On Mon, 17 Nov 2008 12:37:50 +1100, "Phil Allison"
wrote:


"Archimedes' Lever"
"Phil Allison"

** For SED readers.


Clockwise from top left,
1W carbon comp,
2W metal film,
0.5W metal glaze (cermet 150k),
0.5W carbon film,
centre = 0.5 watt metal film.


The two in the center look like 1/4W (USA)to me.


** Yes, the pair in the centre are 0.25 W metal film.

I thought I had a memory issue. :-]

But the even smaller ones in the lower right are 0.5 watt Metal Glaze.


However, I have seen Britished sourced parts where they have a series
that has 1W parts in the same package size as our 1/8W devices here.

** The Metal Glaze types were made in Australia under licence from IRC -
the others are all of Asian origin except for the 1W carbon comps.

The power ratings are for an ambient of 70 C and a max surface temp of 155C
( excepting the carbon comps again.)


Similar to the claim the 2W parts in your pic make. Doesn't matter
what they can continuously dissipate and stay on value with. It matters
more that it is not so damned hot as to ignite combustibles proximal to
it.


** To get the full 2W rating, the resistor has to be mounted proud of the
PCB - either by use of a kink in the leads or ceramic stand-offs.

Yeah... then there are those types that are already packaged into a
metal "heat sink" type can with mounting feet on it. They need a block of
metal to dissipate into.

The trick with metal film and cermet types is they have low enough tempcos
(ie +/-100ppm) to stay within a 1% tolerance when very hot.

Yep. That makes them real good for many applications where a specific
load is needed, and needs to remain the same load you thought it was when
you put it on.

Granted, a good designer never pushes such a part at its rated limit.

50% is a good idea for power resistors.


** Unless you can fully submerge them in water


Of course. That is certainly going to be worlds better than air
dissipation. Water is actually VERY good at removing heat.

- then a 1000% up-rating becomes possible.

Yes... 3 orders of magnitude sounds about right. Works on computers
too, but not quite as good.

I like fluorinert (GE FC-40). One can immerse HV power supplies into
it, and have no fear of arcing. Something like 1200 Volt per mil
strength.

One of these days, I am going to make a deep freeze OC computer that
you'll see photos of in the magazines once I get it done. It will have
close to $600 worth of fluorinert running through it. It will be all
feng shui looking like a waterfall in some places. It is a shame that
goldfish cannot handle sub-zero temperatures.

Hey... I know! I'll get some perfluorocarbon fluid, heavily oxygenate
it, and keep a hamster in it for the photo shoot! On a running wheel!
Tied to a DC PM motor, tied to some LEDs!

I'll let it out for most of its life, feeding and breathing normally.
But I will have the world's best lil' "underwater" electric generator
hamster!