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Which resistor?

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PhoenixMDM

Piano Man
Joined
Aug 21, 2001
Location
Candia, NH
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. :confused: :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!! :p
 

Diggrr

Underwater Senior Member
Joined
Nov 29, 2001
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

PhoenixMDM

Piano Man
Joined
Aug 21, 2001
Location
Candia, NH
Is it
starting - desired / amps = ohms

or

(starting - desired) / amps = ohms

:confused: :confused: :confused:

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

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

Diggrr

Underwater Senior Member
Joined
Nov 29, 2001
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
Joined
Mar 13, 2002
Location
Greensboro NC
The second equation is correct. The first will technically yield a voltage minus a resistance equal to a resistance.
 

Diggrr

Underwater Senior Member
Joined
Nov 29, 2001
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.;)