"UFO" lantern - LEDs in parallel ???

[gentlegreen]

Hi all,

I bought a couple of these for camping and plan to modify one for my bicycle as they are very cost-effective per LED, come in a ready made package and there's room in the middle for a 5 watt, 10 degree MR16 LED.

They appear to break all the rules and consist of 2 concentric banks of 24 5mm cool white LEDs wired in parallel and connected to three AA cells. With new batteries the current drawn is 1 amp - making the average LED current 41 mA.

Switching in the second bank of 24 LEDs makes little difference to the current - or apparent brightness - suggesting to me that the current is being limited by the battery, and also that it would be kinder on the LEDs to run both banks ...

My current bike lights use 4 sets of 3 LEDs in series wired to a 12 volt, 3.3AH NiMH battery via resistors that limit the current to 20mA.

Given that the measured voltage across the LEDs in these lanterns is 4 volts, I'm guessing they have inbuilt resistors ?

My initial plan was to wire them series-parallel with a regulator or resistor to drop the voltage / limit the current.... though my instinct is to use 24 series pairs each with its own resistor - though this involves cutting the PCB tracks in many places :-

Any shared experiences would be greatly appreciated.



My current lighting configuration :-

Front :- 24 x LED, 9 watt fluorescent, 20 watt QH dichroic.
Back :- 59 x LED.
.

 

[Lynx_Arc]

I would not worry about 1 resistor per led but rather put resistors in such that they will limit current to maximum of 30ma per LED and with all running 25ma would be a good value making about 750ma drain or 2-3 hours on alkaline I would guess. If you are going to use rechargables your values will change as under heavier current loads they can supply even more power and at 40ms only the more robust 5mm LEDs can survive long... most do ok at 30ma but over time they deteriorate fast.

 

[gentlegreen]

Thanks - I'll have to stick my meter on my current LEDs - which have served me well over two winters of daily commuting and probably benefit from shoddy construction and hence plenty of ventilation. I'm pretty sure I aimed for 20mA.
I was considering going to a little more trouble this time - though maybe just a smear of vaseline rather than the full-on silicone sealant treatment ...