# Resistor question

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#### yfx4

##### Member
If two resistors are in series I know you add the resistance values together. What about wattage?? If I need 30r 1/2 watt can I use two 15r 1/4 watt resistors??? Seems like an elementary question but I have not been able to locate any info on it despite multiple searches with different search terms.

Thanks!!

#### RCinFLA

##### Well-Known Member
yes, but normally you don't want to run the resistor at more then 50% its rated wattage. Longevity and reliability drops as temp increases.

Last edited:

#### MikeMl

##### Well-Known Member
Ptotal = P1 + P2

P1 = ( I1^2)*R1
P2 = ( I2^2)*R2

Because the resistors are in series I1 = I2
And, you said R1 = R2 = R = 15

Therefore, Ptotal = 2 * [(I^2)*R]

Sure looks like the combination will dissipate twice what either one dissipates.

The max current allowed will be I^2 = 0.5/(2*15) = 0.016667

so Imax = sqrt(0.016667) = 0.129A

#### hostet

##### New Member
Make sure you use 2 x 15R 1/4W to make out a 30R 1/2W. If you use 1 x 9R 1/4W and 1 x 21R 1/4W resistor, it may not work depending on the current within the circuit.

#### yfx4

##### Member
Thank you all. That is what I thought. Sometimes things that seem obvious aren't really so--I thought I should check.

#### Hero999

##### Banned
The highest resistance will always dissipate the most power.

Workout the current and power using Ohm's law: I = V/R, P = I²R

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