How can a wire wound reisitor be non-inductive?
 

I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it would
be an easy matter to find them with enough current rating to do the job but
can't find anything larger than 1 watt carbon (not film, which are somewhat
inductive ) resistors. I can find want is called non-inductive wire wound
resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?

"KC4IH" wrote in message
...
I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it
would
be an easy matter to find them with enough current rating to do the job
but
can't find anything larger than 1 watt carbon (not film, which are
somewhat
inductive ) resistors. I can find want is called non-inductive wire wound
resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?

By winding the wire so the inductance cancels out you get a noninductive
resistor. They are really noninductive at low frequencies. Once you get
above the audio range they become inductive. I doubt they would work for
your case.

KC4IH wrote:
I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it would
be an easy matter to find them with enough current rating to do the job but
can't find anything larger than 1 watt carbon (not film, which are somewhat
inductive ) resistors. I can find want is called non-inductive wire wound
resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?

Either there are two stacked windings, wound with opposite rotations,
or the winding is a zig zag, one turn one way, followed by one turn
the other way.

KC4IH wrote:
I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it would
be an easy matter to find them with enough current rating to do the job but
can't find anything larger than 1 watt carbon (not film, which are somewhat
inductive ) resistors. I can find want is called non-inductive wire wound
resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?



There used to be great big resistors -- essentially carbon composition
-- that were truely noninductive. I suspect they're still available.
My 1988 handbook has a UHF dummy load design that features a Carborundum
part number 889SP500K. With coolant it's good for more than 1500 watts,
so it may be oversized for you...

You can still find the occasional 2W or 5W carbon comp resistor -- if
you're working at HF you can easily parallel these to get the resistance
that you need.

--
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Thanks Ralph, that is what I was thinking but when they are advertised as
non-inductive you would think "non-inductive" wouldn't you. I guess I'll
have to dig around in some old junk boxes or parallel several 1 watters to
get them to work.
I appreciate your information.
Ken

"Ralph Mowery" wrote in message
nk.net...

"KC4IH" wrote in message
...
I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it
would
be an easy matter to find them with enough current rating to do the job
but
can't find anything larger than 1 watt carbon (not film, which are
somewhat
inductive ) resistors. I can find want is called non-inductive wire wound
resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?

By winding the wire so the inductance cancels out you get a noninductive
resistor. They are really noninductive at low frequencies. Once you get
above the audio range they become inductive. I doubt they would work for
your case.

Tim Wescott wrote:
KC4IH wrote:

I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my
6 meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought
it would be an easy matter to find them with enough current rating to
do the job but can't find anything larger than 1 watt carbon (not
film, which are somewhat inductive ) resistors. I can find want is
called non-inductive wire wound resistors. I can't understand how a
wire wound resistor can be non-inductive. Could someone explain this?



There used to be great big resistors -- essentially carbon composition
-- that were truely noninductive. I suspect they're still available. My
1988 handbook has a UHF dummy load design that features a Carborundum
part number 889SP500K. With coolant it's good for more than 1500 watts,
so it may be oversized for you...

You can still find the occasional 2W or 5W carbon comp resistor -- if
you're working at HF you can easily parallel these to get the resistance
that you need.

These are a smaller version:
http://www.ohmite.com/catalog/pdf/a_series.pdf

KC4IH wrote:

Thanks Ralph, that is what I was thinking but when they are advertised as
non-inductive you would think "non-inductive" wouldn't you. I guess I'll
have to dig around in some old junk boxes or parallel several 1 watters to
get them to work.
I appreciate your information.
Ken

Crazy idea but I'll throw it out here anyway.

We know that carbon film resistors have some worriable inductance at 6
meters. If a guy were to take a group of say 1-watters to make up the
power rating, could you count on the consistency of the outer markings
to the extent that you could group them somehow in parallel/inverted so
that they cancel each other's inductance in this application?

-Bill

In article ,
KC4IH wrote:

Thanks Ralph, that is what I was thinking but when they are advertised as
non-inductive you would think "non-inductive" wouldn't you. I guess I'll
have to dig around in some old junk boxes or parallel several 1 watters to
get them to work.
I appreciate your information.

Check out the Caddock and Vishay noninductive metal-foil resistors.
As I understand it they use a serpentine-path foil pattern, which
cancels out the inductance quite nicely. Some of them are advertised
as having an inductance equivalent to an equal-length piece of wire.

