Thermal resistance

The ( absolute ) thermal resistance (also thermal resistance, thermal resistance ) is a heat characteristic value and a measure of the temperature difference ( heat per unit of time or heat output) is produced in an object when passing a heating current. The reciprocal of the thermal resistance is the thermal conductivity of the component. Fiddling with resistance instead of conductance is practical in situations in which resistances occur in series as heat transfer to a heat sink / heat conduction in the heat sink / heat transfer to the air.

Definition

With

• - Temperature difference ( eg between outside and inside of a thermos or between a cooling surface and the ambient air )
• - Heat flow (for example, power dissipation of a heat sink mounted on a component or through a window continuous lost heat)

A homogeneous body with the length of constant cross-sectional area can be calculated on the thermal conductivity of the material:

The unit of thermal resistance, K / W.

Analogy to Ohm's law

Thermal units have analogies to those of the electrical resistance, which are also reflected in their names.

Occur on analogies to electricity, enabling the application of Ohm's law and Kirchhoff's rules in heat transmission. These are:

Application Examples

Building Physics

If in a Styrofoam plate with a thermal resistance of 1 K / W, there is a temperature difference of 20 K between the two sides, then there is a heat flux through the plate by:

Seasonal heat storage

A heat storage at a constant ambient temperature discharges through its own insulation. The course of the temperature difference to the environment over time is

Analogous to the voltage waveform at a capacitor which is discharged via a resistor:

The time constant with which heat storage capacitor is discharged,

Now as a numerical example, most of the results are rounded: heat storage medium is water with 45 % ethylene glycol, 7 m wide, 7 m long, 4 m high:

Is the specific heat capacity of the water - glycol mixture

The heat capacity is the product of output per unit volume specific heat capacity and volume

Thermal insulation is foam glass gravel with a layer thickness. As the surface of the thermal insulation, the surface of the water tank is used:

The thermal conductivity of foam glass gravel is

This results in a thermal resistance

Now can be the time constant of the self-discharge calculate:

After 238 days, the difference between the temperature in the water and in the environment ie to 37% () of the initial value has declined.

Electronics

The design of the cooling of semiconductors or other circuit elements in electronic circuits, the thermal resistance of a concrete heat sink is the definitive characteristic for its selection. It is given by the heatsink manufacturer, eg for free convection.

The thermal resistance of a device without a heat sink to ambient can be used to control whether a heatsink assembly is required at all - he is from the component manufacturer with RthJ / A specified (of English Junction / Ambient. ).

In the semiconductor device itself is a thermal resistance between chip and package - cooling surface occurs. He is from the manufacturer with RthJ / C specified (of English. Junction / Case).

The assembly itself and possibly thermal pad cause further thermal resistances. Is the heat sink within a housing or a rack, it is noted that it transfers the heat to air, the temperature may be well above the temperature of the surroundings.

Of the power loss and the sum of all thermal resistance, the temperature difference between the chip and the environment of the heat sink can be calculated:

If the value is too large, the thermal resistance of the heat sink is to reduce, for example, to be aired by a heatsink with fan or heat pipe, or the cabinet.