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Old 03-24-08, 03:21 PM   #1
chewybrian 
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What the buck puck?

Hey,
I am new to this. I am trying to build a bright, home-made light sytem, but I am a simpleton--not electronically inclined. My working theory is: a 12v, 4.2 ah nimh battery pack, 3-Cree Q5 drop-ins on the front, one 5w 1157 led bulb plugged into a std. auto tail fixture. If my math is good, that's 16 watts, 1.33 amps, @3 hours run, and 6 or 700 lumens front and 300 back(bright tail light, I know...)
Here is where I stumble...
How do I wire these fixtures to the battery? Do I need a fuse? Can I wire each drop-in independently to the battery, with its own switch? Can I run them in a series, with one switch? What is a "buck puck", and why do I need one (if I do)? I can figure the hardware, mounting, sealing, maybe even optics, but can anyone duh-umb down the wiring so I can relate? thank you.
p.s. Cost is just over $100 u.s., not counting whatever the puck I need to wire it together, maybe $50 more if I can do it simply...? It seems like a lot more light for a lot less than a store-bought light.
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Old 03-24-08, 05:22 PM   #2
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The Buck Puck is a constant current regulator designed to drive the LEDs from a battery. If you connect an LED directly to a 12V battery, you will instantly burn them out.

The first thing you need to know is if your Cree drop-ins already have drivers installed and if so what input voltage range they are designed for. If they are intended for 6V max, then you will probably blow them out with a 12V battery.

If your drop-ins do not come with drivers, then you need a buck puck or equivalent. You may be able to use a single buck puck depending on the specifications of how many LEDs it will drive. Connection to the buck puck should be done per the instructions. The taillight should already have a driver and can simply be connected in parallel to the buck puck.

If you have drivers in your drop-ins and they are for a lower voltage than your battery, then you could remove them and use a buck puck in their place.

You'll also need a switch to control the buck puck modes.

In any event, I suggest you buy spare LED emitters because you are likely to burn one or two up in the learning process. Don't forget to heat sink the emitters well.

Whether you need a fuse depends on what will happen if your light module shorts out the battery. If a short is going to cause a fire, then a fuse would be prudent. If you don't know what will happen, then assume it's something very bad and install a fuse.
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Old 03-29-08, 08:50 AM   #3
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thank you for the info.

You sparked a couple more questions. The drop ins are rated to run 3-18 volts. Does this imply that they already have a driver? Would it be harmful to have a driver in the drop in and another buck puck? It would seem, to my uninformed mind, like having 2 fuses on the same circuit--no problem, right? Also, what is heat sinking and why should I be doing it?

thanks again; I am sure these are ez questions to all you veteran electricians.
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Old 03-29-08, 08:54 AM   #4
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one more question...

With the 3-18 volt range on the drop-ins, would you get brighter light at higher volts? I can get a 16.8v battery pack for not much more $. An added benefit would be allowing 16 watts through one, 1 amp buck puck, meaning I could add one more light (why not?).
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Old 03-29-08, 12:14 PM   #5
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What drop in CREE modules do you have? Maybe something like: http://www.dealextreme.com/details.dx/sku.11621. I have one of these on order, I'll be curious to see how bright it is. These should run fine from a 12V battery, or a 16.8V one. They might get a little brighter at higher voltages, but not enough that you will easily notice. I'm guessing from the specs they are running the LED at around 800 mA. So the LEDs will draw around 3 watts, but you loose a bit in the converters.

The modules are probably using a driver that is a variation of this: http://kaidomain.com/WEBUI/ProductDe...px?TranID=2982. The drivers maybe get about 85% efficiency. So say your drop in modules run at about 3.5 watts. 3.5 *3 = 10.5. So add 5 watts for the read light (where did that come from?) and your at about 16 watts.

I wouldn't bother with another buck puck, unless you need one for the rear LED.

With regards to heat sinking, the less covering over the modules the better. Air flowing over metal conducts heat away a lot better than still air. And don't think about sealing everything up in plastic, that just makes matters worse. You can probably find some 1 inch aluminum or copper tubing the modules will fit into (both conduct heat well). You can attach the modules to the tubing with heat conducting epoxy (look for arctic alumina, or arctic silver epoxy). The tubing will give you someplace to mount switches, brackets, etc.

Sounds like a fun and bright project,

Mark
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Old 03-30-08, 10:09 AM   #6
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Originally Posted by chewybrian View Post
thank you for the info.

