Help a newbie out with the LED's
OK, I know there are a bunch of threads out there on these subjects and I've read a bunch of them. I just need a couple of quick opinions and answers so please bear with me!
I'm going to build an Achesalot or Scar type helmet light. I haven't decided on SSC's or Cree's yet. I was going to go all out and just copy the Achesalot design right down to the battery but money is tighter than I was hoping for right now (recent divorce) so I'm trying to save a little and stick with my current battery. It's a 7.2V lion @ 4200mah.
What should I use? Boost or Buck. I'd like to do 3 LED's but would be fine using 2 if it would keep the run times up. I'm only looking for 3 hrs most for runtime. I'd also like to be able to go from 1000ma and dim down almost nothing or off pretty easily since we're not supposed to ride at night in the woods in NJ. What would be everybody's suggestion? Boost? Which one? Or what about what scar did and go with those little pre-regulated boards and some resisters?
Thanks in advance guys!
When using boost or buck drivers you need to give them space to work.
2 x led's = 2 x 3.5 volts each = 7 volts
therefore there is not room for either a boost or a buck driver there to allow for current regulation
A 3 led design with a boost driver and a 7.2volt battery works really well. 3 led's and a buck driver and your ideal battery is a 14.8volt Li Ion
I use a 7.2volt NiMH with my 3 cree cutter kit and a maxflex board. I run it at 700mA as more than that and the driver board needs heat sinking somehow to get rid of the heat generated buy it
hope this helps a bit
A little cheaper than the maxflex, though you still need a rheostat to get dimming...and you still need add a switch to turn off.
OK, so if I went with a boost so I can use my existing battery (which is actually a 4800 mah) I can go with 3 LED's and be fine. That's a good thing. How would I calculate run time? Would I get enough out of it? You mention the overheating issue...would you have the same overheating issue if a buck was used at 1000ma?
The shark looks like a good option. Is there a reason you don't see as many people going with it vs the taskled stuff?
Originally Posted by NJCorky
Battery = 7.2volts x 4.8 amps = 34.56 watts
Light @ 1000mA = 3 x 3.7volts x 1 amp = 11.1 watts
@ 700mA = 3 x 3.5Vf x 0.7 = 7.35
Therefore approx 3.11 hrs runtime at full wack and 4.7hrs at 700mA. Sure you can work out any other combinations from there.
Because a buck driver works differently it doesn't have the same heating issue from what I can gather, I'm not an expert on these things though
Here's a great calculator to figure out runtime: http://www.jtice.com/led_pro/led_pro.htm
To be clear, the Shark is a boost circuit. It would need to be sinked if run at 1000ma.
Buck circuits generally don't need to be sinked. I actually have both Taskled Bflex's and Sharks...but I think most use Bflex since it has a great UI, and does dimming and can turn on/off via one momentary switch (which is included). You could get the Remora board for the Shark, but then the price difference isn't so much. Taskled wasn't offering the Maxflex (boost) for a while, but recently fixed it. In fact, he is releasing the UIB2 interface for it (similar to the Bflex) soon. With the Shark, w/o the Remora board, you need a rheostat (audio taper) and a on/off switch. Hope that helps.
Not really true about buck drivers - they can get plenty hot, too. You also forgot to include driver efficiencies - figure 85% is reasonable. So your 11.1W becomes 13W and your 7.35W becomes 8.65W.
Originally Posted by iggs
You've got 4hr at 700mA and 2.65hr at 1A.
And to NJcorky - you've got a 4 cell Li-ION battery, right?
GMF - Yup, it's a 4 cell.
If I go with a maxflex I get dimming options?
Is heat sinking the board possible in an achesalot design? I know he doesn't and he's running at 1amp but I think he's using a buck instead. I guess that's the difference.
I was trying to avoid spending the money on a 14.8V but if I can't easily get a boost to work at full power I may have to. Are 3 of these suckers at 700ma bright enough compared to 1000ma?
maxflex has dimming...
Pretty sure you could just attach the board to the housing w/ some thermal adhesive (non-permanent type would be best...and after you have soldered all the wires to it). 700ma vs 1000ma....its a bout 30% difference in output...but at 1000ma, you get more heat and less efficiency. I have both, and while you can see the difference, the 700ma triple is still plenty bright.