3 x 3,7V leds into 12v power?

[jacko1]

Hello forum!
I am planning to mod my old Underwater kinetics D8 torch by inserting 3 leds in it. As the D8 torch uses 8 D size batteries it is 12v? So can i use those 3,7V leds in series and just get it working well? 3 x 3,7 = 11,1.
Can my leds handle this or do i need resistors of some kind? I am not
familiar with this stuff at all, so please forgive me!


Underwater kinetics D8 divelight info can be found here: http://www.uwkinetics.com/product/28 .
The leds i am plannin to use are: Cree XR-E Q2 Emitter with Star http://www.dealextreme.com/details.dx/sku.2395
And reflector i am planning to use is this: 48.16mm 3-LED Reflector for Cree Emitters http://www.dealextreme.com/details.dx/sku.1916

Hope somebody would help me out so i wont be soon having one broken D8 and 3 burnt out leds

-Jacko
http://www.uwkinetics.com/product/28

 

[LukeA]

This is a driver that will suit you.

You will need an aluminum plate to mount the LEDs to. A circle cut from 2-inch by 1/8th thick stock available from Lowes/Home Depot will work well.

 

[Gunner12]

Depending on the load, condition of the batteries, and the tempreature, the batteries might be around the Vf of the LED or lower. A driver like the buckpuck LukeA liked to should work but I like to have a definite buck or boost driver which means 2 or 4 LEDs.

Any reason why you aren't using the slightly more efficient Q5 bin? Besides price of course. Also keep in mind the heatsinking needed.

:welcome:

 

[Greg G]

Jacko, if you're using NiMH batteries you'll be able to run it in direct drive, as that battery pack would be 11.2 volts fresh off the charger.

If you run alks you'll benefit from a boost driver as the batteries when loaded will sag a good bit under the Vf of the led string.

 

[jacko1]

Hi everyone and thanks for your help!

would this Aluminium connection board work in my case? http://www.dealextreme.com/details.dx/sku.13914

What is the reason for buckpuc? Is it a resistor or what purpose does it serve in led system?
Where in the circuit should i put this buckpuc? Just in between power and first led?

How much brighter are those Q5´s? Should i get them instead?

-Jaakko

 

[Greg G]

Definitely get Q5's. They are abundant in many tints, and are brighter.

Just my opinion, but you won't need a buck driver or a resistor, but *if* you did install one it would go between the positive side of the battery pack and the led string.

 

[jacko1]

Ok..
I ordered some Q5s from deal extreme and start working with the light when i get the leds etc. Thanks again for your help. I come back to tell you how it turned out!


-J

 

[jacko1]

one more question..

What kind of wire should i use? How thick for this system?

-J

 

[HarryN]

Hi jacko1 - welcome to CPF. The great thing about internet forums is how much time and money they can save by avoiding mistakes. Of course, they also make you want more expensive things.

I really like LEDs, but there is one catch with this setup that is very difficult to get around - heat dissipation.

All light "bulbs" produce both heat and light.

For the incandescent / halogen type, this heat is disposed of as infra red radiation along with the light - out of the front of your flashlight.

For power LEDs (all brands) the light comes out the front, and the heat comes out through the back via thermal conduction. In order to work, the light needs to bring the heat all of the way from the back of the LED to the wall of the flashlight - then either to your hand or to the water. The alumnum mounting plate is a "heat spreader", not a "heat sink" in spite of it sometimes being called the latter.

In the case of many divelights, including this one, the body is ABS plastic - normally an exellent choice, but not for LED based lights. The thermal path is basically not there. I would suggest that for this light, looking at incan bulb options (carley lamps) and battery options would be a better way to improve it.

Search for CPF member barbarin and look at some of the lights he has made for underwater use.

 

[jacko1]

Hello!

You are very very right about the benefits of this kind of forums in sharing knowledge, ideas etc. In my opinion it is the one of best things internet has brought to us.

Ok, back to flashlight.
Yeah, i've been thinking about that heat problem but didn't realize that the heat has to get out the system. clever me!!
I quess i just thought that because there is this star- type "heatspreader" there would be no big problems.

The light would anyways be on just couple of minutes and then turned off. Water in Finland is usually pretty cold in deeper waters +5C etc. And in shallower water i dont need this light. It is supposed to be used as bonuslight for underwater filming (using it as a side beam to forexample illuminate parts of wreck that are further back and behind the reach of our main videolight)

What do you think about this heat. Would it get too hot in couple of minutes? In cold water?

-J

 

[HarryN]

Hi Jacko1 - Yes, Finland water is quite chilly, but the plastic is a pretty good insulator. LEDs (all brands) have decreasing light output with temperature, and at some point, the decrease is permanent.

I tried to post the calculation, but it didn't take for some reason - maybe i was typing too long.

In any event, my first order approximation calculation assuming a 10 cm dia x 1 cm thick heat spreader / heat sink and 15 watts of LED:
- after 1 minute, light output due to thermal effects will have dropped by about 1/2.
- After 3 minutes, the light output drop will be permanently 75% less than a new LED
- Somewhere around 4 - 5 minutes, the LEDs will have less output than a battery indicator LED (forever)

There is nothing like a real test to prove this to yourself. I did some LEDs in this way when they were a lot more money than now.

BTW - are you really sure you want to use white LEDs for underwater photography ? I would use RGB or RCB instead.

 

[jacko1]

I see.. the damage happens fast. I did some research yesterday and found out that ABS plastics are marketed to be very good insulator materials.

So... maybe i just have to abandon this project. The D8 is spare light and i dont need it so i think im going to mod it in some other way instead.

-J