You sparked a couple more questions. The drop ins are rated to run 3-18 volts. Does this imply that they already have a driver? Would it be harmful to have a driver in the drop in and another buck puck? It would seem, to my uninformed mind, like having 2 fuses on the same circuit--no problem, right? Also, what is heat sinking and why should I be doing it?

thanks again; I am sure these are ez questions to all you veteran electricians.
The drop-ins almost certainly have a built in driver. No buck puck should be used with them.
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Old 03-30-08, 10:18 AM   #7
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Originally Posted by chewybrian View Post
one more question...

With the 3-18 volt range on the drop-ins, would you get brighter light at higher volts? I can get a 16.8v battery pack for not much more $. An added benefit would be allowing 16 watts through one, 1 amp buck puck, meaning I could add one more light (why not?).
The purpose of the driver circuit is to provide a constant current to the LED regardless of the input voltage (within the normal range). This keeps the light output constant throughout the battery discharge until the batteries have are just about exhausted. Using higher voltage battery pack is not necessarily any better than a low voltage pack. The key parameters are the voltage, which should be within the range of the drop-in, and the total capacity of the battery, measured in mA-Hr, or A-Hr.
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Old 03-30-08, 10:27 AM   #8
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Originally Posted by Mark Hahn View Post
What drop in CREE modules do you have? Maybe something like: http://www.dealextreme.com/details.dx/sku.11621. I have one of these on order, I'll be curious to see how bright it is. These should run fine from a 12V battery, or a 16.8V one. They might get a little brighter at higher voltages, but not enough that you will easily notice. I'm guessing from the specs they are running the LED at around 800 mA. So the LEDs will draw around 3 watts, but you loose a bit in the converters.

The modules are probably using a driver that is a variation of this: http://kaidomain.com/WEBUI/ProductDe...px?TranID=2982. The drivers maybe get about 85% efficiency. So say your drop in modules run at about 3.5 watts. 3.5 *3 = 10.5. So add 5 watts for the read light (where did that come from?) and your at about 16 watts.

I wouldn't bother with another buck puck, unless you need one for the rear LED.

With regards to heat sinking, the less covering over the modules the better. Air flowing over metal conducts heat away a lot better than still air. And don't think about sealing everything up in plastic, that just makes matters worse. You can probably find some 1 inch aluminum or copper tubing the modules will fit into (both conduct heat well). You can attach the modules to the tubing with heat conducting epoxy (look for arctic alumina, or arctic silver epoxy). The tubing will give you someplace to mount switches, brackets, etc.

Sounds like a fun and bright project,

Mark
Another, very simple approach is to purchase a complete flashlight using a CREE emitter from dealextreme and modify it for an external battery. Drill a small hole in the barrel for the external battery wiring and solder the wires to the rear clicky-switch spring and the center contact for the battery.

However, if you use a flashlight that runs off one or two 18650 3.6V Li-Ion batteries, you'll probably get 3+ hours of use on the high power setting and may not even need an external battery for many applications.
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Old 03-31-08, 01:54 PM   #9
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What drop in CREE modules do you have? Maybe something like...

Yes, that is the exact one. From what 666 said, I guess they include the driver. I would be interested in what you do with them. I have to look into the complete flashlight now, although I am somewhat mentally and emotionally committed to making my own now, although or perhaps because I have no business doing so...
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Old 03-31-08, 02:03 PM   #10
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Thank you, Dark Lord of the American Highway. I have learned from everything you said, and I will look at all options and decide. I think I will go with the external battery pack. Reason being, I am trying to "compete" with some of the super expensive models I have seen. I am keen to see if my simple mind can duplicate the $500-$1000 systems(Arc Li-ion, etc.). I want to acheive the same basic functions, battery life, and brightness for less than $200. One thing I will concede is the weight--I am commuting, not racing. If I can do it, then anyone can. thanks again
--------------------------------------------------------


Another, very simple approach is to purchase a complete flashlight using a CREE emitter from dealextreme and modify it for an external battery. Drill a small hole in the barrel for the external battery wiring and solder the wires to the rear clicky-switch spring and the center contact for the battery.

However, if you use a flashlight that runs off one or two 18650 3.6V Li-Ion batteries, you'll probably get 3+ hours of use on the high power setting and may not even need an external battery for many applications.
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