[sdiy] VCS3/resistor heat

[Ingo Debus]

Am Samstag, 02.07.05 um 15:02 Uhr schrieb Magnus Danielson:

> 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!
>

Magnus and Roy, thanks for your comments. To view a datasheet, go here:
www.welwyn-tt.com
and select WH in the "Select a Resistor Family.." field. I've seen 
another similar data sheet from a different manufacturer (Meggitt) but 
I can't find it on the net right now.

Roy wrote:
> Do they specify either some mfr's part number or some specific thermal
> resistance from the heat sink to the surrounding air?

As far as I can see they don't, and that's puzzling me.
On the first page they spec "Thermal impedance" with the remark 
"Mounted on a standard heatsink". I think this is the heatsink 
mentioned on the third page (30.5cm * 30.5cm). I also think this is the 
total thermal impedance for the resistor/heatsink combo, in other 
words, the temperature rise of the thing when 1W of power is 
dissipated. In the other datasheet I mentioned this parameter is 
actually called "temperature rise". But is this the rise of the 
resistor case or of the wire inside the resistor? The resistive element 
is potted with some epoxy stuff inside the aluminimum case, so the wire 
sure will get hotter than the case. And I still don't know how hot the 
wire is allowed to get. Confusing.

Ok, with the graph on the third page it's possible to choose a heatsink 
by surface area. But usually heatsinks are spec'd by thermal resistance.

> I have some parts salvaged here that are obviously
> meant to dissipate a little power,  they sort of resemble a TO220 
> package in
> that there's a metal tab in there, but it doesn't stick out very far 
> and
> doesn't have a hole in it for mounting to a heatsink

Perhaps meant to be soldered to a large copper area on a PCB?

Ingo