On Wed, 01 Dec 2004 19:57:48 -0800, Roy Lewallen wrote:

budgie wrote:

On Wed, 01 Dec 2004 16:35:36 -0800, Roy Lewallen wrote:


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.


(adds) ..... at the test frequency

Sure. Any statement about a frequency-dependent property like X or Q
applies only at the frequency at which the component has that particular
X or Q.

I only added that because the test frequency was cited as ~700kHz and a casual
reader may have taken the above to mean that the X was insignificant at the
frequency of intended operation.

budgie wrote:

I only added that because the test frequency was cited as ~700kHz and a casual
reader may have taken the above to mean that the X was insignificant at the
frequency of intended operation.

Ah, thanks, I'd missed that. At 700 kHz, you'd never get enough
reactance from a metal film resistor to be bothersome, and I doubt that
you'd even be able to measure it.

Roy Lewallen, W7EL

Duncan Munro wrote:
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.


Over a range from 50kHz to 50MHz, Duncan's two resistors measure about
22 ohms + 31nH, and 33 ohms + 23nH.

To avoid having to construct a special test jig, I measured each
resistor with about 30mm of bent wire leads, which would account for
about 20nH of those measured inductance values.

The very low inductance of the resistor body is completely consistent
with the physical construction. On closer inspection, the metal film is
an almost continuous tube, with a very narrow spiral gap of about 1.5
turns. The gap adjusts the resistance by slightly increasing the overall
electrical path length, but it adds very little inductance.

If you used very short leads instead of the longer lengths I had to use,
these resistors would have a low SWR up to at least 144MHz.

Thanks again to Duncan for supplying the resistors.


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

Ian, if it's not too much trouble, I'd be very interested in how the
measured inductance compares to that of the resistor body/leads only.
I'd think that could be done by coating a similar-size resistor with
conductive paint or foil and measuring with the same method. The
difference between this measurement and the one you made would then show
how much inductance is due to the spiral element, and would represent
the minimum possible inductance for that resistor body type and lead length.

Roy Lewallen, W7EL

Ian White, G3SEK wrote:


Over a range from 50kHz to 50MHz, Duncan's two resistors measure about
22 ohms + 31nH, and 33 ohms + 23nH.

To avoid having to construct a special test jig, I measured each
resistor with about 30mm of bent wire leads, which would account for
about 20nH of those measured inductance values.

The very low inductance of the resistor body is completely consistent
with the physical construction. On closer inspection, the metal film is
an almost continuous tube, with a very narrow spiral gap of about 1.5
turns. The gap adjusts the resistance by slightly increasing the overall
electrical path length, but it adds very little inductance.

If you used very short leads instead of the longer lengths I had to use,
these resistors would have a low SWR up to at least 144MHz.

Thanks again to Duncan for supplying the resistors.

On Thu, 2 Dec 2004 12:01:03 +0000, Ian White, G3SEK wrote:

Thanks again to Duncan for supplying the resistors.

No need, it's thanks to you for taking the time to measure them and post
the results!

At least I now know my linear is not going to fail because the carbon comps
have been replaced with the metal oxide and metal film jobs. Thanks again
Ian.

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

Roy Lewallen wrote:
Ian, if it's not too much trouble, I'd be very interested in how the
measured inductance compares to that of the resistor body/leads only.
I'd think that could be done by coating a similar-size resistor with
conductive paint or foil and measuring with the same method. The
difference between this measurement and the one you made would then
show how much inductance is due to the spiral element, and would
represent the minimum possible inductance for that resistor body type
and lead length.

I could certainly do that, because Duncan has supplied pairs of
resistors: one in original condition, and the other with the coating
cleaned off, just ready for painting.

However, the difference in inductance is going to be very small, and I'd
need to build a test jig that can keep other stray inductances under
control.

Over the weekend, maybe...


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

The idea was to measure the coated resistor under exactly the same
conditions as the regular one, so couldn't you just use exactly the same
setup as before?

Roy Lewallen, W7EL

Ian White, G3SEK wrote:
Roy Lewallen wrote:

Ian, if it's not too much trouble, I'd be very interested in how the
measured inductance compares to that of the resistor body/leads only.
I'd think that could be done by coating a similar-size resistor with
conductive paint or foil and measuring with the same method. The
difference between this measurement and the one you made would then
show how much inductance is due to the spiral element, and would
represent the minimum possible inductance for that resistor body type
and lead length.

I could certainly do that, because Duncan has supplied pairs of
resistors: one in original condition, and the other with the coating
cleaned off, just ready for painting.

However, the difference in inductance is going to be very small, and I'd
need to build a test jig that can keep other stray inductances under
control.

Over the weekend, maybe...

Roy Lewallen wrote:
The idea was to measure the coated resistor under exactly the same
conditions as the regular one, so couldn't you just use exactly the
same setup as before?

Afraid not... In the present setup, the main contribution to the total
inductance comes from the long, floppy resistor wires, and I couldn't
guarantee not to disturb their configuration while applying conductive
paint to the resistor.

To get a reliable answer, I'd need to reduce the lead length and make
the whole thing mechanically more stable.


Ian White, G3SEK wrote:
Roy Lewallen wrote:

Ian, if it's not too much trouble, I'd be very interested in how the
measured inductance compares to that of the resistor body/leads only.
I'd think that could be done by coating a similar-size resistor with
conductive paint or foil and measuring with the same method. The
difference between this measurement and the one you made would then
show how much inductance is due to the spiral element, and would
represent the minimum possible inductance for that resistor body type


I could certainly do that, because Duncan has supplied pairs of
resistors: one in original condition, and the other with the coating
cleaned off, just ready for painting.
However, the difference in inductance is going to be very small, and
I'd need to build a test jig that can keep other stray inductances
under control.
Over the weekend, maybe...


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

I use them with no issues that I know of.

Perhaps someone else has the mathematical and physical explanation.

I measured a few with my LC meter in the L mode and was unable to measure
any L.


"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.


someone please help!

dr. x


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"Brian - KB9BVN" wrote in message
nk.net...
I use them with no issues that I know of.

Perhaps someone else has the mathematical and physical explanation.

I measured a few with my LC meter in the L mode and was unable to measure
any L.

"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.

someone please help!

dr. x

Just a simple explanation. Not a comprehensive review. I do not know what
meter, nor the circuit.

Your depending on the circuit in your meter.
Some Inductance measurement circuits need the resistive component to be
canceled out (balanced by a bridge) to read the inductance. It doesn't mean it
has no inductance, only that he resistance values is much higher than the
reactance of the inductor. Therefore the meter sees the resistance only.

To make it even worse, not all MF resistors are inductive.