# P7 DD with 18650, can i have any problem?



## Daylight (Mar 13, 2009)

Hello, i am a beginner, i want to do a single 18650 flashlight with p7 direct drive, without anymore (resistors, etc)

What are the problems can i have with this configuration?

The p7 will take only the intensity that this datasheet shows?







What will happen with the excess of voltaje of the 18650 AW fresh charged? 

Which of this is the best glue or paste for fix the led to the heathsink?

Thanks, 

Sorry if this thread are out of place or repeated.


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## Benson (Mar 13, 2009)

In a DD, the extra voltage is dropped across the battery's internal resistance, and any resistance in the light body. If things work out right, the voltage drop will match the difference between the battery's open-circuit voltage and the LED's Vf at something close to 3A, and everything will work nicely. In reality, with small cells (typically having a higher internal resistance) and high-resistance switches, springs, etc., the LED will be underdriven -- no problem. With large cells (the 18650 is toward this end, although some people have successfully used it) and low-resistance components, the LED will be overdriven, and run outside spec. Now if your LED is heatsinked well enough, you can overdrive with 4 or 5A through a P7 without immediate harm; the life will be reduced from manufacturer's rating, though, and it becomes harder to keep from overheating the LED.

One of the best options, IMHO, is to use an AMC7135 board to limit the current at 2.8A or 3.15A when the battery is full -- when the battery voltage falls too low to provide that current, the AMC7135 will then let the (lower) current pass through unregulated.

If you do use DD, check what current is actually supplied with a fresh battery, and add a resistor to reduce it if it's too high.


As for thermal epoxy, the back of the P7 is connected to the + side, so you'll either need to use electrically insulating epoxy (Arctic Alumina Adhesive), or make sure the heatsink is insulated (e.g. anodized contact surfaces) or +-connected. (Most flashlight designs result in the heatsink being at -, so using Arctic Silver Adhesive would short the batteries out. Sometimes the batteries may simply be inserted backwards to achieve a positive-ground connection, and the Arctic Silver does have better thermal conductivity.)


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## Daylight (Mar 13, 2009)

Thanks a lot for you detailed and very useful explanation!! 

Then, what do you recomend me to fix the led at the heatsink of all of this?


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## Benson (Mar 13, 2009)

Daylight said:


> Thanks a lot for you detailed and very useful explanation!!
> 
> Then, what do you recomend me to fix the led at the heatsink of all of this?


Depends on your heatsink, and whether you can reverse the battery. I'd recommend the Alumina Adhesive, if you've got a bare-metal heatsink and can't/don't want to use reverse-polarity, and the Silver Adhesive for anodized heatsink or bare-metal with reverse polarity.

Also, if you need to use AA, watch out that you don't make metal-on-metal contact through the epoxy before it sets -- there's a number of tricks suggested to accomplish this (one is to apply a thin layer, let it set, then apply another layer and stick the LED down, another is to lay a couple hairs across the area to make a little thickness...) but I've never used any of these, so I can't make a personal recommendation... (When I did a P7, I was able to reverse-polarity, and so I just soldered it onto the brass pill.)


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## Daylight (Mar 13, 2009)

:bow::bow::bow:


Good!! Thank you very much.


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## moviles (Mar 13, 2009)

Daylight said:


> Hello, i am a beginner, i want to do a single 18650 flashlight with p7 direct drive, without anymore (resistors, etc)
> 
> What are the problems can i have with this configuration?
> 
> ...


make your dd flashlight now and get the power











the battery of my ultrafire c3 p7 are reversed


You bought the tiablo a9?

saludos Daylight


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## Daylight (Mar 14, 2009)

I'm in it, i'll wait to buy the a9 and colimator a few days.

Saludos moviles


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