# Difference power dissipation and actual power dissipation

1. Jul 19, 2013

### mepiace

difference "power dissipation" and "actual power dissipation"

Hi,

Is it possible to explain me the difference between "power dissipation" and "actual power dissipation" for a resistor ? I really cant understand the difference...

Last edited: Jul 19, 2013
2. Jul 19, 2013

### Staff: Mentor

Do you have some context?
Do you have two different numbers for the same resistor in the same setup?

3. Jul 19, 2013

### mepiace

I am reading the standard MIL-HDBK-217F for the reliability of resistors and it talks about "power dissipation" and "actual power dissipation". It just says that "power dissipation" is connected to the power factor and the " Actual Power Dissipation" is connected to the power stress factor but i cant see or immagine any difference between "power dissipation" and "actual power dissipation".

4. Jul 20, 2013

### Bobbywhy

mepiace, Welcome to Physics Forums!

I have downloaded MIL-HDBK-217F at http://www.sre.org/pubs/Mil-Hdbk-217F.pdf and searched the entire section number nine and cannot find those terms “power dissipation” and “actual power dissipation” anywhere. The only words similar are here:

“Section 9.0
The use of the resistor models requires the calculation of the electrical power stress ratio. Stress = operating power/rated power, or per Section 9.16 for variable resistors. ...The rated power for the stress ratio is equal to the full nominal rated power of the resistor.”

Will you please post the link or the exact quotation, in context, to assist members here respond to your question. Thank you.

5. Jul 22, 2013

### mepiace

difference "power dissipation" and "actual power dissipation"

Hi,

Into the page 9-2 is mentioned the "power dissipation" in order to calculate the "power factor'. Into the page 9-3 is mentioned the 'Actual power dissipation" in order to calculate the "power stress factor". I dont know if there is any difference between "power dissipation" and "actual power dissipation".

6. Jul 22, 2013

### AlephZero

In 9-3 I think the "actual" is just to make clear the difference between the power dissipation of the component in the circuit, and the nominal power rating of the component - e.g. a "10 watt" resistor that is actually dissipating 5 watts has S = 0.5.

In 9-2 "power dissipation" presumably means what it says, i.e. the same as "actual power dissipation" in 9-3.
But for a quick conservative estimate, assuming the components have been properly specified, you might use the component power rating in 9-2 instead. The failure rates for most resistors are likely to be low compared with other components in the system.

7. Jul 23, 2013

### mepiace

8. Jul 26, 2013

OH dear god - please do not try to learn from Mil Specs.... haha - in general the "actual" case tends to be a measured or confirmed value.