Determining wattage rating of random resistors

[zjokka]

I need this 1W resitors for the Shaka Tube. I have an old box full of old resistors very big in comparison to my 1/4Ws.
Is there a way to determine their wattage rating? Or is the size the indication?

thanks
zj

[rockgardenlove]

If you have spares you test one by blowing it up.
You can also just use 4 1/4 resistors, that's what I'd do.

[John Lyons]

The size of the resistor will tell you most of the time. A half watt will be twice the size (around) of a 1/4 watt and a one watt will be twice the size of a half etc etc. on up the wattages. A one watt is about the size around of a tic tac mint...

John

[Paul Marossy]

A one watt is about the size around of a tic tac mint...

That's more like a 1/2 watt resistor. A 1 watt resistor is more like a small elongated jelly bean. 

[R.G.]

And now for the dry, dreary techie explanation.

Resistors remove heat from themselves by three means, convection (i.e. air flowing over the resistor) radiation (infrared radiation emitting away into the universe) and conduction. The conduction is limited to the leads and so the heat removed that way is pretty constant. That leaves convection and radiation to vary.

The thing about both convection and radiation is that they are properties of the surface temperature and area. The hotter the surface gets, the more effective both convection and radiation get. The more surface area that there is, the more heat can convect away and the more radiation that comes out of the surface at a given temperature.

If you put a fixed amount of heat dissipation into a resistor, the surface temperature increases until heat out matches power in, and at that point, the surface temperature quits increasing. The temperature of the innards of the resistor, which is what determines what temperature it will burn up at, is only slightly hotter than the surface, so it's all about the surface temperature, and that's all about the amount of surface.

You can make a high power resistor two ways: little surface area and hot, or big surface area and cool(er).

With the old technology of carbon composition resistors, the makers of resistors were hammered into submission into making a standard set of sizes early by government contracts with the military. So they calculated a few size of resistor shells that could dissipate 1/4W, 1/2W, 1W, 2W, etc., with the then-standard phenolic resistor case and keep to a temperature that would not burn up the case or the innards, and supplied these packed to the appropriate resistor density with carbon gook. With a constant-sized case, the power dissipated in a given sized and composition resistor is constant. They picked 200C as the failing temperature for the materials. The surface temperature of a standard CC resistor is then 200C at full dissipation.  The physical size of the old resistors was kept as a goal by modern technology which makes resistors from carbon or metal films on a ceramic body. Again, they have about the same surface, so they get to about 200C at full dissipation.

Some modern resistors do not actually burn out at 200C, and can go higher. So some makers will provide resistors that can dissipate, for example, 1/2W in a 1/4W sized body. There is no magic there - at full power the surface temp is considerably higher than 200C, but the resistor materials can take it. This process is limited because at some point the leads get to the melting point of solder, and you can't keep "overrating" the resistors any more by letting the temps go higher.

It's good design practice to never design for a resistor dissipation of over half the rated power in normal circuit operation. That makes the surface of the resistor be 100C and it's therefore a burn hazard to humans if you design that close.

I'm sure the temps are not exactly as I remember, but they should be close.

It's the surface area!!

[birt]

and if you're not sure about one in a spot in the circuit you can parallel 2 of twice the value so you double the wattage

[zjokka]

thanks guys, so very thorough treatment, I learned both a practical solution and the theoretical background (RG, you're incredible).
for me, this could really go in the faq, so submitted it, hope you guys don't mind
thanks again