How to measure the thermal impedance of a heatsink (3)

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http://queribus.free.fr		 Recording data
You need to record:
 temperatures Th and Ta
voltage V across diodes
current I in diodes

A computer measures at programmed time interval through an analog-digital interface and stores data in a file. It is better to measure more frequently at the beginning. You can measure less often when the systems stabilizes and that temperature varies more slowly.


Curve
Zthha(t) curve is defined for a given type of cooling for a given layout of components. It is necessary to write down the conditions of the test to know its validity.

 
Conclusion of tests
All conditions described previously have an influence on the measure of thermal impedance within a margin of + or - 10 to 20%. Resistances announced by manufacturers do not specify test conditions and you cannot compare precisely several heatsinks when test and utilization conditions are not known. It is safer to test each heatsink in the conditions it is going to be used.
Thermal impedance calculation

Zthjc et Zthch do not play any role in the measurement of Zthha. You have to know them however to estimate junction temperature to choose the current that will flow in diodes without destroying them.


Example of thermal impedance curve Zthha (kelvin/watt) as a function of time (second)
All international units take a lower-case letter (kelvin and watt). Their abbreviation takes an upper-case letter (K and W) when they are named after a person (Lord Kelvin and James Watt). For example gram is abbreviated g because it is not named after Mr Gram. The international unit of mass, kilogram is abbreviated kg and not Kg nor KG. The only exception is litre abbreviated L although it is not named after a Mr Litre. This rule does not apply to imperial units.


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