what size resistor

[pierhogunn]

Hey need some help from the genius pool here

I have located a 4 AA battery holder with a USB out feature to charge cell phones, aparently, it's just a battery holder and that's about it.

it's intended to hold 4 NiMH batteries since they have an output of 4.8V fully charged since that will not overload a USB connector, * which is sposed to run at 4.25v - 5.25v

if I put 4 Alkaline I will get 6V, which, and the voltage would be too high

if I put in 4 lithium batteries, I would get 6.8V ( according to my Volt meter, a fresh AA lithium battery puts out 1.7V)

so with a target output voltage of 4.2 - to 5V what resisters would I use,

would I use a single pole, double throw switch, or a double pole. double throw switch...

also what spec resister would I use? (preferably something I can pick up at the rat-shack

or could I skip the switch, and use a variable resister, maybe something like the 1K ohm trimmer from radio shack...

[DaveS]

Are you looking for a voltage divider setup?

I think you would be better off using a voltage regulator.

[DaveS]

I am all for experimentation for the sake of learning... if that is the case here, then carry one.

However, if all you really require is a AA holder with USB power out, then you would be better off with something like this:

http://www.allspectrum.com/store/pro...oducts_id=1280

Search, Ebay, you can get them for $5, shipped.

However... if this is a learning thing, then... well, carry on.

How about a diode? That will give you a 0.7v drop, and it will also protect against accidental polarity reversal.

[pierhogunn]

I am all for experimentation for the sake of learning... if that is the case here, then carry one.

However, if all you really require is a AA holder with USB power out, then you would be better off with something like this:

http://www.allspectrum.com/store/pro...oducts_id=1280

Search, Ebay, you can get them for $5, shipped.

However... if this is a learning thing, then... well, carry on.

How about a diode? That will give you a 0.7v drop, and it will also protect against accidental polarity reversal.

funny, that is what I was actually going to buy from another source...

what sort of diode were you talking about?

[Tom Slick]

The problem with a simple resister in series is the electronics plugged (i.e. different cell phones) into it change the circuit. If one phone has different resistance (load) than another your simple circuit will have different voltages for each. look up kirchhoff's voltage law and ohm's law if you want to study up on this.

That is why Dave S is recommending something different than a resistor. I wouldn't recommend a simple diode either for the same reason stated above. A voltage regulator circuit would be best because it compensates for differences in load.

[pierhogunn]

so it sounds like I just need to run this thing off of AA Nimh batteries, and forgo using any other type of cell in this holder

so much for that, unless I come up with an $20 lamba electronics DC to DC converter that will take 6.8 - 6V down to 5V reliably, and cut off when the current drops too much . . .

still this problem can't be so hard that a simple component or two can't do the trick...

[x00018]

Can you eliminate one battery and just use 3 lithiums

[DaveS]

$20?

No...

http://www.radioshack.com/product/in...ductId=2062599

You will need a couple small caps as well...

[pierhogunn]

$20?

No...

http://www.radioshack.com/product/in...ductId=2062599

You will need a couple small caps as well...

okay, so would you indulge me and help me with a schematic,

so on the one side input voltage would be 4.25v min to 6.8V max

the business end of the phone is looking for 4.25 - 5.25 V at a max of 900ma

I should probably also throw a loop in this thing somewhere with a momentary switch that will show green if the voltage from the cells is 4.25V or better, and red if it's less than 4.25 showing that i need to replace them...


and I should probably go ahead and order this holder

if the parts are cheap I might be able to build several...

[DaveS]

It is a pretty standard part... so, the schematic for any 7805 should do...

For example...

[Slik Geek]

The 7805 series regulator has a typical dropout voltage of 2V, meaning that to get a 5V regulated output, you need at least 7V on the input. For 4.5V on the output, you would need a minimum input of 6.5V on the input.

You need a "low-dropout" voltage regulator for your application. Search on
www.digikey.com
for low-dropout regulators.

You'll likely need to choose a particular capacitor ESR (equivalent series resistance) for the regulator to operate in a stable fashion.

[pierhogunn]

The 7805 series regulator has a typical dropout voltage of 2V, meaning that to get a 5V regulated output, you need at least 7V on the input. For 4.5V on the output, you would need a minimum input of 6.5V on the input.

You need a "low-dropout" voltage regulator for your application. Search on
www.digikey.com
for low-dropout regulators.

You'll likely need to choose a particular capacitor ESR (equivalent series resistance) for the regulator to operate in a stable fashion.

okay, what about this part from allspectrum?
http://www.allspectrum.com/store/pro...odel+%23+LM317

[LCHIEN]

7805 has too high a drop out, you need something like the LM2940-5 LDO which only drops a maximum of .5V. Don't forget to calculate your heat sink requirements and heat sink accordingly.

[LCHIEN]

okay, what about this part from allspectrum?
http://www.allspectrum.com/store/pro...odel+%23+LM317

bad choice, DO is 1.5v Min and required a slew of external components to set the output voltage.

[pierhogunn]

how about this part?
http://www.mouser.com/Search/Product...NJM2831F05-TE1