They're available in packages such as TO-220, and can be attached
directly to a heatsink (no insulator required).

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Sun, 10 Jul 2005 21:21:34 -0400, "KC4IH"
wrote:

I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it would
be an easy matter to find them with enough current rating to do the job but
can't find anything larger than 1 watt carbon (not film, which are somewhat
inductive ) resistors. I can find want is called non-inductive wire wound
resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?

Look up for bifilar winding.

If your exciter is reasonably clean so that you do not think about
proper termination also for the harmonics, you just need an attenuator
for a single frequency, which should simplify things a bit.

Even if a "non-inductive" resistor has some inductance at 50 MHz, it
should not be too hard to compensate for the inductance and turn the
system into a very lossy series resonant circuit, in which the
impedance is dominated by the resistance in the area around the
resonance. An other alternative would be to put a capacitor in
parallel with the resistor to form a parallel resonant circuit at 50
MHz and the actual resistance would heavily damp the resonance. If the
inductance is sufficiently low, you will end up with capacitances that
are practical (i.e. not affected too much by stray capacitances).

This kind of approach should work at a single frequency(band), but if
multiple bands are needed or if the harmonics also needs to be
correctly matched, this approach does not work.

Paul OH3LWR

On Sun, 10 Jul 2005 21:21:34 -0400, "KC4IH"
wrote:


I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it would
be an easy matter to find them with enough current rating to do the job but
can't find anything larger than 1 watt carbon (not film, which are somewhat
inductive ) resistors. I can find want is called non-inductive wire wound
resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?

I found a graph that I copied off the web sometime back showing the
reactance of various R vs freq for metal film resistors. The 10-1000
ohm range of resistors show as being dead flat at 50 MHz compared to
DC...and well beyond. Above and below values tend to go nuts with
frequency, the high 100k values in particular.

I don't remember where I found this graph but based on it I'd think you
have a good chance of making those particular values work using garden
variety ten-cent 1-watt resistors...thinking along the lines that
manufacturing of plain old carbon film resistors would follow the same
trend.

I think its worth a try.

-Bill

"KC4IH" wrote in message
...
I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it
would
be an easy matter to find them with enough current rating to do the job
but
can't find anything larger than 1 watt carbon (not film, which are
somewhat
inductive ) resistors. I can find want is called non-inductive wire wound
resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?


Wind up approx 40' of RG-174 into a small coil and tie wrap it. It will
handle 100W + at 50 MHz and have a great return loss.
Lots easier than fooling around with resistors of questionable
characteristics and heating problems.
Dale W4OP

Use a length of lossy coax for only 3 db. A lot cheaper. None of
the non-inductive wirewound or laser cut resistors will be truly non-
inductive at those frequencies. Realize that the values need not be
exact, because return loss will keep the SWR down. If you can't get
Carbon, Use the coax. You might also want to adjust Drive or ALC in
the amp.

The Collins 30L-1 used to come with a length of lossy coax for this
reason.

ac6tk
http://tekstuff.freespaces.com

"KC4IH" wrote in message
...
I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to
my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought
it would
be an easy matter to find them with enough current rating to do the
job but
can't find anything larger than 1 watt carbon (not film, which are
somewhat
inductive ) resistors. I can find want is called non-inductive wire
wound
resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?

Now there is a great idea and I have a 100' roll of RG-174 in the closet.
Sounds like a winner. It's a shame that we can't get parts like were
available 30 years ago. I'd like to build some old design tube receivers and
transmitters. I was born in 46 and just got a 1946 ARRL handbook off Ebay
with a lot of things that look interesting.

Thanks to everyone for their input on this matter.... and not one flame in
all of the answers, that's rare on the user groups anymore!
73 to all,
Ken, KC4IH


"Dale Parfitt" wrote in message
news:z4uAe.5729$Zy6.3230@trnddc04...

"KC4IH" wrote in message
...
I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it
would
be an easy matter to find them with enough current rating to do the job
but
can't find anything larger than 1 watt carbon (not film, which are
somewhat
inductive ) resistors. I can find want is called non-inductive wire wound
resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?


