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Resistor question

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yfx4

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