Some questions for my first project

Hello,
I'm a complete newbie but I'm trying to understand how I could build my first torch reading project on this forum and on other websites.

My first project is to completely modify a petzl zoom headlamp (originally with an old halogen bulb) with a high power led. The light is intended to be used for caving and it should normally run at around 150/200 lm, with the ability to quickly burst at around 1000 lm if needed.

This is a similar project: http://www.whitbyseaanglers.co.uk/forum/index.php?topic=30347.0

I want to power it with a 3.7V battery pack (would a 7.4v be better?).

For the emitter I tought about the cree XM-L U3, do you think it's good for my project?

My next two questions regard the driver and heatsinking:

- The driver should have both temperature protection (since I expect it getting very hot at full power) and low voltage protection (some warnings or something that reduces brightness when batteries drop in voltage). Looking on fasttech I found different drivers whit these features but I couldn't find any with both of them, can you help me?

- The petzl zoom has a plastic body so I expect it to get very hot, especially at high lumens. Ok, 1000lm will be only used for seconds so it shouldn't be a huge problem, but what kind of heatsinker do I need to use to run the light at lets say 200lm?

Thank you!

:welcome:


I'll try to answer your questions as clearly as possible.


Question:

giulio1993 said:

I want to power it with a 3.7V battery pack (would a 7.4v be better?).

Click to expand...

Answer: Use 3.7V.

Reason:
The driver has to dissipate the power formed from the excess voltage.
An XML running at 3A takes 3.3V (for 1000lm output).

3.7V battery: 3.7-3.3=0.4V P=IV=3x0.4=1.2W wasted
7.4V battery: 7.4-3.3=4.1V P=IV=3x4.1=12.3W wasted

7.4V for an XML means that your driver ends up wasting MUCH MORE
(12.3-1.2=11.1W)energy than at 3.7V.


Question:

giulio1993 said:

For the emitter I tought about the cree XM-L U3, do you think it's good for my project?

Click to expand...

Answer: It's great, but there are better ones, like the XM-L2 U2. For caving and the like, you'll probably prefer a 2c -ish tint.


Question:

giulio1993 said:

The driver should have both temperature protection (since I expect it getting very hot at full power) and low voltage protection (some warnings or something that reduces brightness when batteries drop in voltage). Looking on fasttech I found different drivers whit these features but I couldn't find any with both of them, can you help me?

Click to expand...

Answer: If you have a decent heatsink, you shouldn't need a thermal protection circuit.

Reason: If your light has to limit itself because it's getting too hot, the heatsink is not good enough. Especially in a caving situation, where you want a light to be reliable in it's output, it needs to be able to deal with the thermal output of the LED for extended periods of time. (Trust me on that, I once built a 6500lm lamp for a friend of mine going caving).


Question:

giulio1993 said:

The petzl zoom has a plastic body so I expect it to get very hot, especially at high lumens. Ok, 1000lm will be only used for seconds so it shouldn't be a huge problem, but what kind of heatsinker do I need to use to run the light at lets say 200lm?

Click to expand...

Answer: Any decent heatsink that you might find as a CPU cooler, for instance, should work well. You'll need to drill out part of the plastic to let the LED sit properly on the heatsink. I'm not familiar with that particular headlamp, so if you have a particular idea for a solution, and you want to run it by anyone here at CPF, I'd advise posting pictures of your light, and heatsink.


Best of luck, and any questions, feel free to ask!

agnelucio said:

The driver has to dissipate the power formed from the excess voltage.
An XML running at 3A takes 3.3V (for 1000lm output).

3.7V battery: 3.7-3.3=0.4V P=IV=3x0.4=1.2W wasted
7.4V battery: 7.4-3.3=4.1V P=IV=3x4.1=12.3W wasted

7.4V for an XML means that your driver ends up wasting MUCH MORE
(12.3-1.2=11.1W)energy than at 3.7V.

Click to expand...

This is only true of a linear driver. With a switchmode driver the efficiency can remain quite high even with large differences between input and output voltage.

That being said, there are other characteristics of a linear driver that you may consider an advantage.

With a switchmode driver, the light output can remain at full power until the batteries are completely spent, at which time the light would suddenly go out. Unless there is some low-battery warning, this could be considered a serious drawback. Some of the better switchmode drivers, like those from Taskled, include low battery warnings by blinking or dimming the light.

A linear driver on the other hand, tends to let the light get dimmer and dimmer as the battery runs out. Depending on the battery, LED, and driver, you may get full output until the battery is 50-70% spent, followed by a tapering off of light. If you let it run until the LED is quite dim, it can run for many times longer this way than it does at full output. Some cavers (especially cave divers) consider this a big advantage. The tradeoff is that you don't get full output for as long as you can with a switchmode driver.

The relative simplicity of a linear driver could be considered a reliability advantage, but unless the rest of the design and construction are of the highest quality, other concerns substantially overshadow any gains by simplicity.

agnelucio said:

If you have a decent heatsink, you shouldn't need a thermal protection circuit.

Click to expand...

I'm not so sure I agree with this one, especially if you include a high output mode that's intended to only be used for very short periods. I do agree that your heatsink should be able to handle the heat from normal operation for extended periods. But things happen. People forget to turn back to normal mode; lights get switched on in your backpack, closet, or car trunk, etc.

Thermal protection is not offered in most drivers. However, there are a vast assortment of thermal switches available that could be used to turn off the driver if something overheats, so it's not a huge problem to add it to your design if you choose a driver that does not include thermal protection.

Running at 200 lm, you are going to need something like 660 mA in the LED. Since AMC7135 drivers come in increments of 350 mA, let's say you settle on 700 mA. When your battery is fully charged, let's call it 3.9V under load, you would be pulling 3.9 * 0.7 = 2.73W of electrical power. Of this, something like 0.7W is coming out as light, the rest, roughly 2W, is converted to heat in the LED and driver.

This should be manageable without too much trouble. I'd have to see what your lamp looks like to be sure, but you ought to be able to do this inside the light.

On the other hand, at 1000 lm, you're looking at 10W of heat. This is too much for a small enclosed plastic case, for long term. For short periods the excess heat can be absorbed by the metal of the heatsink and dissipated over time when you switch back to low power or off.

Thank you for you answers,
I ended up taking a xm-l2 (I took the 20mm version for better heat dissipation) and a 8*AMC7135 (Nanjg 105c) driver. Now, are normal flashlight driver pillar enough to that kind of heat (running at 200lm)?