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 can be suitable for some RF applications. If you are thinking of
the wire-ended ones up to 2-3W, they are a flat spiral of just a few
turns of the metal film, on a ceramic core.

Plugging the number of turns, diameter and length into the usual formula
suggests inductances of less than 0.1uH, even for the larger ones. This
means they have inductive reactances rising up to about 10 ohms at
30MHz.

For example, if you wanted to use say a 47 ohm resistor as a dummy load,
the SWR would be quite reasonable up to 30MHz.

However, the inductance of the higher-value resistors goes up much less
rapidly then the resistance does (they use a thinner film of a more
resistive material, but not a lot more turns). The inductance of a 470R
metal film resistor is not a lot higher than that of a 47R. So if you
were to use ten 470R resistors in parallel to make your dummy load, the
inductive reactance would be divided by 10 and the SWR would be much
better.

I have used multiple-paralleled 2W metal film resistors as high as
50MHz, and up to144MHz in situations where the inductance could be tuned
out.

The ones I tested can also be overloaded to red heat for several seconds
without significantly changing resistance when cold - unlike the cheaper
carbon film resistors which burn (as carbon does), or wire-wound
resistors which generally develop a hot-spot and burn out.



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

On Wed, 24 Nov 2004 20:57:59 +0000, Ian White, G3SEK wrote:

Plugging the number of turns, diameter and length into the usual formula
suggests inductances of less than 0.1uH, even for the larger ones. This
means they have inductive reactances rising up to about 10 ohms at
30MHz.

Ian, I've just measured a couple of resistors bought as spares for the
FL-2100Z, 33 ohm 3W metal film used as a grid stopper, and 22 ohm 3W metal
oxide used on the anode, again as a suppressor, with a coil wound round it
(both of the originals were carbon).

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 have quite a surplus of them, so if you wish I can send one of each in
the mail for you to check on your VNA.

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

Duncan Munro wrote:
On Wed, 24 Nov 2004 20:57:59 +0000, Ian White, G3SEK wrote:

Plugging the number of turns, diameter and length into the usual formula
suggests inductances of less than 0.1uH, even for the larger ones. This
means they have inductive reactances rising up to about 10 ohms at
30MHz.

Ian, I've just measured a couple of resistors bought as spares for the
FL-2100Z, 33 ohm 3W metal film used as a grid stopper, and 22 ohm 3W metal
oxide used on the anode, again as a suppressor, with a coil wound round it
(both of the originals were carbon).

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 have quite a surplus of them, so if you wish I can send one of each in
the mail for you to check on your VNA.

Those values seem surprisingly high, so yes, please do and I will
measure them.

Based on the standard inductance formula (and there seems no reason why
that shouldn't apply in this case) it would seem to require very large
numbers of turns to achieve 1uH.

My posting was based on the ones I scraped the paint off in pre-VNA
days, so it would be useful to collect more data on typical 'coil'
dimensions and numbers of turns, together with the actual effective
inductances.


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

On Thu, 25 Nov 2004 07:32:27 +0000, "Ian White, G3SEK"
wrote:

Those values seem surprisingly high, so yes, please do and I will
measure them.

Please post the results here when you've done so, Ian. I for one would
also be interested.


--

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

On Thu, 25 Nov 2004 07:32:27 +0000, Ian White, G3SEK wrote:

Duncan Munro wrote:
I have quite a surplus of them, so if you wish I can send one of each in
the mail for you to check on your VNA.

Those values seem surprisingly high, so yes, please do and I will
measure them.

OK, will post them off tomorrow night. I'll include the manufacturers info
(they were bought recently from Mouser in the US), and put two of each in
so that you can crack one of each value open and see what they're made of
;-)

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

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.

73,
Mike, KK6GM

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/

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.