Setting up LED

[lxlarkin]

Hi, I'm new to electronics and want to build a flashlight.
My plan is to use a CREE XLamp XM-L LED with a simple 30VDC/0.5 SPST Sub-Mini Slide Switch from radioshack and a AA alkaline battery. I was wondering if this design works, or if there's a better way to do it.

Thanks for the help

Links to the pages:

CREE
http://www.cree.com/LED-Components-and-Modules/Products/XLamp/Discrete-Directional/XLamp-XML

RadioShack
http://www.radioshack.com/product/index.jsp?productId=19964156

 

[KrisBlueNZ]

That won't work for several reasons.

• That LED has a forward voltage V_F of 3.1V. This means you need to provide 3.1V to it. A single AA cell will only supply 1.5V. There are various ways around this problem, including using three 1.5V cells in series, and a current regulator.
• That LED requires 3A (three amps) of forward current to run at maximum brightness (10W; ten watts). An AA cell - even a C cell - will struggle to provide 3A for more than a few minutes; even an alkaline one will have trouble. And the terminal voltage, at a 3A load, will drop significantly - probably below 1V (that's just a "gut feeling" estimate; I haven't checked any data sheets to confirm this).
• High-power LEDs are best driven with a regulated current. This means circuitry in between your power source and the LED to provide a steady, constant current, regardless of variations in temperature, input voltage, or any other parameters.

You need to investigate a current regulator to drive the LED with the specified current, if you want to run it at its maximum rated brightness. These come in several types, but probably your best option would be a buck switching converter with regulated output current. These run from a higher voltage (between 10V and 30V would be appropriate) and step the voltage down, while boosting the current, and regulating the current. Google the keywords in this paragraph. You can probably get them through eBay, but that doesn't mean that you should

You also need to look for batteries that can supply 10W for a useful time period. Actually, because of the loss of power in the current regulator, the battery will need to supply at least 12W, maybe as much as 15W. The battery voltage needs to be matched to the input voltage requirements of the current regulator, as mentioned above. I imagine you will want to use a Nickel Metal Hydride (NiMH), Lithium Ion (Li-ion) or Lithium Polymer (Li-Pol) rechargeable battery.

P.S. Welcome to Electronics Point

 

[BobK]

That LED is pretty extreme for a flashlight, with an output almost as high as a 100W incandescent bulb.

Bob

 

[lxlarkin]

Well thanks for all that info, and the warm welcome! I have two questions for you though.The "Nitecore Tiny Monster TM26" has 4 of these LEDs and runs off of four 3.7v rechargeable batteries. Now how is that possible? Does the voltage on the batteries stack?

The second question is what kind of circuit would be required to provide a steady and constant flow of electricity?

 

[KrisBlueNZ]

The "Nitecore Tiny Monster TM26" has 4 of these LEDs and runs off of four 3.7v rechargeable batteries. Now how is that possible? Does the voltage on the batteries stack?

If the batteries are connected in series, the voltages add together. If they're connected in parallel, the currents add together. It could be done either way; the fact that the flashlight can be used with only one or two batteries for emergency situations implies that they're in parallel.

If that's the case, the batteries will supply 3.7V which will be regulated down to about 3.1V by four separate current regulators, one for each LED. This is probably not the most efficient way to drive the LEDs, especially if you want a simple solution; this flashlight is an expensive product and the manufacturers may have gone to significant expense to maximise the conversion efficiency.

The second question is what kind of circuit would be required to provide a steady and constant flow of electricity?

I answered that question.

You need to investigate a current regulator to drive the LED with the specified current, if you want to run it at its maximum rated brightness. These come in several types, but probably your best option would be a buck switching converter with regulated output current. [...] Google the keywords in this paragraph.

 

[BobK]

Wow, that is one hell of a flashlight!

A Li Ion battery like the 18650 will start at 4.2V and go down to about 3.2V as it discharges, so it is great for a 3.1V LED. A buck regulator regulating current would power this from a single 18650 for maybe 30 minutes. You can get buck regulators (also called DC to DC converters) cheap on Ebay. Get one that has a current limiting function and set the current to the desired level and the voltage to something above 3.1 and it should work fine.

Also, watch out when buying 18560 batteries, the cheap ones are likely fraudulent. I bought a couple of "Ultrafire" batteries with chargers and they were useless, maybe had 1/10th the capacity advertised.

Bob