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Re: Are metal oxide resistors non-inductive?

>"Michael" >> >> Are your typical Radio Shack metal oxide resistors non-inductive? >> > >** Resistors are described as "' non-inductive" if their design prevents any >excess inductance being created. > >Examples are carbon composition and bi-filar wound wire resistors. > >All capacitors are " non -inductive " by design - ie the wound foil strips >are connected all along the edges.

This one's not. The cool thing about polystyrene caps is that you can see inside them, and not speculate.

ftp://jjlarkin.lmi.net/Poly_1.jpg

ftp://jjlarkin.lmi.net/Poly_2.jpg

It's pretty obvious how this is constructed: they stack strips of film and foil, attach wires to one end, and roll it up, starting from the other end. So the wires are near the outside of the spiral.

So, two foil strips separated with an insulating film, connected on one end, must form a transmission line. Here's a TDR of this cap:

ftp://jjlarkin.lmi.net/Poly_TDR.jpg

Note the initial inductive spike at cm 2 on the screen, from the leads themselves, and the initial plateau, at the second cursor meatball, cm

2.3 roughly. That's the transmission line impedance, about 5.7 ohms, about 1 ns in duration, which sounds reasonable.

This 5.7 ohm transmission line effect behaves sort of like ESR. If the foils were all connected on the ends, you wouldn't see this.

John

Reply to
John Larkin
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"John Larkin" "Phil Allison"

** Those pics show nothing of the internal construction.
** Makes it much the same construction as a typical aluminium foil electro.

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** What is the circuit model for a SHORT open ended transmission line ?

A pure capacitor, below the 1/4 wave frequency.

Here's a TDR of this cap:

** So the wound strip inside the cap is about 30cm long.

Makes the " 1/4 wave stub " frequency about 250 MHz.

IOW - way above the self resonant frequency of the same cap.

** Sort of like the same way chalk is just like cheese.

..... Phil

Reply to
Phil Allison

You can just see the ends of the wrapped foil in the second pic, and easily see the spiral ends of the body. Trust me, the foil ends are not connected along their edges.

It's a transmission line. And to a 25 picosecond TDR step, it's not short.

Well, in the time domain it's a transmission line. Which is why it looks like 5.7 ohms for the first nanosecond, and *then* it starts to charge up like a capacitor.

We're probably seeing the round trip as about 1 ns, at maybe 3/4 c, so that makes it 4 or maybe 5 inches.

The ringing along the charging slope is about 300 MHz. I probably could have made the leads a little shorter, but the way I soldered it is about how an axial cap like this would be mounted on a PC board.

Pretty good cap.

John

Reply to
John Larkin

"John Larkin"

** That is not the SR frequency of that cap.

The cap's ESR is not 5.7 ohms either.

And none of this has any relevance to my post.

.... Phil

Reply to
Phil Allison

The part that's relevant to your post was the fact that you said that

which clearly isn't so.

TDR is an easy way to test film caps and infer their internal structure.

This is interesting, different ways to terminate a transmission line...

LEAD LEAD foilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoil

dielectricdielectricdielectricdielectricdielectricdielectricdielectric

foilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoil LEAD LEAD

or

LEAD LEAD foilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoil

dielectricdielectricdielectricdielectricdielectricdielectricdielectric

foilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoil LEAD LEAD

or

LEAD LEAD foilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoil

dielectricdielectricdielectricdielectricdielectricdielectricdielectric

foilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoilfoil LEAD LEAD

all of which will have different high-speed behavior. My polystyrene cap seem to be the middle of the three.

John

Reply to
John Larkin

I also thought of polystyrene caps as an obvious example on non-extended foil construction. I don't have a TDR but I do have an LCR meter than can go up to 5 MHz and a network analyzer for up to 3.6 GHz.

There is another type of cap that definitely has non-extended foil construction, which is a low cost mylar. See the lower image, type EIL here:

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I have a vendor's assortment of those from years ago. I'm going to make some measurements to see if I can detect a difference.

It would be interesting if you would do a TDR measurement on some caps known to be extended foil construction, such as stacked mylar film, or perhaps silver mics and post the results.

Do you have any larger polystyrenes? A comparison of various types but of the same capacitance would be instructive.

Reply to
The Phantom

These Caddock resistors are flat to about 6 GHz. An Agilent 54006A scope probe is just one of these and some SMA stuff. I think thay want about $3K for it.

ftp://jjlarkin.lmi.net/Caddock_450.JPG

John

Reply to
John Larkin

"John Larkin"

** Fuck off nut case.
Reply to
Phil Allison

No, John Larkin, polite contributor to newsgroup. Phil Allison, impolite twit.

Reply to
Tom Jones

Tom Jones, troll baiter.

Hope This Helps! Rich

Reply to
Rich Grise

You alway snip the parts that are true.

John

Reply to
John Larkin

When he's not foaming and swearing, he can be helpful. Even nice. Bad chemistry, likely.

John

Reply to
John Larkin

It would be a very lonely world for Phil if he didn't snip!

P.S.

I've had Phil on my black list for a while now, I don't miss him! ;)

Jamie

Reply to
Jamie

When is that? Ok, I may have seen it once.

Bad apple.

Reply to
krw

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