Need help with my custom lightstick

[kerulberul]

Hi,

I'm new here and this is my first post. Created an account to ask for help here. I'm a total newbie in electronics.
So my situation here is that I'm modding my lightstick (those used for concerts) to make them brighter as its very dim and it uses one 5mm led.

The way I'm modding it is replacing the 5mm led with 5 3W leds http://www.ebay.com/itm/180930028142 , ea with a forward voltage of 3.6-3.8V and current of 700mA.
For the source I'm using 2 10440s http://www.ebay.com/itm/150995625943 in series.

My problem now is that I keep screwing up with my resistor calculations and burnt my last 2 resistors.my last one being ~7ohm 1/2 watt. So I need to find a proper resistor for my circuit or is there any other way of solving it?

This is my circuit diagram.

 

[LilKevin715]

From what I see there are a few problems. Running those 5 LED's in parallel with your power source in series (direct drive) isn't the best way to do it. Ideally you would need a buck circuit instead (more efficient compared to direct drive) since your battery input voltage is much higher than the LED's Vf. If you had to stick with the schematic then you would need a 1 ohm 13 watt resistor. This is assuming you want to drive the LED's at their rated spec of 700ma @ 3.7v. It would be much better if you could parallel the 10440 batteries, that way you probably wouldn't need the resistor at all.

However since the LED's are wired in parallel they will draw 3.5A, WAY too much for 10440 cells as the maximum safe discharge rate for li-ions is 2C (0.6A X 2 = 1.2A). The original design used the 5mm LED's as their current draw was most likely much less. If you want to use those LED's you linked then your power source will be the problem.

 

[kerulberul]

From what I see there are a few problems. Running those 5 LED's in parallel with your power source in series (direct drive) isn't the best way to do it. Ideally you would need a buck circuit instead (more efficient compared to direct drive) since your battery input voltage is much higher than the LED's Vf. If you had to stick with the schematic then you would need a 1 ohm 13 watt resistor. This is assuming you want to drive the LED's at their rated spec of 700ma @ 3.7v. It would be much better if you could parallel the 10440 batteries, that way you probably wouldn't need the resistor at all.

However since the LED's are wired in parallel they will draw 3.5A, WAY too much for 10440 cells as the maximum safe discharge rate for li-ions is 2C (0.6A X 2 = 1.2A). The original design used the 5mm LED's as their current draw was most likely much less. If you want to use those LED's you linked then your power source will be the problem.

So if i were to not stick to the schematics and make it as basic as possible, eg batteries, resistors and leds, what would be the best way to set it up?ouh and i can change the batteries to 3xN-size (1.5V ea). Or would the best be the buck circuit?

 

[LilKevin715]

You would need at least 4 10440 cells in parallel, or a larger size li-ion like a 18650 in order to run those LED's @ 700ma each. Your biggest issue is the power source as the LED's will draw 3.5A when wired in parallel. N cells (assuming you are referring to a LR1 alkaline) are a poor choice IMO in terms of output performance and cost.

 

[kerulberul]

Hmm so would you still recommend me to use a buck led driver? do you have any recommendations for one?

 

[datiLED]

I think that you are overlooking a very important aspect of your LED upgrade. Heatsinking the LEDs. The LEDs in your design are driven at about 10 watts total, which will generate a lot of heat. You might want to consider using a single high quality LED driven at a modest current to keep heat in check. You can still produce a lot of light (for a light stick), but will be able to reduce the requirement for a massive heatsink. Your battery requirement can be lessened, too. Using a simple AMC7135 (buck) regulator set to 700mA will produce well over 200 lumens, and require that you only disspate 2W worth of heat from the LED. The higher efficiency CREE LEDs will generate less heat, and more light than cheap e-Bay LEDs. Keep in mind, you will still need a heatsink.

 

[jason 77]

To the OP, It might help if you show a picture of what your glow stick looks like?

 

[kerulberul]

sure. the stick also comes with a pcb for the led and resistor. it fits 2 AA or 3 N-size/A23 batteries.

 

[LilKevin715]

I don't see any metal for the body of the host unless the pink portion is anodized... If there isn't any metal on the body of the host I'd pretty much forget about using a high-powered LED as the LED would overheat from the lack of metal for heatsinking.

 

[datiLED]

I don't see any metal for the body of the host unless the pink portion is anodized... If there isn't any metal on the body of the host I'd pretty much forget about using a high-powered LED as the LED would overheat from the lack of metal for heatsinking.

+1

 

[Norman]

Hmmm....the nail clippers are the only thing I recognize in your pic, and it doesn't really help with the scale. Do you own any other flashlights? I wonder if the tubular portion of the white wand would fit over a 2AA MagLED. If it's too small, perhaps an 18650 or a Costco 3AAA Teklite would fit in the bottom portion of the wand. I think a 2C MagLED would be the next size up after that, but you'd have to unscrew the head for that. Just a few easy alternatives if you have them available.

 

[Darvis]

For a glow light, I honestly think a basic Joule thief circuit with a 5mm LED would be plenty bright and it looks like it would fit in that host. Heat would be non-existent with this setup.