Need help with Cree XML voltage/amperage from 5v source

Hi Everyone,

I'm a first time poster although I have lurked from time to time

I have looked thru the stickies and did do some searches but couldn't find the answer to my question.

I'm new to LEDs but familiar with component level electronics. I really have several questions and would appreciate any guidance from the guru's.


My Config:

I have two bike headlights that I want to hook up on my electric scooter. Here's a link to the lights and a link to the LED specs. The headlights each have a Cree XML LED and driver/switch built into the housing/heatsink. Each light is powered by a 8800 mAh, 3.7V Li-ion battery pack wired to the unit. The pack is composed of four 18650 cells in parallel. The battery can vary from 3.5 volt dead to 4.2 volts fully charged.

The lights work just fine in this config but it's a pita to keep charging the small batteries. So i figured my electric scooter has a 48V 10ah battery pack, i'll just connect the lights to that.

I installed a DC voltage converter to take the 48 volts down to 5 volts, 5 amps and connected the lights to this power source. The lights lit up but the housing/heatsink got very hot very quickly and the lights then went out. I opened the housing/heatsink and discovered that the thin wires from the LED driver to the LED had melted. I soldered on new wires, connected the lights to the original battery packs and they work fine so it looks like i dodged a bullet there.

So i figure i'm feeding the lights to much voltage.

I did the LED load calcs and it looks like if I add a 1 ohm, 10 watt resister to the negative leg of each light I will get to 3.1 Fv at 1500 mA. This seems excessive to have two huge resisters when the lights ran fine with the 4.2 volt batteries. (Keep on mind the load calcs are for a bare LED and do not take into account the driver built into the housing.)

Question one: Why do the lights run fine with a varying battery voltage of 3.5 to 4.2 but don't run on a 5 volt source? Does the driver in the housing takes the varying input voltage and reduce it to the 3.1 volts the LED needs?

Question two: The spec sheet shows that the LED's will run from 2.9v @ 700mA to 3.35v @ 3000mA. What specifies the mA draw? The input voltage? In other words if I supply the LED with 2.9v it will draw 700mA but if I supply the LED with 3.35v it will draw 3000mA?

Question three: If I put a 1 ohm 10 watt resister on the negative wire between the 5V source and each light will this reduce the voltage enough so the light runs properly.

Thanks in advance for any guidance anyone can give me.

Z-Ya Later

I can tell you right away that your problem is the 5 volts going to your driver. Not 5 volts going to your LED. I dont know the voltage tolerance of the ones installed in your light but it sounds like they dont like being over driven with the extra .8 volts. And some drivers are very voltage sensitive at least in a maximum rating. Any chance you could swap out the driver/regulator board for one with broader load levels? You can get some at FastTech or DealExtreme pretty cheap.

Z-Ya Later said:

Question one: Why do the lights run fine with a varying battery voltage of 3.5 to 4.2 but don't run on a 5 volt source? Does the driver in the housing takes the varying input voltage and reduce it to the 3.1 volts the LED needs?

Question two: The spec sheet shows that the LED's will run from 2.9v @ 700mA to 3.35v @ 3000mA. What specifies the mA draw? The input voltage? In other words if I supply the LED with 2.9v it will draw 700mA but if I supply the LED with 3.35v it will draw 3000mA?

Question three: If I put a 1 ohm 10 watt resister on the negative wire between the 5V source and each light will this reduce the voltage enough so the light runs properly.

Thanks in advance for any guidance anyone can give me.

Z-Ya Later

Click to expand...

1. Without knowing the specifics of the driver built into the housing, you're talking about just a little over 1 volt more, but realize that is 25% greater. If the driver is not designed for that I am not surprised it is overheating. Also, make sure you're actually measuring 5V and not 6 or 7. A lot of times these DC-DC convertors are not terribly accurate, and will include a pot to dial it in more exactly.

2. The mA draw is determined by the LED characteristics. It's not linear with respect to voltage, and every generation of LED is different. You just have to design for a certain current and provide the correct power source. Try not to think about applying a voltage to an LED, since what you really want is a constant current supply. Yes, technically if you apply those voltages you will get those currents. Thing is, it's really hard to modulate within 0.3V of range very accurately and your current will fluctuate wildly.

3. Yes technically you can drop some voltage this way it's just not very efficient and won't account for voltage fluctuations in your scooter power supply.

Overall though, if you are melting wires that is really bad. That means something went way wrong. The thing is your scooter might have a 48V power supply, but under load and perhaps other conditions, the voltage is going to fluctuate a bit. If you want to hard-wire the lights in, I would recommend using a relay to isolate the lighting circuit from the scooter so that when it's off you reduce the risk of fire or draining the battery if something is wired improperly, and then use a constant current driver board designed with a wider range of input voltages.

I appreciate the replies Guys, very helpful. I realize the specs of the led's don't matter so much since i'm actually powering the driver.

I tried putting 10 watt diode inline to drop the voltage but it only dropped it .3 volt. Then my 48 to 12 convertor died. I'm in process of getting a new one now.

The voltage convertor input is 40v to 60v and outputs 12v to run some 12v accessories. That matches well with my battery range. 53.4 fully charged to 46.4 discharged. That 12v output goes to another dc-dc convertor that outputs the 5v i've been playing with. The 5v is actually very steady according to my meter.

I'm not as concerned about efficiency as I am size. I'm blowing thru 10ah in an hour or so. The draw from the led's is minuscule. Also the entire lighting circuit is isolated by a switch.

The stock driver is a ZJD-067. I couldn't find much about it. It's a 22mm round PCB with 3 modes. I'm going to try to find a driver that I can feed eith 5 or 12 volts that will work on the stock location.

Thanks again for the replies.