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USB Project


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#1 biferi

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Posted 13 March 2014 - 08:34 PM

I was going to conect the Black and Red wire from the USB cable to the Black and Red wire from my Battery Packs.

 

The Battery Pack will take 16 D Cell Batteries in Series.

It will give 24VDC at 4,000mAh.

 

And the other end of the USB Cable I was going to put to my Tablet.

 

I know my Tablet takes 5VDC to run and yes the USB Port would only give 500mA so can I not use a Resister to get the Voltage I need?



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#2 mattsowders1989

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Posted 13 March 2014 - 08:41 PM

well if you really want to do that, you would need a switching voltage regulator to step 24v down to 5v. SVR is the most efficient way to step down voltage



#3 biferi

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Posted 13 March 2014 - 10:11 PM

Why can I not use a Resister or a Car Adapter?

 

If I use Resister it can make my 24V 5V.

 

And if I take a Car Adapter I can take the part that goes into the Cigarette Lighter apert.

 

And conect the two wires in that to the two wires on my Batery Pack.



#4 mattsowders1989

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Posted 14 March 2014 - 01:58 AM

are you trying to get connectivity to your computer while using your batteries ran in series to power/charge the device?

 

in that case, your idea seems more sufficient.

 

could you give me some more details? i find what your doing pretty interesting



#5 biferi

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Posted 14 March 2014 - 12:19 PM

I am not going to be conectting the Battery Pack or the USB Cable to the Compuiter.

 

The Micro USB end of the Cable will be pluged into my Tablet.

 

The other end of the USB Cable that you would plug into the Computer I will cut off.the USB conecter.

 

Now after I cut off the USB conecter I will open up this end of the Cable.

 

I will take just the Red and Black wires and conect them to my Battery Packs Red and Black wires.

 

I will be useing 4 Battery Packs and each Battery Pack can hold 4 D Cell Batteries.

And the Battery Pack hold the D Cell Batteries in Series so I will end up with 24Volts.

 

But the mA will stay the same so I will get 4,000mAh.

 

Now this is what I ment I know I need 5Volts not 24Volts so if I use a Resister or a Variable Resistor to ajust the Voltage down to 5Volts I will be ok.

 

But you say the Current will be to Hight because USB Ports only putout 500mA.

 

But can I not use Resisters to drop the Current as well?



#6 mattsowders1989

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Posted 15 March 2014 - 12:08 AM

ah. i understand. that is a good idea....

the amperage from your battery packs are irrelevant. your tablet will only draw the mAh that it requires. so in basic terms, your battery packs would last quite a while...

 

the only thing i would do different if i were you would be to run your battery packs in parallel, you would have 6 volts, only having to step it down by one volt, plus you would have more amps. more amps = longer lasting



#7 x64

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Posted 15 March 2014 - 02:41 AM

Mainly butting in here to offer a WARNING:

Running the battery pack is parallel will not help - the (slightly) higher voltage batteries will try to 'charge' the lower voltage batteries and best best drain them and at worst they will catch fire. Somewhere in the middle of the risk curve is a blown up tablet...

 

Re other suggestions. 24v via a resistor. You'd not get fine enough control, and you'd just be wasting all of the volts between your battery and output. The regulator in first reply would fix the output voltage but still no point in starting from 24V.

 

The best idea is the use of the circuitry from a car usb charger device (in which case there is little point in starting with more than 12-14v). This is by far the safest option as well. There is a lot of confusion of various electrical terms batted around above - I'd not recommend trying to design the circuitry yourself without a far greater understanding of electronics (I have a 30year old degree in electronics).

 

x64



#8 biferi

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Posted 15 March 2014 - 01:12 PM

I wanted to conect the Batteries in Series because I know the mAh will stay the same as one Battery.

 

And I thought High AMPs would Burnout the Tablet.



#9 mjd420nova

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Posted 15 March 2014 - 01:22 PM

High MAH means the unit can run longer at the set voltage.  The unit will determine the current draw.  Messing with anything supplying an external voltage to the internals of the machine is destined to go up in smoke.



#10 x64

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Posted 15 March 2014 - 01:25 PM

mAh is a measure of battery capacity, not voltage or current. A 100mAh battery lasts half as long as a 200mAh battery...

 

Seriously - I'd strongly recommend that you do not guess at this stuff... The only idea above that stands anything like a chance of working is the re-engineering of a car adapter.



#11 rotor123

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Posted 15 March 2014 - 02:15 PM

Hi, What You propose to do is very dangerous to the Tablet. It is designed to be fed 5 volts from a USB port. There is a huge possibility that You will feed to much voltage and burn it up. Yes a resistor can be used to drop voltage, However with one limitation. To drop the Voltage a known amount You need a constant load and Your tablet is not that kind of load.

 

Then You will most likely need a large resistor to dissipate the heat being produced. The wattage it needs to handle will depend on Voltage across it and the current through it. If we figure that the laptop draws 1/2 amp AKA 500Ma and that we are dropping from 24 volts to 5 volts we end up with 9.5 watts. Calculator From here

My guess is a 10 watt resistor would run hot.

 

Bottom line use a different method. I personally would just buy something such as shown here.

8 External Batteries to Keep You Charged on the Go From here

 

is designed to charge tablets, smartphones, MP3 players, portable game consoles and other USB devices. All you need to do is connect the battery to a power source until it's fully charged, and then it's ready for use.

 

Something like this is designed to do what You want and should be 100% safe.

 

Good Luck

Roger


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#12 biferi

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Posted 15 March 2014 - 05:18 PM

Thanks to all for the help.

Keep up the good work.



#13 mattsowders1989

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Posted 15 March 2014 - 05:44 PM

amps will not damage the device. it will only draw what amps are needed, for example. i soldered a battery pack together to power a router set up as a repeater. the pack has 9 4v batteries in it, they are ran in series of 3. then, the packs of 3 are ran parallel for more amperage. the router only requires 1 amp and the battery pack all together has 9 amps. their for, the pack lasts much longer






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