# SMD Resistor Power Rating Calculation

I'm extending my question from previously.

How to find SMD resistor and capacitor value and correct size

I'm doing some calculation and find something confusing below is the calculations:

V = 5

R = 1000

I = V/R = 5/1000 = 0.005A

P = IxV = 0.005A x 5V = 0.025W = 1/40W

1/40W is output for me to find the right size of SMD now confusion kicks in when I change the values see below

now let's change this to

V = 5

R = 10000 (10k)

I = V/R = 5/10000 = 0.0005A

P = IxV = 0.0005A x 5V = 0.0025W = 1/400W << that's 400 m'i doing something wrong from the SMD power rating chart i cant any bigger than 1/20 ? how to resolve this?

Now let's come to my real problem see below.

V = 3.3

R = 0.10 ohms

I = V/R = 3.3/0.10 = 33A << ??

P = IxV = 33A x 3.3V = 108.9W = 108 9/10 << ??

See the Circuit and focus on R2 and R3 value. Can someone help to understand me this? or correct me what I'm doing wrong? There is no way its 33 Amps of current for this what will be the correct power rating for this R2 and R3 based on the inputs mentioned on the circuit, V = 3.3 and R2, R3 = 0.10 ohms

if anyone can show me the calculation for the above problems and advise me the correct way of calculation, I have created an excel sheet to make life easy but unable to figure it out whats is wrong when changing the value and if any of you can help me to improve that sheet for the public I'll appreciate

• why you assume that over r2 and r3 would be 3.3v? help me to understand this. – marcosbc Jan 31 at 1:38
• I really don't know why I assume... hmm, so I need a value across the resistor not what I'm supplying as input pfft, poor me! @marcosbc – Adeel Rizvi Jan 31 at 1:51
• no problem friend, we are here for help. lines below Frog gives you the exact answer! – marcosbc Jan 31 at 1:56
• 1/400 is much smaller than 1/20 which in turn is much smaller than 1/8, so a 1/8 W resistor is more than ample for either your 1 K or 10K examples. Look at the decimal values rather than the fractional values. – Peter Bennett Jan 31 at 2:31
• Righto! That's the first mistake I'm doing not reading decimal!! good point out @PeterBennett – Adeel Rizvi Jan 31 at 2:37