# Resistor power dissipation

Discussion in 'General Electronics Chat' started by stoopkid, Jan 8, 2013.

1. ### stoopkid Thread Starter Member

Mar 3, 2011
136
1
So say I have 12v with 300ohm resistor. That's (12^2)/300 = 480mW. I think.

So say it's 3 100ohm resistors in series. Is the power dissipation for each 480mW or is is 430/3 each?

Say it is 6 200ohm resistors, each 2 in parallel and each pair in series. What is the dissipation of each resistor there?

Thanks.

2. ### toffee_pie Active Member

Oct 31, 2009
162
7
for part #1 its 48mW, part 2 Pt = 432uW

3. ### bertus Administrator

Apr 5, 2008
15,799
2,386
Hello,

You can calculate the voltage accross each resistor.
The resistors are equal so the voltage on each resistor is 12 Volts / 3 = 4 Volts.
The power in each resistor is (4 Volts * 4 Volts) / 100 Ohms = 0.16 Watt each.
For the 200 Ohms resistors it is (4 Volts * 4 Volts) / 200 Ohms = 0.08 Watt each.

Bertus

4. ### toffee_pie Active Member

Oct 31, 2009
162
7
for part 2, 100R in series, PT = 4.3W, or 1.43W each.
?

5. ### takao21203 Distinguished Member

Apr 28, 2012
3,578
463
Everyone learns math in school. 13 year old kids learn to solve weird equtations.

But applying this maths knowledge does not happen all that often.

I don't want to sound arrogant, but applying maths, think what if you use 10,000 x 0.1 Ohms resistors.

What if each dissipates 0.5 Watts. Suddenly 5000 Watts are dissipated from nowhere.

So you can solve the question for yourself pretty easily.

I don't want to criticize you. It is more like I want to criticize the education system, and how they deal with maths.

6. ### toffee_pie Active Member

Oct 31, 2009
162
7
dont know what im smoking but this is what you want, as bertus said.

been so long since i did this stuff..

7. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
While learning to work the problem mathematically is important, it is also important to learn to estimate (or, in this case, solve) problems more intuitively and to ask if the answer makes sense.

All of you variations result in an effective resistance of 300Ω, so the circuit supplying the 12V can't tell the difference between them and delivers the same amount of power, namely 480mW, overall.

The concervation of energy means that the sum of the power dissipated by all of the resistors must equal 480mW. Symmetry says that, in each configuration, there is nothing to distinquish one resistor from another and, therefore, each resistor is dissipating the same power as each of the others. So in the first case the single 300Ω resistor is dissipating 480mW, in the second case, each of the three 100Ω resistors is dissipating 160mW, and in the third case each of the six 200Ω resistors is dissipating 80mW. If you were to take sixteen 300Ω resistors and arrange them in four parallel strings of four resistors each, then each resistor would dissipate 30mW.

Now make sure that you can work the explicit math to get these results.

8. ### Audioguru New Member

Dec 20, 2007
9,411
896
No.
Your simple grade 5 arithmatic is wrong.
The power is (12 squared)/300= 480mW, not 48mW.

9. ### stoopkid Thread Starter Member

Mar 3, 2011
136
1
Woah this thread is getting hostile.

Thanks guys, it was more of a resistor concept question than a math question. I wasn't sure they would divide so linearly.

I think my question has been answered, thanks again.

10. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
The power divides evenly only because all of the circuits described are perfectly symmetric. Let's say that you too seven 300Ω resistors and hooked four o fthem up in a 2x2 array (two strings in parallel with each string consisting of two in parallel) and then hooked this array in series with another resistor and put this combination in parallel with a string made up of the remaining two resistors in series. The end result would still be a 300Ω effective resistance and a total power dissipation of 480mW, but now the four resistors in the 2x2 array would dissipate 30mW while the remaining three resistors would dissipate 120mW each.

11. ### toffee_pie Active Member

Oct 31, 2009
162
7
hence me putting up a photo with the correct answer, i was still hung over from the new year on my earlier post it seems