Hi,
i got some questions, i hope you guys do not mind to help me answer them.
I can i run a DC12v to dc6v, 3amp converter from aliexpress(http://www.aliexpress.com/item/DC-to-DC-Converter-12V-Step-Down-To-6V-5V…) connect from an automobile batt(14V) to run direct drive to 2xml connected in series.
I would like to know what will be the voltage and current across each LEDs.
Appreciate your kind reply
regards,
philip
It seems to me automobile electrical systems are about 12 V on the battery or 14V charging. There is a mil. spec. for automotive electrical systems, and it is sobering to read, so I am not sure direct drive is the way to go if the battery is part of an automobile. If it is detached, then the power is very clean but the voltage depends somewhat on the state of charge.
Since the converter is rated at 3 amps, the same as an XM-L, it probably won’t blow them out, maybe not even in a vehicle.
According to the data sheet, an XM-L draws about an amp at 3 V and 3 amps at about 3.3 V. So two of them would run at about an amp at 6 V, and not burn out as long as the converter does not put out more than maybe 4 or 5 amps at well over 6 volts.
I have already done just what you are talking about. Here is a video of 2 XM-L’s in series connected to a 12V source. Actually it is set to 13.5V as the system in a car rises to about 14.4V while charging. I am using a LM2596 board that may be similiar to the one you are thinking of using.
I ran a test on that board and made a video. What I did was hook one of the boards up to 2 – XM-L’s in series. I have decided that it is more efficient than a board for each emitter. A board for each emitter only generates twice the heat.
The left most meter is reading the voltage off of a 12V battery. The meter to the right of it is measuring the current through the emitters. Next, the yellow meter is hooked to a thermocouple that is clamped to the back of the board at the thermal vias. The meter to the far most right is reading the voltage output of the board. I set the LM2596 board to output a constant 6.15V across the 2 XM-L’s. At 6.15 initially, 1.60A flows thru the emitters, a rather modest amount. But as the LED’s heat up, more and more current will pass. This is the nature of LED’s when applied to a constant voltage source.
The yellow meter that is reading temperature is reading the temperature of the LM2596 board, as at that time that is what I was concerned about. The constant increasing current that flows thru the emitters is NOT because of that. In fact notice the voltage reading during the whole test, it remains unchanged! Even though that board became really hot, the voltage output remained unchanged.
The current rises until about 2.7A and does not go higher only because that board had it’s current limiting protection kick in and limited it to 2.7A. Had it not been for that, the current, i believe would have kept rising until the LED’s burned out.
So what does this all mean? It means that because the amount of current that an LED will draw is dependent on its temperature, we don’t want to use constant voltage source but rather constant current source.
That is what are used in the flashlights that we use. So instead of looking for CV boards, look for a CC board. I ended up using this board as is because in effect it acted as a CC source once the protection kicks in. Not really the best way to do it, but it works. 
BTW, the temp of the board seemed to stop at the end and hold steady at 204F, but that was not the case. What happened at the end of the video is that the plastic yellow clamp that I used to hold the themo couple to the board melted. When that happened, there was no more contact between the thermo couple and the board.
The question thats bogging me is, if i use that aliexpress converter(rated output of 6v n 3amps), direct connected to 2 series xmls, would i ideally get 3v n 1.5amps to each led, and how bright would each run?
Assuming the batt is constantly being charged at 14.4v.
If they are connected in series, each gets half the total voltage (if they are identical) but each gets all of the current. It’s like water in a hose, the current is like the water current and the voltage is like the water pressure. If they are connected in parallel, the current is split (equally if they are identical) and both get the full voltage. Series does seem the better choice, as the ratings match more closely that way.
The harder question is just what will be the total voltage and total current under load. That depends on the details of the power source, the converter, the leds, the temperature and the connecting wires. It sounds like it would be reasonably safe to try it and measure voltage and/or current if you want to know it. If you want to be safe, start with a resister of about an ohm in series or with longer than necessary wires. Leds don’t make good resistors, as incandescent light bulbs do, because the current changes too fast with changing voltage.