two part question using 3.8 VDC batteries (dc to dc converter?)

[Enginyr]

Steven here. Mechanical engineer. I have a pretty nice mechanical design here using a 3.8v dc battery supply and a 3.4v 13A LED. I need to use some 5v fans.

So my question is....

how to get the 3.8v down to 3.6V to 3.3V

and....

how to get the 3.8V up to 5V-6V

There are no electrical engineers to fix my problemo here

 

[itsmoked]

First try the fans on 3.3V many would spin and it would save a headache.

Next was that 13A? What size is this battery? 13A is non-trivial.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[TurbineGen]

It's easier to step down voltage than step it up when talking about DC. Can you use a 5V battery instead of the 3.8V? If so you can use a switchng regulator or resistor to power your LED cicuit.

Did you mean 13 mA on the LED? 13 Amps sounds like quite an LED array. That'd be either one monsterous fan cooled LED or an array of around 500 standard LEDs. If you meant 13 mA, use a resistor. 5V-3.4V/.013 = 120 ohms. 1/4 Watt resistor will be plenty.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[Enginyr]

"First try the fans on 3.3V many would spin and it would save a headache."

"Next was that 13A? What size is this battery? 13A is non-trivial." 4 D cell 11,000mA

The 3.3v fans spin but I need to overpower them to get the correct 15 CFM. 5.5v would be lovely.

13a is The current draw but might get to about 15a if I run them at 3.8v which is why I need to step it down a bit.

5V-3.4V/.15 = 10 ohms. ? Watt resistor (how do you decide what wattage resistor to us?

 

[ScottyUK]

Most computer-type fans have the speed set by the electornics. Applying more voltage doesn't make them go faster, it just lets the magic blue smoke out. Add more fans or find more powerful ones.


13A into an LED array? What is it, a searchlight?


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If we learn from our mistakes I'm getting a great education!

 

[TurbineGen]

Enginyr, I think you are going to need to move to a 6 cell battery and step your voltage up to 5V to make this work better. I'm assuming your configuration is 2S2P? 3S2P would get your fans to the needed CFM, then a simple resistor will step down the voltage for your LEDs.

Sizing your resistor is simply a power funtion I^2 * R or V^2/R. I'm going to use the V^2/R.

First you'll need to determine a value for your resistor:

5V battery - 3.4V LED = 1.6V drop in resistor.
Since we know the LEDs are requiring 13A, we apply V = I*R and get:

R = 1.6/13 = .123 ohms - This is not a normal resistor value, so we'll need to make it from some parallel or seies resistors. I'll go with parallel. If using all the same value the resistors parallel rating will be R/Number resistors. So: 5 X 0.63 ohm resistors in parallel will work. So will 8 X 1 ohm resistors in parallel.

Now for the Wattage requirements:

We know P = V^2/R (The I^2 *R yields same result) and we know V drop on the resistor bank is 1.6V. We also know R is ~.123.
So you'll need a resistor bank rated for at least 21 Watts. Since the power dissipation is divided equally among parallel resistors the wattage value needed is:

Wattage of bank/#parallel resistors.

So if you use 8 X 1 ohm resistors, they'll need to be 3 watts or more. If you use 5 X .63 ohm resistors, they'll need to be 5 Watts or more.



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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[Enginyr]

oh my you know your shiat. Thank you so much. I found a dc to dc converter from v-infinity that takes 3 to 3.6 and brings the voltage up to 5vdc 80% efficient so that closes this case. Attached is the going to be a super bright led flash light using light pipe recycling and adjustable focus.

 

[Enginyr]

Here is the finish product

http://www.enginyr.com

http://www.enginyr.com

 

[itsmoked]

That link is to an inscrutable site. Makes no sense to me.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Enginyr]

3rd Image to the right and one down. "click"

http://www.enginyr.com

 

[itsmoked]

Do you understand that every time one opens that site the pictures are randomized?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Enginyr]

oops. lol sorry. It's the picture with the green leaf 50 watt led one

http://www.enginyr.com

 

[itsmoked]

World's brightest flashlight??!

I'm skeptcal.

http://peakbeam.com/index.html

Keith Cress
kcress -

http://www.flaminsystems.com

 

[TurbineGen]

50 Watts of power though and LED for a flashlight? That's no flashlight, it's a searchlight! Thank goodness you are cooling this monster. My MAG light has a 3 Watt LED in it and the beam illuminates quite well.

The picture suggests this might be carried in you pocket. Is this really meant to be carried around? 4 D-cell batteries is enough weight to bring my pants to my ankles.



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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[Enginyr]

It is actually just used to demonstrate our light pipe recycling technology. I have designed it for a 3D 3.4VDc 35 watt or 4D 4V 50 watt Maglight. It has a wide zoom and uses an aspherical lens in two position. Anyone want a $1,500 flash light

http://www.enginyr.com