Wind up approx 40' of RG-174 into a small coil and tie wrap it. It will
handle 100W + at 50 MHz and have a great return loss.
Lots easier than fooling around with resistors of questionable
characteristics and heating problems.
Dale W4OP

At 50MHz, you should find that metal-oxide resistors will work fine.
You're just making a power-wasting pad, not a precision attenuator.
(Carbon comps wouldn't be appropriate for precision work, either...)
The low-value metal-oxides I've tested on RF component analyzers have
all been fine for non-precision applications out to 150MHz. Try, for
example, Vishay FP69 series 2W parts or TT/IRC GS-3 series.

A couple years ago, I went through my stock of carbon composition
resistors and found more than half of them to be out of tolerance at
DC. They don't age well at all, even while not dissipating power.

1/8-W SMT resistors are so cheap that you could just use
series-parallel combinations of them to get what you need, too. For
more serious power dissipation, Caddock (and some others) have power
film resistors in TO-220-like packages which are low enough inductance
to be quite useful up into VHF and maybe above.

Cheers,
Tom

K7ITM wrote:
At 50MHz, you should find that metal-oxide resistors will work fine.
You're just making a power-wasting pad, not a precision attenuator.
(Carbon comps wouldn't be appropriate for precision work, either...)
The low-value metal-oxides I've tested on RF component analyzers have
all been fine for non-precision applications out to 150MHz. Try, for
example, Vishay FP69 series 2W parts or TT/IRC GS-3 series.

A couple years ago, I went through my stock of carbon composition
resistors and found more than half of them to be out of tolerance at
DC. They don't age well at all, even while not dissipating power.

The other half of them would probably be out of tolerance too, after
having been soldered.

I just came across an attempt to use carbon comp resistors (obtained
with some difficulty) as RF impedance "standards" - completely hopeless.
Small wire-ended metal film resistors would have been just fine, because
they don't change value significantly when soldered.

Wire-ended metal film resistors can be quite good for RF, but the
parasitic inductance can vary. They are made by cutting a spiral track
into a basic resistive tube, and the pitch of the spiral is changed to
produce a range of resistor values. When the spiral becomes too fine,
the process steps up to a higher-resistance 'blank' and the whole cycle
starts again with about a one-turn spiral.

Thus the number of spiral turns can vary between about 1 and maybe 10.
You have to scrape the paint off to see. The inductance can then be
estimated using the standard formula based on diameter, number of turns
and length, and it varies from a few nH up to maybe 100nH.

The problem is that different manufacturers make the steps at different
resistance values, so resistors of the same DC value may have very
different values of parasitic inductance. If parasitic inductance is
critical, circuits that worked perfectly well with one make of resistor
may have problems with a different make (as Elecraft recently
discovered).

1/8-W SMT resistors are so cheap that you could just use
series-parallel combinations of them to get what you need, too. For
more serious power dissipation, Caddock (and some others) have power
film resistors in TO-220-like packages which are low enough inductance
to be quite useful up into VHF and maybe above.

Yes, easily. The only problem with the TO220 packages is the shunt
capacitance to the grounded tab - and the power dissipation is very low
if the tab is not grounded.



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

I just found an old Narda 3db pad in my junk box. It checks good and is
rated at 5 watts. Since I am concerned with SSB only, I can cut the carrier
back on CW, I think the Narda will work with the SSB signal since it won't
be continuous duty and the pad is rated at 5 watts continuous. I'll give it
a try... I don't even remember having this thing!


"KC4IH" wrote in message
...
I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it
would be an easy matter to find them with enough current rating to do the
job but can't find anything larger than 1 watt carbon (not film, which are
somewhat inductive ) resistors. I can find want is called non-inductive
wire wound resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?

On Mon, 11 Jul 2005 18:22:37 -0400, "KC4IH"
wrote:

I just found an old Narda 3db pad in my junk box. It checks good and is
rated at 5 watts. Since I am concerned with SSB only, I can cut the carrier
back on CW, I think the Narda will work with the SSB signal since it won't
be continuous duty and the pad is rated at 5 watts continuous. I'll give it
a try... I don't even remember having this thing!

Fifty feet of RG-174 would do it too.


"KC4IH" wrote in message
...
I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it
would be an easy matter to find them with enough current rating to do the
job but can't find anything larger than 1 watt carbon (not film, which are
somewhat inductive ) resistors. I can find want is called non-inductive
wire wound resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?