I am bit confused as to determine the wattage for a resistor. They come in
1/4, 1/2, 1, 2 watt etc.. I want to to a 80ohm resistor between a 300v dc supply and lets say ground. I'll have 3.74amp and 1125 watts. How do I choose the wattage of the resistor ? The difference in the resistor is almost none, so a 4 watt resistor ?
What do you mean by "the difference in the resistor"?
If you put 300V across an 80Ohm resistor, it'll dissipate 1125 Watts. So with a safety margin you'll need a 2kW resistor. I imagine that that would be one expensive beast!
To boost a 4 Watt to 10 Watts I simply Setup up a Plaster of Paris or Clay Box around the Resistor, and let it setup or bake it in the oven.
For larger resistors I use Ni-Chrom wire, cut it to length needed for the correct resistance and set it in Plaster of Paris. Much Cheaper then buying New.
According to the CRC handbook of chemistry and physics the k value for air is 0.0262. For Clay it is ~0.25 or 9 to 10 times higher. With Plaster of Paris k value around 0.42 or 16 times higher.
Air is very efficient thermal insulator and in general most solids are much more efficient thermal conductors. Air flow can improve its conductivity, but that requires constant power.
The material is not preventing the heat from escaping, it is providing a larger surface area for its conduction into the surrounding body (air). Because the material is more efficient at conducting the heat away from the resistor then air, and the substantial increase to the resistors overall surface area, more heat can and will be dissipated. Period.
As stated above the thermal conductivity of Plaster of Paris k value around
0.42. The heat generated by the wire is absorbed into the Plaster of Paris and dissipated by air from the structure. Plaster of Paris will decompose if to much energy is being attempted to be dissipated over to small of an area. Other conductors can also be used such as SS wire. Thus my suggestion "Of course this is not recommended for someone without knowledge of electronics as confusion often leads to pain." Perhaps this was not sufficient. I should have stated "Of course this is not recommended for someone without knowledge of Thermal Dynamics as confusion often leads to pain." as it is apparent you should have a capable grasp of electronics by now and should be familiar with the High Wattage Ceramic Resistor by now.
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