# Resistors Wattage

#### Cerkit

Joined Jan 4, 2009
287
What is physically different between two resistors with the same resistance but different wattage rating??
And if it is thickness then how do you still get the same resistance rating??
Don't understand what the different factors are that allow the wattage rating to be independantly altered with regards to the resistance.

Thank you

#### joeyd999

Joined Jun 6, 2011
4,433
It has to do with how the resistor, or any power dissipating component, dissipates heat.

A higher wattage part is designed to either dissipate more heat into the environment, or to operate normally at a higher temperature, or both.

Generally, the higher the power handling capability, the larger the surface area.

#### thatoneguy

Joined Feb 19, 2009
6,359
You determine the value by the current going through it, adding a little bit of wiggle room.

$$W=I^2R$$

#### crutschow

Joined Mar 14, 2008
25,640
Generally the higher the wattage, the physically bigger the resistor substrate. How the correct resistance value is generated depends upon the type of resistor. If it's a film type resistor, the resistance material goes in a narrow spiral path on the substrate from one end of the resistor to the other. The resistance value is determined by the resistance of the material, the thickness of the material, and the length and width of the path.

#### thatoneguy

Joined Feb 19, 2009
6,359
Higher wattage resistors will also have an aluminum anodized casing with integral heatsink fins.

Some high wattage, low value resistors are formed by wrapping resistive wire around a core. These wirewound types have the drawback of adding parasitic inductance and capacitance to the resistance.

#### mcgyvr

Joined Oct 15, 2009
5,394
In general a resistors wattage should be 2 times required. So if your calculations show your resistor should be 1W you would pick a 2W resistor.

Joined Dec 26, 2010
2,148
For a given choice of materials, wattage is mainly a matter of surface area. The resistor needs to have sufficient area so that the heat produced can be lost to the surroundings without the temperature rising too much.

The resistance is given by the material resistivity, multiplied by the resistor element length, and divided by its cross-sectional area. Thus the thickness of a resistive surface coating such as a metal film can be chosen to set the resistance, without changing the power rating.

Other methods of changing the resistance include cutting spirals in cylindrical film resistors, and L-cuts for rectangular ones. These may slightly reduce the power rating, but mainly affect the resistance value.

#### bountyhunter

Joined Sep 7, 2009
2,512
You can't always guess wattage by overall size. A 3W metal oxide resistor would be about 5 times the size of a 3W wirewound resistor. It all depends on how the resistor dissipates heat and how hot it can safely operate.