question- 14.4v and 5 crees

[Robatman]

Inspired by "10 crees makita",
http://candlepowerforums.com/vb/showthread.php?t=158356&highlight=makita

I want to do something similar using a 14.4v cordless drill battery.

I will mount the 5 cree stars on a PC heatsink/fan.

My question is can someone tell me the current each emitter will be getting and estimated lumens?

The fan uses 0.06a and the actual voltage is over 15 volts off the charger.

I would really appreciate the calculations as even though i have bought the stars, need to know if it will be worth the effort as i will be making a number of pieces on my wood lathe as well as fiddling around soldering etc.

NewB confession: I dont even know the diff between series and parrallel, I was just going to put positive from the switch to one positive on a star, then join the negative of that star to the positive on the next etc. Would this work????

Thanks
Robert

 

[uk_caver]

With 5 Crees in series (positive on one to negative on the next, like you mentioned), running in direct drive from a battery that was at 15 Volts, that's only 3 Volts per LED, so they may not be incredibly bright.

The two problems with direct-driving LEDs (running them straight from a battery) is that if the battery voltage is just a *little* too high when it's freshly charged, that can result in the LEDs being overdriven, which can seriously reduce their life. If the battery voltage is such that it's not overdriving the LEDs when freshly charged, then it will probably be driving them at less than maximum power for much of the time.

Very roughly, the way Crees (and other high-power white LEDs) work, they need maybe 2.7Volts to start giving any light out at all, about 3Volts for medium light, all the way to maybe 3.5-3.7Volts for full brightness, with 4 volts likely to overdrive them. However, the precise figures do vary significantly from one LED to the next, possibly even within a batch of the same type you buy at once. The figures also vary with the temperature of the LED, which in turn depends how much power it is consuming. The upshot of that is that it isn't really easy to say how bright an LED will be based on the voltage across it (apart from maybe "4 volts will likely damage it, 3 volts will likely not give anything like full output)

With the 10-Cree light running off an 18V battery as 2 strings of 5, when the battery was actually at 18V, that'd be 3.6V per LED - likely somewhere near the voltage needed for full power, and probably not too much, but even then, running in direct drive is a bit of a gamble, depending what the battery voltage is when freshly charged.

However, it's easy to reliably drive LEDs by controlling the current they draw, and if you want to build a light that gets maximum power from your LEDs without risking overdriving them, you'd be better using a current-regulating circuit - even though it would add to the cost, you could end up with a much better light, and have options for variable brightness in case you don't always want to run at maximum.

There are circuits around which can either increase or decrease voltages, so you could (for example) run a string of 5 or more Crees off a boost regulator which gives a higher voltage out than it takes in, or run fewer LEDs from a buck regulator which converts the voltage downwards.
Some of the dealers on here sell such circuits, but I haven't used any of them so I can't really make recommendations, but there will be people around who can.

 

[Robatman]

Thanks Uk Caver, just thinking i may be better off doing it with only 4 crees to raise the voltage,

Also read a bit about clipping the edges off the star to isolate the volatge from the sink... i thought the back side of the star wouldnt have any current, is this wrong. does this mean i have to use an extra thick layer of Arctic silver epoxy to insulate?

 

[uk_caver]

Thanks Uk Caver, just thinking i may be better off doing it with only 4 crees to raise the voltage,

Also read a bit about clipping the edges off the star to isolate the volatge from the sink... i thought the back side of the star wouldnt have any current, is this wrong. does this mean i have to use an extra thick layer of Arctic silver epoxy to insulate?

With 4 Crees, you might be pushing things a little - 15 Volts would give 3.75 Volts per LED, which might be a little high. There's a reasonable chance things would work at least for some time, but there is an element of gambling there. As the LEDs heat up, the current they can draw at a given voltage also goes up, so they could start off fine, and end up getting overdriven after heating up, though your heatsink might help in that respect.
A lot depends on what the absolute maximum battery voltage is, how the battery voltage sags under load and changes over time, and the precise characteristics of the LEDs you have.

The clipping of edges to avoid unwanted connections does apply to the bare Cree emitters, which have contact strips running along the back and the front of the short sides, but I doubt it would apply to a star, since stars typically have an electrically isolated back surface.
The only obvious reason I can see for trimming a star is to make one fit in a light where it's otherwise too large. It's always possible there are some strange stars around, but I'd be doubtful.
It's probably worth double-checking what you read, and if I'm wrong, do say so.

 

[yellow]

making an extra thick layer of whatever means decreasing heat transfer --> leds will get damaged quick.

When using Stars, there is no voltage at all on the back side, when using emitters its better to somehow disconnect upper and lower contact strips (I scrape the lower off the ceramic holder)
then mount the emitter/Star with just enough heat transfer paste that base is covered and finally glue them in with epoxy around emitter (Stars: use screws)

PS: when using the 14.4 V batt, 5 emitters, Shark and Remora board, You receive easy available three levels of stable output

 

[Robatman]

thanks guys- slowly getting a picture in my head....

Is there a convertor board/boost that will give me more than 1000mA? (the shark appears to go to 980) Given glockboy can run 1.8a with his i figure the heatsink/fan can control the heat, is there a cheap boost circuit that can go to 1.5 or even higher?

Thanks again.

 

[DonShock]

.....Is there a convertor board/boost that will give me more than 1000mA? .....

If you are going to drive 4 Crees off of a 15V source, you will probably need a step down converter. When I run four P4 Crees at around 1A, I get 12.6V for the total Vf of the LEDs. The Sandwich Shoppe offers a SOB Blank Converter that you can customize to your desired current output. The SOB is a step down converter with 16V max Vin and 1.5A max Iout. There is a resistor calculator link that you can use to find the the needed resistors ($0.25 each) for your desired current output and it only costs $3.00 extra to have the Shoppe install them for you.

 

[uk_caver]

thanks guys- slowly getting a picture in my head....

Is there a convertor board/boost that will give me more than 1000mA? (the shark appears to go to 980) Given glockboy can run 1.8a with his i figure the heatsink/fan can control the heat, is there a cheap boost circuit that can go to 1.5 or even higher?

Thanks again.

Glockboy is running two strings of 5 LEDs. Within each string, the LEDs are in series, but the two strings are run in parallel, so it's only ~0.9A per string.
1.8A through a single string of LEDs would likely wreck them extremely quickly.

 

[glockboy]

Glockboy is running two strings of 5 LEDs. Within each string, the LEDs are in series, but the two strings are run in parallel, so it's only ~0.9A per string.
1.8A through a single string of LEDs would likely wreck them extremely quickly.

It's 1.8A through a both string of LEDs, run time is 42 minutes for 3.0A battery.
Crees is cheap, push them hard, and still going.

 

[uk_caver]

Ah, I'd read the start of the thread, and it seemed like it was 1.8A total.
1.8A each really is going some. I guess it's only the serious heatsink that keeps everything from frying very quickly.

 

[92fs]

Robatman, please keep us posted on how your project is progressing. I have the same Makita batteries and would like to do a similiar project.

 

[Mash]

MHO regarding mounted emitters:
Stars (as in any emitter mounted on a base) seems to be the easiest way to use Crees, without having to worry about base electrical contact etc..., and I would think would be an even better conductor of heat away from the emitter, if attached to the sink with a thin layer of heat conducting medium. I say this cuz the emitter is soldered onto the star, which gives a very direct immediate dissipation path.
You can now get "stars" ie mounted emitters on tiny boards (square, 13mm across), barely larger than the emitter itself, so I find them ideal for space limited applications, as a bonus they can also be screwed down for a perfect fit. So dont neccesarily think of "stars" as the one size star pattern board we all think of.