Wow, thanks for that post. Things are making sense now.
I see on that chart how just a a few 0.1v's make a HUGE difference in the current draw. Now the need for a voltage regulator makes crystal clear sense.
So, how does the Meanwell determine how much current/voltage to send to the LEDs?
In other words, if a Meanwell is rated at 700mA (non-dimmable), does the Meanwell simply alter the voltage output to about 3.5v per LED (increasing by 3.5v per LED added, up to the max voltage)?
If so.... what if you had XRE and XPG on the same circuit? I have been told you can mix LED's that have the same/similar forward voltage. But at 3.5v the XPG should draw nearly 1400mA, yet the XRE is 700mA.
I appreciate your responses, VERY helpful Applesnail!!!!!
The biggest problem you'll run into with resistors is that resistive voltage modification is wasteful, and you rarely have the voltage you want. If you get 10v and cut it down to 6.6v to drive two XR-Es at about 350mA, you've got (10v-6.6v = 3.4v) going to waste. 3.4v times the drive current (.35A) equals waste heat at the resistor - 1.1 watts out of 3.5 watts of power are pure waste. If you add an LED to that setup, you get dim XR-Es, outputting about 60-70 lumens apiece. You'd have to drive a pair of LEDs (saving you $6 for the LED left out) at just about 500 mA to get more light. Spending about $2.30 for the LM317T at Ratshack's high prices is well worth it, especially if you aren't lucky enough to have good wall warts. Let me emphasize - you could solder the wall wart +DC to the first XR-E, the other two in series, and back to the wall wart - that'd work fine, especially if you put your switch on the AC power.
Constant-current drivers are, to me, magic. They do feedback of some kind to maintain a constant current output no matter what voltage is required (within their own limits). LEDs want constant-current, while incandescent bulbs want constant voltage; LEDs are sensitive to voltage while bulbs die from small current changes. Given a particular voltage (as in high-end lights), an incandescent bulb's resistance defines the current, at a level that won't blow it out. With LEDs, a particular current is what you care about. The LED's Voltage and Current are exactly related - no matter the voltage, if only 350 mA goes into an XR-E, it will see 3.3 to 3.9 volts. On the other hand, if you fix voltage on an XR-E then because of the Vf; varying from 3.3(typical) to 3.7v at 350 mA, you have little idea of what current the LED will see. The LM317 keeps 1.25 volts between the Vadj and the Vout pin - putting a resistor there defines the current pumped through the chip (up to its voltage, supply, and wattage limits). Meanwells work with a different electronic setup to do the same thing - set any voltage necessary to maintain the current selected.
The Meanwell driving one LED will pick out its forward voltage at that drive current (within the limits of its precision to drive at .700 Amps). If it drives two LEDs then it will step up voltages - so don't have a switch adding and subtracting LEDs from a string to cleverly dim a meanwell - it may not adjust voltage quickly, and your dimmer might smoke the LEDs left in the string. The LM317T imparts a voltage drop to do its work, making it no good for flashlights. Think of regulation as holding a stack of books at an exact height. As long as the stack stays the same, it's easy to keep it in place. But then some goofball throws books on and takes them of - only very strong people could keep their hands very still, and only fairly nice electronics will adjust quickly enough to save the LEDs. This includes powering up a driver without a load on it - shorting it out can fry some driver's electronics, and connecting some to an LED while the driver is powered can kill the LED - it was trying to push its rated load through a air gap - it'll never reach 10000v per inch (between LED+ and LED-), but it'll charge to the highest voltage it can, dutifully trying to push its current. Then you'll toast your LED string and be sad (Voice of experience!). With that said, generally you would connect the LED to the driver and then power up the driver, and if you mess up the order then unplug the driver and discharge any capacitors it has through a resistor.
Mixing LEDs.
Mixing LED Vf IN SERIES works acceptably, although it can get icky from an efficiency standpoint. Let's say you have an XP-G and an XR-E, they usually have different Vfs (I'd be surprised to find a matched pair). If we drive both with constant current at 350 mA, the XP-G will have 3.0v(typical) to 3.75(max) Vf. The XR-E will have the 3.3 to 3.9 Vf. If we put these in parallel, they will not get the same current. They aren't resistors and I don't know the semiconductor math, but the low Vf of the XP-G will make it suck up much more current than the XR-E. No matter what, I'm driving the XP-G hot and inefficient - CREE LEDs are always less efficient at higher power levels (above 20 microamps), while the XR-E is going to be running at a very low current. Depending on the current imbalance I may burn the XP-G out and then the XR-E will see the whole current and immediately smoke. If the power supply is voltage-based (Batteries, wall-wart), low-Vf LEDs will die and high-Vf LEDs might live - depending on the source voltage.
LEDs in series are more friendly. You sum the Vf, and they all get the same current. It's up to you if you want them to run at the same current; that's a design choice (and affects the light output and heatsinking and so on).
Edit: For LEDs you don't really want to regulate on voltage. Your design decisions for a lamp should be based on how much light you want. Once you pick a particular LED that can deliver that, in a color of white that you want, you pick a current to give you the output you want for this LED - in my lamp I wanted about 150 lumens. With a Q4-brightness XR-E (100 lumens at 350 mA), I wanted 150% of that. Page 8 says that it's 800 mA. So I picked my resistor to make the LM317T deliver 800 mA. If I'd picked a voltage instead, I'd have no control over the brightness, except by fiddling with the voltage. Since wall warts rarely give you the voltage you want, it's kind of a shame to waste $6-per-unit LEDs when another few bucks will give you complete control. In short, picking the current lets you choose how bright the LEDs will be, and ensures that you won't toast them.
which I am partaking of :twothumbs (for driving 13 XPG)
