Question about multiple LED

[jaygrant]

Well this might not be the correct forum, but here goes, since it will lead to questions about a homemade light.

I'm not real good on DC and I'm nothing on electronics, so my question is novice and I have done some searching and read stickys, but I have not found the answer I need.

I understand series and parallel in battery packs, but what about LEDs?

I will use a hypothetical:
6 leds in series. Each one using 3.5Vf = 21.0 Volts
Now each led needs 2.5 amps = ??how many amps total?? See in a battery pack, the amperage would stay the same, only the voltage increases. Does it work for the demand side too? I'm not seeing it. I would think the demand side would need amperage equal to the sum of the leds, but I don't know.

That's my first question.

 

[Packhorse]

2.5amp

Now if you wired them in parallel you would need 3.5 v @ 15amp

Either way same watts and same theoretical lumen.
But getting a 3.5 volt power supply that can hadle 15 amp including the wiring switch etc could be a problem.

 

[kuksul08]

It's just like the battery pack. In series they will all receive the same current which in this case is 2.5, but the required voltage sums.

 

[jaygrant]

Thanks for the answer.

So now I know how a group of leds will act in series.

Now about the heat produced. I read that the heat in any high powered light is a killer because it can't be dissipated quickly enough, but since I don't have much experience it's harder to get it. Heat is created by the excitation of the electrons moving in the stream? and from the resistance of movement to the stream, i.e. resistance in wires, switches and the led dies themselves?

Since the wattage remains the same, the amount of heat stays the same in either parallel or series?

 

[Gunner12]

What LEDs are you using?

The heat should be the same both ways. I think it is caused by the die and probably the phosphor. The wires might heat up more with the lower voltage but that isn't significant.

You should wire the LEDs in series because LEDs can have very different Vfs, from 3v at 350 mA to 3.7v at 350 mA(maybe higher). Even if they are in the same Vf bin they could have enough of a difference to cause one LED to take a lot more current then another.

:welcome:

 

[jaygrant]

Well I would favor series. Series is the way I should go. As to which leds, I am not 100% sure yet. Either MC-E or P7. Most likely P7, but it's going to be a while till I actually do it, so it might be some other led.

It's not going to be a modified flashlight. It will be from scratch. It's not really for a personal need, just for a personal accomplishment. I don't know for sure if it will be traditional flashlight style, or more like a lantern style.

My original thoughts were along the lines of an all copper light. 3" diameter head, 2" diameter body. Copper heatsink, ducted cooling fan, seven P7 emitters. Or a box style body with a lantern style head. I don't know for sure yet. I am just thinking of how I would use the components, so that I can decide on how I want the housing to be.

It is pretty much for WOW factor, since the heat will cut down usefullness. Just something to say I did it. I have not gotten to the driver or switch thoughts yet. That's later on to figure out. I do not want a single stage on/off switch and I rather fancy a dimmer switch, but I have not looked much for any DC Dimmers or Potentiometers yet. I wanted to figure out the series wiring for the emitters first, which I have now, thanks to you gentlemen confirming the Amp & Watt figures.

 

[jaygrant]

I see that supplying the power for 7 P7 emitters in series might be a challenge and may make me automatically go for a box style light body, with a lantern style head.

I have been reading a lot here and I see disadvantages to each way. I have never used any type of lithium style battery and I can see that my "style" probably wouldn't be good for those. I sure do not want to have to check the battery voltages, or worry about amp draw. I really don't want to take the time and effort for all that, so that leaves nimh and sla. If I do nimh, it would take a bunch of them, like 24 of them and then it gives issues from the sheer number of them together, meaning I am back to checking each battery's condition. Sorry, I charge them, they work and if they don't I chuck them. That's as far as I want to go with a battery.

So it would seem that using two small sla 12v batteries in a small box would work. Then I could use a sealed beam style lantern head and make it all up for something more like the big ones my dad used to use. The ones that fit on top of a battery with a head and handle and off you go.

