# COMMUNITY : FORUMS : BURNING QUESTIONS

## LED Mountain Bike Light

I'm designing a home made LED mountain bike light.  I Intend to use three CREE XPG 3UP stars which have three individual cree XPG LEDs mounted in series on a MCPCB star shaped board. Each individual LED when driven at 1500mA (the intended max current) will have a forward voltage of 3.5V. Therefore each 3UP MCPCB star will have a forward voltage of 10.5V when driven at 1500mA.

If I wire the three MCPCB's in parallel the current drawn from the battery will be 4500mA - correct? I was planning to buy an 11.1V / 6000mAh rechargable lithium-ion battery pack.

The battery pack will be connected to the LED's via a b2Flex LED driver that limits the max current to 1500mA but can be set to run at 350mA, 700mA, 1000mA and 1500mA max by programming the b2Flex driver. The light will probably be set to max current for short periods of riding but spend most time at 1000mA.

Does anyone see any problems with the slight voltage overdrive (i.e 11.1V from battery vs. 10.5V that he LED's will theoretically require)

I'd really appreciate any feedback on this.

Cheers Todd.

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trhnelson1970 (author) 6 years ago
I've just realised that this makes no sense. if then b2Flex LED driver limits current to 1500mA then this will be divided up between the 3 MCPCB's which will recieve 500mA - this will seriously reduce light output, and create alot of heat (the MCPCB's and led driver willl be connected to a heat sink but I'm more worried about maximising lumen out.

Will have to think things through some more.

Cheers

Todd.
6 years ago
Since the LEDs are all in series, they will all see the same current passing through them. If you put them in parallel, which is not recommended, the current will be divided. In this case, you must use additional circuitry to make sure the current actually is divided evenly.

Here is a decent link if you would like further explanation:
steveastrouk6 years ago
...so put them in series......
And don't, ever, parallel LEDs.

Steve
trhnelson1970 (author)  steveastrouk6 years ago
Steve,

If I wire the 3 MCPCB stars in series wont that require a total of approx 30V to drive them? i.e i will have a total of 9 x 3.6V LED's all in series (3 individual LED's mounted in series on the boards then the 3 boards mounted in series too).

I cannot use a Lithium ion battery pack greater than 14.8V.

Cheers

Todd.
6 years ago
Oops. My mistake, You'd need a "boost" not "buck" driver. Try these instead ?