Dropping voltage - how?

[deejayspinz]

Hey gang,

So, one of my lights on my custom LED bike light set has crapped out. I want to replace it with another P7 and was looking at the Aurora P7 @ DX http://www.dealextreme.com/details.dx/sku.15739 I already have one of these and want to use it for the bike.

Now, here's my dillema. My battery pack is 4 x 18650's wired up to provide 7.2V. However, the Aurora maxes out at 4.2V. I need to drop the voltage, but want to maintain the current. Can anyone suggest a circut / components for a voltage regulator? I found this (http://www.robotshop.ca/dimension-engineering-de-swadj3-switching-regulator.html), but dont want to spend the $$$ and it doesnt provide enough current.

 

[Justin Case]

Why do you say that the regulator you found doesn't provide enough current? It is rated to deliver 3A and up to 25W, which is above the 2.8A rated max for the P7 (and the usual ~10W). All of the usual candidate high amp buck drivers cost that much -- Shark Buck, hipCC.

 

[deejayspinz]

@Justincase... Good pt..

Maybe I should reword the request more along the lines of - are there components I could purchase and DIY which would be cheaper than buying the the $30 ready made unit? I've read about an LM317, but I dont think it can support the current draw of the P7. Essentially, im interested if there are any other options out there?

 

[MorePower]

@Justincase... Good pt..

Maybe I should reword the request more along the lines of - are there components I could purchase and DIY which would be cheaper than buying the the $30 ready made unit? I've read about an LM317, but I dont think it can support the current draw of the P7. Essentially, im interested if there are any other options out there?

An LM338 can be used to limit current in the same way an LM317 is used, but can handle 5A continuous output.

 

[Justin Case]

You could try this.

http://focus.ti.com/docs/prod/folders/print/ptr08060w.html

Doesn't seem to be constant current, which is probably the main drawback.

 

[rmteo]

Two options:

a. Re-configure your battery pack from 2S2P to 4P - no need to drop the voltage and most efficient solution.

b. Use the TI that Justin recommended - less expensive ($9 vs $27), higher current (6A vs 3A). You do not need it to be constant current as the light has a driver built-in.

 

[deejayspinz]

Thx for the suggestions everyone.

 

[deejayspinz]

Will this work?

http://www.dealextreme.com/details.dx/sku.20330

Would just have to make sure it fits within the Aurora P7 (link in first post)