Planning ahead... LED Questions

Hi everyone, I have been researching for hours and hours about LED's on the forum, and google, and have a few questions. My goal is to build a very powerful LED bike light that will have some good flood, and also good throw with a couple hours of runtime. Can anyone please help me out with the following parts?

LED: I am thinking of using SSC P4's. They are cheap and bright, and I have seen lots of people use them. I think an array of 6 or so would be enough. Any better suggestions?

Heat/current: From my understanding, the more amps you feed these LED's, the hotter they will get. They are rated for more lumens the higher the current, but what is actually a good current to use? Lots of people seem to use 350mA, some use 600mA....all the way up to 1A. I don't plan on going crazy with the heatsinking, and don't want to drain a battery in 5 minutes...so what is the sweet spot?

Power Source: I have to buy a remote control airplane battery anyway, which is a 7.2V 3000mAh NiMh battery... does anyone use this type for these lights, or is there something better? It will go in my camelback.

Optics: From what I understand, I can simply buy the reflector corresponding to how much flood or throw I want and throw it on...right? So maybe a few narrow angle and a few wide angle reflectors would be best for this application?

Drivers: I finally understand what this is (kind of)! ...but are they necessary? Couldn't I just use a resistor in series with the LED's to bring the voltage down, or do I need the driver to regulate current? Looking online I see a bunch of them with different current modes, but how do you change between the modes? Also, people often recommend one driver per LED instead of controlling the whole array with one driver...why is this?


I know, lots of questions but with the advice of others, I'm just asking away and hopefully you guys can help educate me If, and when I have further questions, I will post them in this thread.

SSCs will work ok if they're on stars.

I like 700mA as a high-level drive current.

That battery will work fine. 6 SSC P4s at 700mA will give you about 90 minutes or so of runtime from it.

Mode switching is accomplished by turning the power off and back on, typically within three seconds or so.

The recommendation to use one driver per LED (or series string of LEDs) has to do with the likelihood that one LED (or string) will take more current than the others and burn out. Then the next LED or string gets too much current and burns out. This continues until you shut it off or all the LEDs are kaput.

For a driver solution I like AMC7135-based drivers. Here's a nice link that shows how to wire those drivers so that you can use series strings with the driver. Substitute "P4" for "P7", "3 NiMH cells" for "1 Li-ion 18650" and substitute "2" for "3" where there is three of anything. Oh, and use 2xAMC7135 drivers instead of an 8x.

You would want three AMC drivers running in parallel from the battery, with each one running two P4s in series.

That's how I would do it. It also happens to be the cheapest way. It's just one mode. I'm sure others can suggest other ways to drive your LEDs.

LukeA said:

SSCs will work ok if they're on stars.

I like 700mA as a high-level drive current.

That battery will work fine. 6 SSC P4s at 700mA will give you about 90 minutes or so of runtime from it.

Mode switching is accomplished by turning the power off and back on, typically within three seconds or so.

The recommendation to use one driver per LED (or series string of LEDs) has to do with the likelihood that one LED (or string) will take more current than the others and burn out. Then the next LED or string gets too much current and burns out. This continues until you shut it off or all the LEDs are kaput.

For a driver solution I like AMC7135-based drivers. Here's a nice link that shows how to wire those drivers so that you can use series strings with the driver. Substitute "P4" for "P7", "3 NiMH cells" for "1 Li-ion 18650" and substitute "2" for "3" where there is three of anything. Oh, and use 2xAMC7135 drivers instead of an 8x.

You would want three AMC drivers running in parallel from the battery, with each one running two P4s in series.

That's how I would do it. It also happens to be the cheapest way. It's just one mode. I'm sure others can suggest other ways to drive your LEDs.

Click to expand...

Thanks a lot, Luke(?)! That helped out a lot.

So, they won't get too hot with 700mA? All I plan on doing would be using some thermal glue between the star and some aluminum, and hoping the airflow is sufficient.

edit: Looking at the data sheet, it seems the maximum input voltage for that 7135 driver is 6V, so hooking up a 7.2V source probably isn't the best idea right?

see post below

bump

Question: In a direct drive circuit, LEDs in series will each receive a fraction of the source Voltage, correct?

So... how does a driver know how much voltage to put out? (In cases where you can put anywhere from, say 1-3 LEDs in series from a driver)


Question: How do you determine how many LEDs a driver will support? (say...a 700mA 7135 driver from DX)


Thanks guys

In a direct drive circuit there is no driver. The way LEDs work (with series drivers) is that the driver puts out a constant current at a voltage greater than 3 LEDs' combined Vf and gets back the remaining voltage.

The number of LEDs in a buck series circuit (this circuit is buck) is: minimum battery voltage - 1 = total forward voltage of LED string. That means 2 LEDs per series here.

LukeA said:

In a direct drive circuit there is no driver. The way LEDs work (with series drivers) is that the driver puts out a constant current at a voltage greater than 3 LEDs' combined Vf and gets back the remaining voltage.

The number of LEDs in a buck series circuit (this circuit is buck) is: minimum battery voltage - 1 = total forward voltage of LED string. That means 2 LEDs per series here.

Click to expand...

Thanks man

2 LEDs in series would mean 7.2V, then +1 means I actually need a 8.2V min battery though,right

kuksul08 said:

Thanks man

2 LEDs in series would mean 7.2V, then +1 means I actually need a 8.2V min battery though,right

Click to expand...

Well, your average pair of SSC P4s (depending on Vf bin) will have a total Vf of 6.4-7.0V at 700mA, so your NiMH pack will work fine. It will dim at the end though. But that lets you know if it's running low.

The standard 1-mode 700mA AMC7135 board has a drop across it of .14V. It's more if you don't remove the protection diode, which you definitely can do.

LukeA said:

Well, your average pair of SSC P4s (depending on Vf bin) will have a total Vf of 6.4-7.0V at 700mA, so your NiMH pack will work fine. It will dim at the end though. But that lets you know if it's running low.

The standard 1-mode 700mA AMC7135 board has a drop across it of .14V. It's more if you don't remove the protection diode, which you definitely can do.

Click to expand...

Looks like from this graph that its more like 3.6V, and then what about the +1V from that equation thing above? I really appreciate the help, just want to make sure I'm not missing anything before I go buy parts


Those 700mA drivers you showed said they want an input voltage of 3.6-4.5V, so the 7.2V battery would burn them up right? Would these be a better choice in my case? http://www.dealextreme.com/details.dx/sku.3256


Now that I understand the forward voltage relation to the source voltage, some people have still mentioned there being limits to the number of LEDs that a driver has the capacity to run. Does this vary from driver to driver?

:thanks::thanks:

P.S. I got my soldering iron today, and I suck at it

kuksul08 said:

Those 700mA drivers you showed said they want an input voltage of 3.6-4.5V, so the 7.2V battery would burn them up right? Would these be a better choice in my case?

Click to expand...

I posted a link earlier showing the successful usage of the 7135 with over 4.5V.

LukeA said:

I posted a link earlier showing the successful usage of the 7135 with over 4.5V.

Click to expand...

Oh I see now, I need to play around with configurations by drawing some schematics :twothumbs

Thanks again