K7ITM wrote:

Adding to Ian's good words, the larger resistors (1W, 2W and sometimes
more) are commonly metal-oxide. They also work well, and have the same
few-turn spiral structure that the metal film ones do.

I should have made it clearer that metal-oxide resistors exist, and are
generally similar to metal-film in their construction and low
inductance.

However, we can only say "generally". If the very lowest inductance is
important, you need to remove the coating and check for yourself.

Several years ago I measured a little dummy load I made from four 200
ohm 2W metal-oxide parts in parallel. The construction was "tight" so
that the leads didn't contribute more inductance than the resistors
themselves. I don't have the numbers with me at the moment, but I
recall a return loss measurement equivalent to about 1.1 or 1.15:1 at
150MHz, and around 1.5:1 at 450MHz.

The metal-oxide resistors have another interesting characteristic. They
can dissipate enough power to glow red and do a very respectable job
maintaining their resistance value--though expect some change if you
let them get that hot.
[Moved]
It's very unlikely that a carbon composition resistor, or a carbon
film, or even a normal metal film, will be able to hold its value as
well if it gets that hot.


Metal film resistors do seem to behave well in similar tests. Like Tom
(and Tim in another message) I have deliberately overloaded samples by a
factor of 10 to see what would happen. From glowing bright red, the
resistance returned to a few percent of the original value.

In parallel tests, modern carbon film resistors simply burned up - which
is exactly what you would expect carbon to do. I didn't waste time on
carbon comp, as any OT already knows they cannot tolerate even moderate
overloads without oozing organic binder materials, accompanied by smoke,
an awful smell and a large, permanent increase in resistance.

The other low-inductance power resistors that haven't been mentioned are
the flat-film types, designed to be bolted down to a heatsink. There's a
nice constructional example of a large RF power attenuator using these
devices at:
http://granta.digital-crocus.com/Attenuator.php3

What surprises me is that carbon comp resistors are still available as
specialist items because of their claimed performance in pulsed
applications. With so many alternatives available, that have almost
equally low inductance and vastly superior power handling, it would be
interesting to learn what the specific advantages of carbon comp might
be?


That's a double-edged sword. On the one hand, it's nice to know they
will be pretty stable, but if you mount one on a circuit board, you
need to make sure that it won't dissipate too much power, because it's
quite capable of burning a hole in the board.

These modern power resistors are also much smaller than traditional
types... but this too can be double-edged. I sell a kit that includes a
number of 1W, 2W and 3W metal film resistors, and occasionally receive
e-mails saying: "There are several small resistors left over - and where
are all the power resistors?"


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek