Mike Silva wrote:
Duncan Munro wrote in message
. ..

The metal film 33R measures 6.5uH and the oxide 22R measures 4.5uH on the
aade.com meter. Both values (if the readings are correct) would represent
a high ratio of X to R at HF frequencies...

I wonder, though, if the AADE meter is not getting confused by the
resistance of the resistor. The fact that the measured inductance is
just about proportional to the resistance might be evidence for that.

Duncan has kindly sent a couple of samples, with duplicates that have
had the paint scraped off. I just arrived home from a few days away, so
haven't had time to measure them yet.

Each one is only an open spiral of about two turns along the whole
length of the 3W resistor body, so you can see immediately that there's
no way the inductance can actually be more than a few hundred nH.

This actual, physical inductance is in *series* with the resistance.
What seems to be happening is that the AADE meter displays the
resistance and reactance in their equivalent parallel form, which is a
function of the measurement frequency (which varies, but is understood
to be in the order of a few MHz).

Guessing a frequency and then doing the parallel - series
transformation on 22 ohms in parallel with 4.5uH produces results in the
right ballpark: R is still around 22 ohms but the *series* inductance is
100-200nH.

I will try to measure the resistors tomorrow.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

On Tue, 30 Nov 2004 21:04:28 +0000, Ian White, G3SEK wrote:

Guessing a frequency and then doing the parallel - series
transformation on 22 ohms in parallel with 4.5uH produces results in the
right ballpark: R is still around 22 ohms but the *series* inductance is
100-200nH.

There is an additional complication in that there is another inductor in
the box itself of 680uH, LX (or should I say RX) is in series with that.
It's late now, but I will try and work out what's going on tomorrow night.

I will try to measure the resistors tomorrow.

If you get the opportunity, it would be much appreciated.

--
Duncan Munro
http://www.duncanamps.com/

On 30 Nov 2004 12:37:57 -0800, Mike Silva wrote:

Duncan Munro wrote in message .. .

The metal film 33R measures 6.5uH and the oxide 22R measures 4.5uH on the
aade.com meter. Both values (if the readings are correct) would represent
a high ratio of X to R at HF frequencies...

I wonder, though, if the AADE meter is not getting confused by the
resistance of the resistor. The fact that the measured inductance is
just about proportional to the resistance might be evidence for that.

Mike, I think you've hit the nail on the head. To be fair to AADE, they
warn that the 'Q' of the inductor has to be reasonable to get a sensible
measurement.

The kind of measurement frequencies we are talking about are in the order
of 700kHz. At that frequency, the inductance of the 'indicated' 4.5uH is
19.8 ohms, not a million miles from the 22 ohms of the resistor itself -
this is not what I would call a reasonable 'Q' value. Fair play to AADE,
it's designed to measure the inductance of inductors, not other components
;-)

--
Duncan Munro
http://www.duncanamps.com/

Duncan Munro wrote:

The kind of measurement frequencies we are talking about are in the order
of 700kHz. At that frequency, the inductance of the 'indicated' 4.5uH is
19.8 ohms, not a million miles from the 22 ohms of the resistor itself -
this is not what I would call a reasonable 'Q' value. Fair play to AADE,
it's designed to measure the inductance of inductors, not other components
;-)

If the reactance is much lower than the resistance, it's generally
inconsequential in a practical application. I think that's almost always
the case for carbon film resistors, and I suspect it's nearly always the
case for metal film resistors.

Probably, if the Q is so low as to make measurement difficult, it's
probably low enough that the X isn't important in a practical application.

Roy Lewallen, W7EL

I've used "Flame Proof" resisters to build signal splitters, the
outputs were within .1 db of a commercial unit (made by Suhner) @ 850
Mhz.

That ain't bad .

Howard.

On Wed, 24 Nov 2004 18:59:16 +0000 (UTC), "James Bond"
wrote:

are metal film resistors wirewound or not? I've been trying to find this one
out. Someone who I know says they're not so are suitable for RF but Maplin
catalog seems to say they are.


someone please help!

dr. x


---
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On Wed, 24 Nov 2004 18:59:16 +0000 (UTC) "James Bond"
wrote:

are metal film resistors wirewound or not? I've been trying to find this one
out. Someone who I know says they're not so are suitable for RF but Maplin
catalog seems to say they are.

They are not WIREwound, but the resistive element IS usually a helical
film that is a layer on a ceramic body.

As someone else mentioned, there are some types which are serpentine
rather than helical, but these are always special and sold as such.

Film resistors generally have fewer turns in their helix than
wirewounds, so they will have more inductance than carbon or ceramic
bulk resistors but less than wire wound resistors.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

"James Bond" wrote in message
...
are metal film resistors wirewound or not? I've been trying to find this one
out. Someone who I know says they're not so are suitable for RF but Maplin
catalog seems to say they are.

Metal film or metal oxide? There is a big difference. High ohmic values are
almost always Oxide types and these are usually the continuous layer type with
the occasional 'trim'. Really low value metal film are also a continuous tube
or nearly so. It is the upper half of the metal film rage (100R-100k) that is
spiral cut & consequently of appreciable inductance. Unfortunately, these are
the values that are most needed.

matt wilson wrote:

"James Bond" wrote in message
...
are metal film resistors wirewound or not? I've been trying to find this one
out. Someone who I know says they're not so are suitable for RF but Maplin
catalog seems to say they are.

Metal film or metal oxide? There is a big difference. High ohmic values are
almost always Oxide types and these are usually the continuous layer type with
the occasional 'trim'. Really low value metal film are also a continuous tube
or nearly so. It is the upper half of the metal film rage (100R-100k) that is
spiral cut & consequently of appreciable inductance. Unfortunately, these are
the values that are most needed.

I think metal film resistors are made by depositing metal vapor onto
ceramic rods in a vacuum. Then they are attached to metal end caps
with leads and either laser or abrasive engraved to set the final
resistance. Some have only a turn or two, some have more. They are a
lot less inductive than wire wound devices, and the best low
inductance versions have a serpentine pattern engraved in the film, to
keep the inductance very low.
e.g.
http://www.caddock.com/Online_catalog/power/power.html
--
John Popelish

On Sat, 27 Nov 2004 19:31:37 -0500, John Popelish
wrote:
d the best low
inductance versions have a serpentine pattern engraved in the film, to
keep the inductance very low.

Yes, minimal inductance, but doesn't that pattern give rise to more
parasitic capacitance?
--

"What is now proved was once only imagin'd." - William Blake, 1793.

Paul Burridge wrote:

On Sat, 27 Nov 2004 19:31:37 -0500, John Popelish
wrote:
d the best low
inductance versions have a serpentine pattern engraved in the film, to
keep the inductance very low.

Yes, minimal inductance, but doesn't that pattern give rise to more
parasitic capacitance?

Some. More than an inductive pattern? Not much, if any.

--
John Popelish