In my quest to find the best, easiest, cheapest way to run 2 XM-L's from a 12v source (my riding lawnmower) I've come across these 12v DC to 3.7V DC regulators I found on Ebay. It seems to me that I could drive two XM-L’s in parallel from it, giving 2.5A to each XM-L, right? Am I safe in “direct driving” from this module since it’s only rated up to 5A output? Or is an LED driver still recommended to limit current pulled from that module? What could be expected connecting 2 parallel XM-L's to this module?
EDIT - It's actually a little cheaper from this seller with free shipping (in case anyone else likes this idea wants to buy it).
Those are adjustable voltage, so you just set the voltage to whatever level allows the desired current through whatever LEDs you attach to the outputs. Less voltage = less current, more voltage = more current. You could even have two of those boards, one set low and the other set high, and switch between them with an on-off-on switch for high/low beams.
Which, the ones you linked or mine? Yours are not adjustable, the current will depend on the LED's forward voltage requirements. With a fixed voltage to the LEDs they will run at whatever current corresponds to that voltage, up to the 5A limit of the regulator. If the LEDs need more voltage to run at 5A than that unit will provide, they will not use the full 5A capability.
Mine will do up to 3A but being adjustable, you can wire the LEDs in series. Adjust the voltage pot to whatever current you want it to run at. Set it and then slather in clear silicone for waterproofing, or use a small sealed project box.
LEDs aren't like a filament bulb. A LEDs voltage depends on the amperage, and the amperage depends on the voltage. They work like a water faucet. Voltage opens the tap wider so more water(current) flows. You cannot shove more current through if the voltage doesn't also increase. With a fixed regulator it will do whatever it's gonna do and if that doesn't give the output you want, nothing you can do about it.
Gotcha. I was referring to my link. So mine is regulated at 3.7v (assume it will be exactly 3.7v) with up to 5A output. Right now I have first generation XM-L's on standard aluminum stars. I don't know what current 3.7v corresponds to (and I'm on my phone now making it hard to lookup a vF chart). Lets assume 3.7v corresponds to 3.5A. Since I'm running two in parallel they will be trying to pull 3.5A each (total 7A), but since the regulator is only rated to 5A it will only provide 2.5A to each, correct? This is my suspicion and I just want to make sure it works this way. I won't burn up the regulator trying to pull 7A from it, right? I understand I'll be limited to whatever current/voltage combination this module will put out, but my suspicion is that it will put out plenty for my needs (i.e. 2.5A per emitter).
Cree - Flashlight Wiki has a link to the data sheet. From it I get that it only needs 3.3 V to accept its 3 amp. maximum current. So you would be using the supply as a current regulator rather than a voltage regulator at its maximum current output and reduced output voltage. It seems to me likely that it will overheat with that load, but you could try it. It is less likely, but it might also put out over 6 A and overheat the leds.
A current regulated output supply would seem ideal, but you could add a small resistor or variable resistor to control the current. According to the graph, at 2 A an XM-L takes just under 3.2 V, so you need to lose about half a volt in the resistor. At 4 amps. total current, that would take 0.125 Ohm and 2. watt, which could either be a resistor or, easier, a moderately light pair of copper wires, maybe longer than you need to make the connection.
I don’t have data at hand, but I think the problem is going to get the resistance low enough to drive the XM-Ls as you like them. 12 to 16 inches is more than one usually uses in flashlights. To be safe, you could find an Ohm meter and measure the wire or find data on resistance per length for your wire size. It would be nice to have a meter with thick leads to measure the current.
How much current to use depends on how much light you want and on how the leds are cooled.
Ok, ok, maybe I'll try those other boards. As I checked out those boards on Ebay I saw numerous versions. Are there ones to stay away from? Some better than others? If those boards took 18v AC as an input they'd be perfect (I'm still using another module for the 18v AC to 12v DC conversion). I saw a post on FastTech where someone said they put those modules inside those plastic 18650 battery cases. I guess it's ok to seal them away from airflow with no heatsinking?
You don't need to drop it to 12v, unless you need that to run something else. Just a plain rectifier to get rid of the AC is all you need for these LM2596-based boards.
There are dozens of versions, some have independently adjustable voltage & current pots, you don't need that - LEDs limit their current based on the voltage; some have additional controls to handle battery charging and have an auto shutoff function - you don't need that either. Just the plain ones the same format as in my link, they're cheaper per unit if you buy 10 at a time (and they are useful for all kinds of things you won't realize until you have them and experiment with them a while).
How about this rectifier? I'm sure the amperage rating on this one is fine, but what kind of amperage rating do I need? The input voltage is higher than my final output voltage. Isn't the input current also going to be lower? But how low? I guess considering the price difference isn't much different I should just get the larger one like I linked. (I do understand that the rectifier doesn't control current, just passes it and the current rating is just the max it can pass before burning up).
I'm sitting here thinking of what old electronics I have that I may be able to pull a rectifier out of. I have a bunch of old PC power supplies, could look in those.
I would try it first with one module and the two LEDs in series, these boards will do a max of 3.3-3.4A in my testing but may go higher at higher input voltages than I tested with (4 18650s in series). They don't make a lot of heat but I also did not run it long term like that to see what happened. If it gets too hot, use two boards with one LED each. They are probably smaller than you're expecting, around the same size as a USB thumbdrive. Two will fit in a little battery box no problem.
As for the rectifier, I honestly have no idea. I would use one from my junk box collection of pre-scavenged parts, or build one from 4 diodes. A ready-to-use rectifier is really nothing more than 4 diodes stuck together in a particular layout and in a single package for convenience. See here: http://en.wikipedia.org/wiki/Bridge_rectifier Search ebay for 'bridge rectifier', 'diode bridge', or similar. Searching for just 'rectifier' will turn up lots of motorcycle replacement parts, though if you find one in the right format (just 4 wires, no other 'features') and for cheap enough that might not be such a bad solution (they will be in a sealed, finned AL housing and might save you some work).
Yes, ATX PSUs usually have nice beefy rectifiers, mine in my junk box came from PSUs and old home stereos and the like. That Radio Shack part is similar if not identical to what I've pulled out of PSUs.
Convert everything to watts to compare input to output... since you know the desired output, say 6.6 volts at 3 amps, round up to 20 watts. An overly-pessimistic estimate on efficiency of the regulator board would be 90% (it will in real life not be nearly that bad), so it'd need an input of 22 watts to get 20 watts out. 22 watts at 18 volts is only 1.2 amps.
edit: There will also be some losses through the rectifier but as you can see, the numbers are so small even if it's horrible it will still be low enough current to not really even worry about.
Ok, thanks. I'll see what I can find in the power supplies. Radio Shacks website is showing all those bridge rectifiers as "web only". Ugh! Remember when Radio Shack used to actually sell electronic parts?!
Its gonna take me awhile to get those modules ordered and received. I'll likely proceed with my current setup (my AC to DC module & KD 3-18v drivers) in the meantime to see how that goes.
Ok Comfy, I'm now considering this setup. What kind of switch (I know an "On-Off-On" type, but do you have a link or more details)? Would I wire the ground wires together and just run the positive leads to the switch?
EDIT - Am I looking for an SPDT (single pole, double throw) type switch like this one? Or is there a reason I should go with a DPDT and run the ground wires through the switch too?
Another thing, should I consider adding a fuse inline somewhere?