[sdiy] VCS3/resistor heat

[Magnus Danielson]

From: Ingo Debus <debus at cityweb.de>
Subject: Re: [sdiy] VCS3/resistor heat
Date: Sat, 2 Jul 2005 12:21:12 +0200
Message-ID: <039DEE8C-EAE3-11D9-8B8C-000A9571C136 at cityweb.de>

Ingo,

> 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.

Hmm... I know the deal about the transistors and chips, but doesn't have a
clue about what funnieness in power-resistor datasheets you are talking about.
Would you care to give a few examples of funny datasheets, now you got me
interested!

Cheers,
Magnus - digging for oil(tank) in the back yard