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# need help for a project (resistor question)

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#### nitrothugg

##### Newbie level 1
i need help selecting the proper resistor for my application. any help would be appreciated.

i have a battery source of 7.2 volts and i need to drop it down to 1.3 volts at 2 amps. can someone please figure out the resistor i would need to accomplish this?

If you know ohms law its very simple.

Voltage you want to drop using resistor is 7.2 - 1.3 = 5.9 v and the current flowing through your resistor is 2 amps. To get 5.9v across resistor at 2 amp use ohms low R = V/I. You need 2.95 ohms

Well in this case your resistor should be rated for 5.9 * 2 = 11.8 watts (i.e. for DC, P = VI) if you use resistor with less power rating it will burn.

Mind you this is a criminal waste of power. What's your exact application? There has to be a better way around throwing away 11.8 watts of power. Especially when it's a battery. There's no garuntee that your battery can even source that much current to begin with. 1.3 volts at 2 amps only sounds like 3watts, but to get that 1.3 volts you have to waste 11 watts of power. At 2 amps a 1000mah 7.2volt battery will provide that power for about 30 minutes, and that's not going to be anywhere NEAR a linear voltage/current for the discharge. For a NiMH battery pack this would be strainging the pack more than you should. Can you detail more of your project? I'm sure a much better sollution could be found if you supplied more information.

hi

why would you like to use a resistive down conversion. u can use a zener or u can use commonly available power regulators for typical values 1.8v, 3.3v to down convert your voltage. im not sure whether 1.3 volt supply is available. u can use a 1.8 v converter and use a resistive down conversio. as it would'nt eat much power.
i hope im getting to the point. if im wrong plz correct me

Or you can use 2 high value resistors to get your voltage and have that voltage pass through a buffer that can supply your amps at 1.3V.

for example

use 1.3 : 5.9 ratios for your resistor. then buffer this through a opamp with high current output.

1.3 volts at 2 amps
use ZENER diode.. and then drop it further by resistors..
don't use small resistors directly across battery, it will sink a lot current...
another option is use approach like the following..
let u have 6 Volt battery..
use 3 high resistance resistors, so that it will sink very less current..each will have a drop of 2 volts.. then u can use this 2 volts by further implementation of this 2 volts in parallel..
there are infinite ways.........to implement anything

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