 

[gillestugan]

50mA is actually enough in most situations when used at home. Enough to read and walk, or whatever you want to do as you already know where to put your feet and where to find things.
It just seems a little unnecessary bulky.
I mean , how often does an emergency last for several months?
Anyway. There is nothing to lose in modding it if you already have the battery and the leds.

 

[Lynx_Arc]

At a nominal 50mA with a (say) 56 ohm resistor, you could expect the following runtimes:

El-cheapo 4D lantern battery: <100 hours
4F General purpose (standard duty) eg Eveready 409: <140 hours
4F Heavy Duty (eg Eveready 509): 160 hours (1 full week)
4F Super-Heavy Duty (zinc chloride) (eg Eveready 1209) 220 hours
4F Alkaline (eg Energizer 529): 520 hours (yes Virginia, that's three weeks continuous)

may run even longer as the voltage drops on batteries the current to the LED drops and the runtime can increase more unless you have a regulated circuit.