Meaning of power ratings.!!

Thread Starter

Gow

Joined Sep 9, 2017
4
I was thinking about an analogy to understand the physical meaning of the term ‘Power’. Let’s take two resistors of same resistance (Say 5 Ω) in two different circuits. First one is the carbon resistor with low power rating and the second one is a wire wound with high power rating. When a DC 5v is applied to the each of them, the current flowing in both the circuits will be 1A. Now, if the voltage is increased in both circuits to 10v, the current is increased to 2A in both the circuits. Say, now at this current, the carbon resistor heats up and catches fire while the wire wound doesn’t (though it heats up very quickly). Does the ‘high power rating’ of a device mean, it’s ability to withstand the heat that was generated due to the highly energised electrons ?
 

wayneh

Joined Sep 9, 2010
17,496
Welcome to AAC!

Yes.
Ditto. Energy is released by the electrons losing energy as they "fall down the hill". It must be dissipated by the device that caused that to happen. We exploit that in a lightbulb to make light, or in a motor to provide shaft work. The energy transformed into heat and/or work can be calculated as I^2•R or as V•A.
 

nsaspook

Joined Aug 27, 2009
13,081
It does mean a ability to withstand/dissipate the heat without physical damage.

Electrons:( Pedantic mode enabled. Not a fan of the "fall down the hill" analogy.


The energy of this heat wasn't supplied by highly energized electrons as current in wires from your power supply to the resistor, they get highly energized inside the resistor. The 'current' just by itself supplies almost no energy to the hot resistors. ELECTRIC ENERGY IS NOT CARRIED BY INDIVIDUAL ELECTRONS (usually) in wires but that's a subject for a different day. In the circuit loop from power supply to resistors(s) the current is the same but the voltage drop across the connection wires from the power source to resistor is low so when we take the product of current and voltage the power/energy flowing into this wire is low. Across the resistor we have almost the entire power source voltage and the series current of the circuit when we take the product of current and voltage to calculate power/energy flowing into the resistor it is high. This resistor voltage drop electrically accelerates free electrons in the resistor, these electron collide with the atoms in the resistor and release the transfered potential electrical energy as kinetic energy to the atoms (Joule heating).
 
Last edited:

Thread Starter

Gow

Joined Sep 9, 2017
4
Ditto. Energy is released by the electrons losing energy as they "fall down the hill". It must be dissipated by the device that caused that to happen. We exploit that in a lightbulb to make light, or in a motor to provide shaft work. The energy transformed into heat and/or work can be calculated as I^2•R or as V•A.
So, its basically every material has an upper limit at which the energy released by the electrons damage that device. An another example I can think of is 'water heaters'. The heating element should be rated to withstand high voltages and high current. Right.?
 

MrChips

Joined Oct 2, 2009
30,708
Both resistors have to dissipate the same amount of power given the same voltage or current for the same resistance.
The power generated is given by any of the following three equations

P = I x I x R
P = V x V / R
P = I x V

Hence, with 5V @ 1A the power dissipated is 5W

For both resistors, the temperature of the body of the resistor will rise. How much the temperature rises depends on how rapidly you can remove the heat being generated.

There are three ways to remove the heat:
  1. conduction
  2. convection
  3. radiation
There are many ways in which heat removal can increased:
  • choosing resistor with higher wattage rating (physically larger resistor- 1, 2, 3)
  • mounting on a large piece of metal (large heat sink- 1, 2,3 )
  • painting the heat sink black (3)
  • using conductive heat paste (1)
  • increasing air flow (standing the resistor away from the PCB for improved air circulation - 2)
  • forced air ventilation (fan - 2)
  • forced fluid ventilation (water cooled pipes - 1, 2)
  • any combination of the above
 

Thread Starter

Gow

Joined Sep 9, 2017
4
Wonderfully explained. This will help me in an organised study. Thank you. I guess, nsaspook covered the 'conduction' part. Will get back if any doubts.
 
Top