[sdiy] VCS3/resistor heat

[Roy J. Tellason]

On Sunday 03 July 2005 09:49 am, Ingo Debus wrote:
> 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.

Got it,  but had to fiddle with the orientation to get it to display right 
here. 

> 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 wire.  The spec given for the WH50 for example is 3.5 degress C / watt, 
which is usually how this stuff is expressed.  They also seem to be saying 
that this is the case when it's mounted on "standard heatsink",  that being 
one that has the dimensions given on the third page,  having an area of 544 
sq. cm.

> The resistive element is potted with some epoxy stuff inside the aluminimum
> case, so the wire sure will get hotter than the case.

Yes,  but one would assume that the potting compound is designed for good heat 
transfer.

> 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.

The graph is showing you how the allowable power dissipation goes down when 
smaller heatsinks are used.  I haven't looked at any sites for heatsinks 
lately,  but it wouldn't surprise me to find them specified both ways.

> > 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?

Not this one (it's a 2SD1258,  I think,  though I'd have to pull them out to 
be sure.  I've salvaged some FETs that are of the type you're talking about, 
though.  Slight variations on the package,  some on that board were mounted 
to heatsinks and some were soldered to the board directly.