How do you determin what wattage a resistor is?
David

:How do you determin what wattage a resistor is?
:David
Hi Dave
Good question, other than physical size I honestly don't know.
Mark

:How do you determin what wattage a resistor is?
:David

Use this handy guide from my site:

http://www.philcorepairbench.com/resistcompare.htm

Chuck

(R*R)/E then double the calculated value for safety margin. R is the resistor value in ohms and E is the voltage drop across the resistor. Or, I*E where I is the current through the resistor and E is the voltage drop across the resistor. Again double the calculated value for safety margin. Current can be calculated from E/R. If you Google Ohm's law or Watt's law you should find something. Resistance is never futile. Hope this helps

MRO

:How do you determin what wattage a resistor is?
:David
Hi All!
OOPS! its been so many years since I used Ohms Law I totally forgot about it! blind is me! LOL, yes IxE will give you the wattage, sorry about that.
Mark

Do a web search for 'ohm's' law. There is a site where you can insert 2 of 4 unknowns and get the two others. Neat place.

:How do you determin what wattage a resistor is?
:David
Hi David: I glance at this site on occasion and was puzzled a little by your question. Are you referring to an existing resistor installed in a circuit OR a resistor in your parts drawer OR a resistance value needed in a particular circuit application?

The power rating of a resistor is a measure of its ability to dissipate heat. This power rating is in watts AND at what ambient (surrounding) air temperature the resistor is operating within. If the surrounding air temperature is "hot" then the resistor cannot dissipate heat as fast, OR, if the air pressure is less (thin air), such as in an aircraft, the resistor cannot dissipate heat as rapidly and must be "derated". Energy moves from hot to cold and not vice-versa. You can increase the ability of any resistor to dissipate heat by mounting it to a larger structure such as a chassis. The old "candohm" resistors used this technique to dissipate heat very effectively. Power transistors, diodes, etc also use this method of "heat sinking".
The power being dissipated by an operating resistor is most easily calculated by squaring the voltage across the resistor and dividing this square by the resistance value.
Remember that the Watt is a unit of Power, most people use the word power incorrectly. Power refers to the RATE AT WHICH WORK IS PERFORMED. The electric company actually furnishes energy and you determine the power by the rate you consume it.
When a resistance conducts current (amperes) the internal atomic friction produces heat at the rate of about 3 BTU's per watt. A resistor dissipating 10 watts produces 30 BTU's of heat.
Since I know you are confused now I will tack on this little axiom concerning Watts (power) There are 746 watts in a horsepower - - -multiply this by 2 and realize that Columbus was a 2 horsepower guy!!

OH!! I get it 1492. one of Columbus sailing/docking dates.

marv

::How do you determin what wattage a resistor is?
::David
:Hi David: I glance at this site on occasion and was puzzled a little by your question. Are you referring to an existing resistor installed in a circuit OR a resistor in your parts drawer OR a resistance value needed in a particular circuit application?
:
:The power rating of a resistor is a measure of its ability to dissipate heat. This power rating is in watts AND at what ambient (surrounding) air temperature the resistor is operating within. If the surrounding air temperature is "hot" then the resistor cannot dissipate heat as fast, OR, if the air pressure is less (thin air), such as in an aircraft, the resistor cannot dissipate heat as rapidly and must be "derated". Energy moves from hot to cold and not vice-versa. You can increase the ability of any resistor to dissipate heat by mounting it to a larger structure such as a chassis. The old "candohm" resistors used this technique to dissipate heat very effectively. Power transistors, diodes, etc also use this method of "heat sinking".
: The power being dissipated by an operating resistor is most easily calculated by squaring the voltage across the resistor and dividing this square by the resistance value.
: Remember that the Watt is a unit of Power, most people use the word power incorrectly. Power refers to the RATE AT WHICH WORK IS PERFORMED. The electric company actually furnishes energy and you determine the power by the rate you consume it.
: When a resistance conducts current (amperes) the internal atomic friction produces heat at the rate of about 3 BTU's per watt. A resistor dissipating 10 watts produces 30 BTU's of heat.
: Since I know you are confused now I will tack on this little axiom concerning Watts (power) There are 746 watts in a horsepower - - -multiply this by 2 and realize that Columbus was a 2 horsepower guy!!

I was trying to find the wattage of some old resistors that I have laying around. I am restoring a few pre-war radios and they come in all diffrent shapeas and sizes. I can look at todays resistors and know what wattage they are by there size and shape.
David