[sdiy] VCS3/resistor heat

[Ingo Debus]

Am Donnerstag, 30.06.05 um 09:33 Uhr schrieb Magnus Danielson:

> The power-supply of Mk I VCS3 and Synthi-A runs hot, since it uses 
> shunting
> regulation. In that powersupply the resistors run hot. My best 
> recommendation
> is to mount either larger resistors (more mass and heat-radiating 
> surface) and
> make sure it has alot air-contact for convection.
>
While we're on the subject, does anyone know why the power 
derating/heat sink requirements for power resistors are spec'd in such 
a funny way?

I mean, with semiconductors usually the maximum die temperature is 
given, and the thermal conductivity from die to case. If the power 
dissipation and the ambient temperature is known, it's easy to 
calculate what kind of heatsink is required.

But recently I saw some datasheets of wirewound power resistors, and 
the specs were quite different: there's a "standard heat sink" assumed 
(with given size and material) and the thermal conductivity 
("temperature rise" or so called in one data sheet) for the 
resistor/heatsink combo. Also the power derating is given for this 
special heatsink.
This is convenient when this very heatsink is used, but what to do if I 
want to know which (different) heatsink I would need in a different 
situation?? Am I missing something obvious or do I interpret the data 
sheets uncorrectly?
I'd think the mechanism in a power resistor is the same as in a power 
semiconductor: the die (or wire for the resistor) can stand only so 
much temperature, there's a thermal conductivity between die/wire and 
case and therefore for a given power dissipation of the die/wire 
there's a maximum allowed case temperature.

Ingo