# Resistance dissipation power

#### electronicsLearner77

Joined May 26, 2012
125
Generally in specifications of the resistor wattage is specified. According to me this is the power dissipation or V*I. So the resistor is capable of sustaining this much heat. Am I correct?
Does this heat will be immediately dissipated and it becomes cool again?
Will this heat does not further increase the heat in the resistor, i mean to say some kind of recursive effect?
Can the resistor sustain any number of hours, if the heat dissipation is below the specifications?

#### Picbuster

Joined Dec 2, 2013
990
The power in the resistor = I^2 x R.
The so generated power is radiated.
The thermal resistance in combination with heat sink and environment ( dry wet hot cold or a combination thereof ) will transfer the power(heat).
Remember that heat takes time to flow resulting in a high temperature shot at the source.
This phenomena could destroy components when not calculated for.

Picbuster

Joined Mar 10, 2018
3,913
Generally in specifications of the resistor wattage is specified. According to me this is the power dissipation or V*I. So the resistor is capable of sustaining this much heat. Am I correct?
Yes, if its rating is N watts and you are driving N watts into it. Note this depends
on ambient T. It has a power derating factor. Its P rating is stipulated up to some T,
then a derating factor has to be applied to it.

Will this heat does not further increase the heat in the resistor, i mean to say some kind of recursive effect?
No, the R achieves equilibrium by transferring heat radiantly and by conduction back

Does this heat will be immediately dissipated and it becomes cool again?
If power applied is steady state the R temperature will achieve a steady state with its environment.
A flux of energy will be established that transfers heat (energy) into environment. If the power
applied is a pulse then heat will rise exponentially, then when pulse terminates decay eponentially.

Will this heat does not further increase the heat in the resistor, i mean to say some kind of recursive effect?
No, see above.

Can the resistor sustain any number of hours, if the heat dissipation is below the specifications?
Yes for its ratings. Although it has a MTBF rating as well.

Energy is measured in Joules. Power is measured in Watts = Joules/second,
heat in BTUs, "The British thermal unit (Btu or BTU) is a traditional unit of heat;
it is defined as the amount of heat required to raise the temperature of one
pound of water by one degree Fahrenheit. ... Heat is now known to be equiv-
alent to energy, for which the SI unit is the joule; one BTU is about 1055 joules."

Regards, Dana.

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#### nsaspook

Joined Aug 27, 2009
6,968
For an analogy you can think of heat (energy) as a liquid flowing into the top of a container (resistor) with a drain hole on the bottom (dissipation/cooling). The physical construction of the container determines the rate of fluid movement (thermal resistance) to that incoming flow.
https://www.sciencelearn.org.nz/resources/750-heat-energy

The flow rate of energy (heat) into (V*I) and out (heat dissipation) of a X sized resistor over a period of time (power is energy divided by time) determines the amount of thermal energy stored in the actual resistor usually expressed as a temperature measurement in some test specification.

#### MrChips

Joined Oct 2, 2009
20,351
Definitions

Check the definition of power, energy, heat, and temperature.

Power is instantaneous energy, i.e. energy consumed or emitted at that instant. Power in a resistor can be calculated as
P = I * V = I * I * R = V * V / R
Energy is the total power over time, i.e. it is power x time.
Heat is the transfer of energy.
Temperature is the average kinetic energy of the molecules in the material.

In your case, you are interested in the temperature rise of the resistor. This will depend on many factors. Heat will be dissipated via conduction, convection, and radiation and will be affected by many factors:
• physical size of the resistor
• type of construction
• size and length of the resistor leads
• placement on the circuit board
• spacing between the resistor and the circuit board
• amount of heat dissipating material on the circuit board
• any heatsink material
• vertical or horizontal mounting of the resistor
• vertical or horizontal mounting of the circuit board
• airflow
The temperature of the resistor will rise and continue to rise above ambient temperature as long as power is being delivered to the resistor.
Eventually, the temperature will reach a state of equilibrium when the energy to the resistor is equal to the energy removed from the resistor by all of the methods stated above.

Resistors are specified by a power rating (wattage).
Standard practice is to derate the power rating to ½ its specified rating (i.e. use a resistor rated for twice the dissipated power).
For example, if the calculated power is ¼W, use a ½W resistor.

#### Tonyr1084

Joined Sep 24, 2015
4,197
Heat is energy
Energy moves from the higher potential to the lower potential
The greater the difference in potentials the faster the heat energy is transferred.

Take an ice cube and toss it on the driveway. In the summer time the ice will melt within minutes (depending on the size of the ice cube). But put the same size ice cube on the driveway in spring or fall and it will melt much slower. Put it there in winter (assuming the temperature does not fall below freezing) and the ice cube will take several hours to melt. How does this relate to resistors? It's all in the transfer of heat energy. Higher wattage resistors are larger resistors. They have greater surface area and can expend that energy more easily. But if you put that resistor in the oven then it's not going to dissipate much heat at all. It itself will run hotter, and likely closer to its breakdown temperature.

The analogy of a bucket with a hole in the bottom is a good one. Only, imagine the hole is in the side of the bucket, down at the bottom. Put an inch of water in the bucket and the water squirting out the hole will merely dribble. Put several inches of water in the bucket and the water will stream out in an arc. The more water you put in it the further out the arc of water will flow. Now - if you over fill the bucket - in the case of a resistor - you have failure. Putting more energy into the resistor than it can expel means it will burn out. Run it at max load on a daily basis and it will eventually fail. Run it at half the temperature and it will last far more than twice as long. I don't know this for a fact, but I imagine that a resistor will last four times longer at half its rating. That is four times longer than if you were to run it at full power. So if a resistor can stand operating at max temperature (wattage) for 10,000 hours, at half the wattage it should (in theory) last 40,000 hours. And I'm probably wrong about that. It could be on the order of 16 times longer (inverse cubed law [I think]).

Bottom line, if you need a 1 watt resistor - use a 2 watt resistor.