Any way I go, there will have to be some type of regulation, but so far I don't find a lot of stuff that would work for that high of a voltage. If there is a recommendation, it would be nice.

Then I got to thinking about switches and I think I really like the idea of either a multi-position rotary switch or a variable rotary switch (dimmer). I don't see to many of those here either, that would do for 24V.

Any thoughts about all that? If anyone already has done that kind of thing, I just haven't found it in the searches yet.

 

[Oznog]

Heat is generated at the back of the thermal pad of the LED. No heat of note generated at the LED's electrical contacts. We don't just want to stay well within the "absolute maximum rating" for die temp, we want to keep them as cool as possible to lengthen their life and raise efficiency. Lumen output goes down quite a bit with high temps.

P7 is quite difficult to cool due to the extreme power level. 7x is really really difficult. There's 2 problems, one is effectively spreading the intense heat away from the small backing and two is being able to cool the sink through contact with the air.

Right, you'd have a LOT of trouble supplying 7x P7's with a "reasonable" number of batteries. Your SLA could be very heavy. Even SLAs have limits on their current output, BTW. That's over 5A out of 12v and a small SLA, like 5AH, that's a 1C discharge rate. Won't hurt it much but you won't get as much capacity as the nameplate suggests since that AH is for a 20-hr rate.

I suggest you start with a more modest, sensible project. I mean you're kinda jumping into a project that needs a strong understanding of batteries, machining, electronic drivers, thermal and optical design or else it'll probably bomb. ONE P7 is impressive, trust me.

 

[jaygrant]

That's over 5A out of 12v and a small SLA, like 5AH, that's a 1C discharge rate. Won't hurt it much but you won't get as much capacity as the nameplate suggests since that AH is for a 20-hr rate.

I suggest you start with a more modest, sensible project. I mean you're kinda jumping into a project that needs a strong understanding of batteries, machining, electronic drivers, thermal and optical design or else it'll probably bomb. ONE P7 is impressive, trust me.

I thought from the answers first posts that amperage was determined to be 2.8 amps with 7 emitters in sereis and that voltage was 7x3.7v= 25.9V and Watts would be about 72W, was that incorrect?


Well some times I'm sensible, but this is just for fun.

Let me try it this way, members of CPF how would you go about doing 7 SSC P7 emitters in a flashlight? Has it been done before and how was it done? If you have thought about it, how would you do it?

 

[Calina]

Well some times I'm sensible, but this is just for fun.

Let me try it this way, members of CPF how would you go about doing 7 SSC P7 emitters in a flashlight? Has it been done before and how was it done? If you have thought about it, how would you do it?

I'm not sure you realize how much light this would provide. You will need a very large heat sink and a wheelbarrow to carry your batteries if you want to have more than a few minutes of run time, plus it wouldn't be a thrower. Multi dies LEDs are very hard to focus tightly, they are good flooders though.

 

[jaygrant]

Like I stated before, I know it's not useful, it's only for a wow factor, nothing more. I have seen posts where members are already working on 7- P7's and probably it's already been done, so it's nothing new. I just haven't found the posts yet, but I keep searching. I know someone here will do it and then I can probably figure it out from there.

Or not.

 

[VegasF6]

Like I stated before, I know it's not useful, it's only for a wow factor, nothing more. I have seen posts where members are already working on 7- P7's and probably it's already been done, so it's nothing new. I just haven't found the posts yet, but I keep searching. I know someone here will do it and then I can probably figure it out from there.

Or not.

Quick 1AM reply (means it isn't very well thought out.)

If I was determined to do this, I would buy two 12V RC car battery backs.
Something like this:
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2699
But I would take the time to find out who makes quality cells. RC car stuff has a tendancy to really be on the cutting edge of battery tech.

They would be wired in series with something like deans plugs so I could take them apart for charging.

For heat sinking, I would look into something along the lines of what you would see on a high power car amplifier maybe. I would have a good look here:
http://www.mmmetals.com/pages/mediu...medium_and_high_power_extruded_heat_sinks.htm

Probably wouldn't lean towards fans, but then again, maybe I would.

