# Selecting the power rating when purchasing an SMD resistor

Hi everyone,

I am having a hard time selecting the right power rating for SMD resistors. With through hole resistors I never encountered running into problems with resistor power ratings as through hole resistors have relatively high power ratings and I was never really thinking about them when using them in my 5V Arduino's projects before.

In my project I will be using the ATMega 328P and as an example lets take this Arduino nano schematic. Let's look at pin 17 on the ATMega328P which is connected to the resistor RP2A a 1K ohm resistor array, a yellow led and ground. Lets ignore that RP2 is a 4x1K ohm resistor array for now and look at RP2A as a single resistor.

Using: V / R = I so 5V/1000 ohm = 0.005A and P = I x V so 0.005A x 5V = 0.025W = 1/40W

So the minimal power rating would be 1/40W for this resistor? So using a resistor with a 1/10W power rating (typically seen on imperial 0603 SMD package sizes) would give me a good safety margin?

What is confusing me is that when I do the same calculation for a 10K ohm resistor instead of a 1K ohm resistor I get a power rating of 1/400W (Using the same calculation as above). I was expecting power ratings to go up as the resistor value goes up. More resistance produces more heat and therefore energy right??

TheAliw1: I was expecting power ratings to go up as the resistor value goes up. More resistance produces more heat and therefore energy right??

No, more resistance means more resistance, and the circuit its in then determines the power dissipated.

From the basic equations: V = I R P = I V

we get P = I^2 R = V^2 / R

So for a fixed voltage power goes down as resistance goes up (the current decreases), for a fixed current power goes up as resistance goes up (the voltage increases).

MarkT: So for a fixed voltage power goes down as resistance goes up (the current decreases), for a fixed current power goes up as resistance goes up (the voltage increases).

So if I understand correctly my calculations are right and the theoretical minimum power rating for the 1K resistor = 1/40W and the 10K resistor = 1/400W at a fixed voltage of 5V.

I will probably go somewhat higher than this. Like mentioned before I will probably use 1/10W as a safety margin.

I'd use 0805 size so you can actually work with it without a pick-and-place machine!