# Which resistor?

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#### PhoenixMDM

##### Piano Man
Alright, i know about Ohm's Law, and stuff like that, but i'm a bit iffy when it comes to what u do with the solved equasion.

Let's say i have a 12v power supply, and i wanna tone it down to 7v. I'd do the following, if i'm correct:

12 12
7= ----- then, 7x=12 then, x= -----
x 7

Which reduces to 1.71. Do i need a 1.71ohm resistor, or a 171ohm resistor? I heard somewhere that you multiply ur result by 100, but now i'm just confused.

Also, i put this in gen hardware since it is sorta a power supply question, but not really, and had no clue where else it should go. Don't hurt me mods!!

#### Diggrr

##### Underwater Senior Member
Starting voltage - Desired voltage / Device amperage = Ohms

Easy enough? You just gotta know the devices amperage in Amps, not miliamps. 1 Amp = 1000 mA (miliamps), if the device uses 500MA, then use .5 Amps in the equation above.

Have fun!

OP

#### PhoenixMDM

##### Piano Man
Thanks, it's good to hear the right formula from an experienced person!

OP

#### PhoenixMDM

##### Piano Man
Is it
starting - desired / amps = ohms

or

(starting - desired) / amps = ohms

Order or operations is a sorta important thing most of the time, lol.

And with my project (you will soon see ), for some reason a 12v input, to 5v at 1a, just dosn't seem like 7 ohms is right.... Maybe it's just me

#### Diggrr

##### Underwater Senior Member
I don't think it matters which order you do it the math in, I just work it straight through on the calculator.

Yeah, 7 ohms seams like it's wrong, but it is correct. The lower the amperage, the more ohms are required to shift the voltage. Just make sure your wattage rating on that resistor is okay...at 5 watts of power, a 5 watt resistor is okay, but I'd use a 10 watt resistor so it doesn't get too hot.

Have a good'n

#### Xaotic

##### Very kind Senior
The second equation is correct. The first will technically yield a voltage minus a resistance equal to a resistance.

#### Diggrr

##### Underwater Senior Member
Yeah, I just tried it both ways with some different numbers plugged in, it yielded a negative number when done the first way (wrong answer too), so the second way is definately the way to go.

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