Leds would be wired series of course, and each would have a big ol series resistor. Maybe like a 10W. For values I would spend some time on a resistor calculator which I don't feel like doing right now.

Somewhere on here there is a spotlight that was built recently that had an all aluminum body with a wooden handle. It was awesome. Forget the details but it was stupid bright, and not very long ago. Help me out with this one guys. I think it was 4 or 5 P7's.

I wouldn't expect to run it very long at all, would simply be called "shock and awe" and I probably would keep currents down, maybe 2.5 amps at full battery charge. Resistor values and drive current would both be dependant on what those batteries charged up to anyhow. Probably come off the charger around 1.45V per cell.

Just for giggles instead of a finned heatsink maybe I would use about a 3/8 aluminum flat plate and stick dry ice behind it. That oughta keep it cool :) Or who knows, maybe something with water blocks.

But, if I was serious about wanting that much light I would look into doing a 2x50W HID instead.
Like this one:
http://www.candlepowerforums.com/vb/showthread.php?t=199214

 

[jaygrant]

10 Crees I believe:
http://www.candlepowerforums.com/vb/showthread.php?t=158356&highlight=makita

Yes it's an awesome light. Yes that's the kind of thing I am talking about. It's funny, if you search here long enough, most everything has already been done, multiple HID, multiple LED, I guess maybe I should do Flourescent. (nevermind).

That cured me, I don't want to do it any more. Already been done and done well.

Thanks for the post, I found it after about 15 minutes of searching.

I wish there was a way to search here (newest to oldest), it would make searching a lot easier.

 

[VegasF6]

There is so much data here, it can be awful hard sometimes to find what you want. Good job serching, cuz I sure couldn't find it again. I am surprised it was so OLD! Now that I think about it, I was researching that Makita dock for another thread I was answering, and that was how I found it. But, my memory failed me again.

Speaking of, after I went to bed last night I realized I had written about putting a resistor in line for each of the leds on the multi p7 build. Well, that was obvisouly wrong for a series application. Duh, I was sleepy.

Plus, I started thinking, if one was really serious, I guess 2 shark bucks from the sandwhich shoppe would be another option.
If you still wanted to read up on it, check here:
http://www.candlepowerforums.com/vb/showthread.php?t=211599

 

[Calina]

10 Crees I believe:
http://www.candlepowerforums.com/vb/showthread.php?t=158356&highlight=makita

Yes it's an awesome light. Yes that's the kind of thing I am talking about. It's funny, if you search here long enough, most everything has already been done, multiple HID, multiple LED, I guess maybe I should do Flourescent. (nevermind).

That cured me, I don't want to do it any more. Already been done and done well.

Thanks for the post, I found it after about 15 minutes of searching.

I wish there was a way to search here (newest to oldest), it would make searching a lot easier.

You should use advance search.

This light is nothing like the one you were planning on. It uses 10 single die Crees. Yours would use multiple dies Crees.

 

[jaygrant]

You should use advance search.

This light is nothing like the one you were planning on. It uses 10 single die Crees. Yours would use multiple dies Crees.

Yes, 7 multiple die P7 emitters. Way too hot to control. I mean the one that had 10 single die emitters "got hot" in less than a minute. I can now imagine the heat produced in 7 P7's. Sort of uncontrollable in any type of enclosed space from the looks of it. Heck, that light was out in open air with a huge heatsink and a 120mm (I think) fan. In an enclosed space, it would fry itself in a heartbeat., but heck I don't know that. I don't know anything about Electronics, just like about 90% of us newbies comming here I would guess, so I'm not alone huh?

So is there any type of "rule of thumb" for how much heat is put out from an LED emitter? (or for Incans as well), any type of formula or calculator where you can "plug in the numbers" and have some idea of heat output? That would be helpful and If I had found info like that, I probably wouldn't have gone so far with this whole thing.

Even though it might be a pain for the knowledgeable guys having a newbie ask dumb questions, it's a good thing I asked before buying anything. It would have cost me a bunch of money for nothing.