Equation for converting thermal conductivity to thermal resistance?

[IanB]

so basically this:

Rth​=thickness / (thermal_conductivity * are_contact​ )

That seems correct, yes. But make sure you keep all your units of measure consistent.

[TimFox]

In Imperial units, thermal conductivity is measured  in  1 Btu/(h⋅ft⋅°F) = 1.730735 W/(m⋅K), where "h" = hours.
For engineering purposes, I have seen this converted to use thickness in inches and area in ft², also time in seconds.
(I usually distinguish between Imperial and US Customary units, but British Thermal Units sound Imperial to me.)

[IanB]

In Imperial units, thermal conductivity is measured  in  1 Btu/(h⋅ft⋅°F) = 1.730735 W/(m⋅K), where "h" = hours.
For engineering purposes, I have seen this converted to use thickness in inches and area in ft², also time in seconds.
(I usually distinguish between Imperial and US Customary units, but British Thermal Units sound Imperial to me.)

Yup. This is why it is not fun to study engineering in the USA. There are so many conversion factors in the standard equations. You invariably need a standard table of formulas to avoid having to remember all the magic numbers that crop up everywhere.

As for the Btu, US Customary units were derived from Imperial units, and the Btu was inherited from the Imperial system. Nobody has quite managed to agree on a universal definition of its value, but 1.055 kJ is close enough for government work.

[TimFox]

I believe the original definition of a BTU is the heat required to raise the temperature of 1 lb av of water by 1 F^o, analogous to the original definitions of the metric calorie or kilocalorie.

[IanB]

I believe the original definition of a BTU is the heat required to raise the temperature of 1 lb av of water by 1 F^o, analogous to the original definitions of the metric calorie or kilocalorie.

Yes, but there are different ways to do this, resulting in various different values. See Wikipedia for details:

https://en.wikipedia.org/wiki/British_thermal_unit

[TimFox]

"original definition"

[Jeroen3]

In Imperial units, thermal conductivity is measured  in  1 Btu/(h⋅ft⋅°F) = 1.730735 W/(m⋅K), where "h" = hours.
...

Today in cursed units!

British Thermal Unit per hour feet squared Fahrenheit

[Sniper1]

I need to dissipate 5-7W max it is a APFC mosfet .
I NEED to isolate the 400Vdc it has from the heat sink since this is a real safety problem ( i am afraid of making a mess with mica+paste but that is option 2 since i have a mica kit already .
I will check that product on mouser and depending on stock and price and what it is i may try it.

7W from a TO220 are easy going if you just have a heat sink that´s large enough. For this dissipation you can also use a fully isolated package FET.

I had a had time picking the proper fet , i went through 15-20 in order to get 4 usable ones. So finding a isolated one with proper characteristics is probably not wort the time since those sil pads are not that expensive.

[SteveThackery]

Units drive me crazy! I usually use a dot for products. You also have to be careful with Newton-meters for work (1 Joule) vs torque, which is completely different. And then I'm never sure what to capitalize. Fortunately there's this- https://physics.nist.gov/cuu/Units/checklist.html

I think it's a brilliant system, carefully designed to be clear and unambiguous, with simple numeric relationships between them.

Yes, a force of 1 N applied over a distance of 1 m converts 1 Joule of energy, but you never refer to that work as Newton-metres.

[Phil1977]

I had a had time picking the proper fet , i went through 15-20 in order to get 4 usable ones. So finding a isolated one with proper characteristics is probably not wort the time since those sil pads are not that expensive.

The isolated FET´s for sure are more expensive than standard packages + glimmer sheets. It´s just for the convenience that it´s easier to assemble and potentially safer to handle.

[TimFox]

Units drive me crazy! I usually use a dot for products. You also have to be careful with Newton-meters for work (1 Joule) vs torque, which is completely different. And then I'm never sure what to capitalize. Fortunately there's this- https://physics.nist.gov/cuu/Units/checklist.html

I think it's a brilliant system, carefully designed to be clear and unambiguous, with simple numeric relationships between them.

Yes, a force of 1 N applied over a distance of 1 m converts 1 Joule of energy, but you never refer to that work as Newton-metres.

The N-m in torque refers to a force that is perpendicular to the radius.
For N-m in energy (Joules), the "scalar product" of the force times distance traveled gives you the work done:  maximum for force and distance vectors collinear.
The normal unit for energy or work in English units is lb-ft.

[TimFox]

A side comment about expressing equations and units:
In careful usage, a letter in an equation for a variable or parameter should be italicized.
The same letter, used as a unit, should be in plain Roman.
This is covered in books such as the mathematical typesetting chapter of University of Chicago Press Chicago Manual of Style  https://www.chicagomanualofstyle.org/home.html
(Some modern publishers, such as Springer, ignore this and potentially cause confusion.)
For example V in an equation is a voltage or potential, while V represents the unit "Volts".