tape contuctance

[NightWinder]

I was just wondering how to check paper tape for conductance? 1/4 in black crepe. This was a topic a bit back, but there was never any how too.......

[Possum]

easy

just take your ohmeter and put the two leads on the tape with the leads near eachother, if you get any kind of reading at all don't use the stuff, you should get a zero reading.

[Joe Gwinn]

just take your ohmeter and put the two leads on the tape with the probes near each other, if you get any kind of reading at all don't use the stuff, you should get a zero reading.

I know what you mean, but it isn't zero ohms that one should see, it's open circuit (infinity ohms).

I doubt that any reading exceeding 1,000 ohms could have any effect.

[David Schwab]

I know what you mean, but it isn't zero ohms that one should see, it's open circuit (infinity ohms).

I doubt that any reading exceeding 1,000 ohms could have any effect.

Yeah, you want to see an open circuit. Zero ohms would be very conductive!

In the past I've shielded pickup coils with copper tape. I can't say I ever heard any detrimental effect from having the foil around the coil. I'd imagine it must raise the capacitance a little, but would that be enough to matter? I would think it would be swamped by the capacitance of your patch cord.

[kevinT]

meter setting?

To confirm...

Would you have your meter set the same as if you were checking the DC Resistance of a pickup?

[David Schwab]

To confirm...

Would you have your meter set the same as if you were checking the DC Resistance of a pickup?

yes.

[Joe Gwinn]

In the past I've shielded pickup coils with copper tape. I can't say I ever heard any detrimental effect from having the foil around the coil. I'd imagine it must raise the capacitance a little, but would that be enough to matter? I would think it would be swamped by the capacitance of your patch cord.

Yes. Copper foil will have little effect (aside from raising the capacitance if the outside of the coil is hot) IF the copper foil does not make a complete turn. Overlap is OK, so long as the ends are insulated from one another. This is a standard shielding technique.

Some kinds of copper tape have conductive adhesive, some do not. Do you recall which kind you used?

[David Schwab]

Yes. Copper foil will have little effect (aside from raising the capacitance if the outside of the coil is hot) IF the copper foil does not make a complete turn. Overlap is OK, so long as the ends are insulated from one another. This is a standard shielding technique.

That's interesting. I'll have to keep that in mind. Makes perfect sense now that I think about it.

Some kinds of copper tape have conductive adhesive, some do not. Do you recall which kind you used?

Yes, it didn't have adhesive at all. It was actually copper flashing I cut into thin strips.

The Stewmac stuff has conductive adhesive.

[NightWinder]

just take your ohmeter and put the two leads on the tape with the leads near eachother, if you get any kind of reading at all don't use the stuff, you should get a zero reading.

How close are the leads? Inch or 2? I just want to make sure Im understanding correctly. Thanks for the responses too guys

[Joe Gwinn]

Yes, it [the copper foil around the coil] didn't have adhesive at all. It was actually copper flashing I cut into thin strips.

Oof. That's pretty thick stuff. I bet it was a bit awkward to use.

What might be easier to handle is brass shimstock. Electrically, any thickness will work. Mechanically, the stuff that's ~0.003" thick is convenient. This stuff can be cut with ordinary paper-cutting scissors, is easy to solder to, is easily formed to shape with the fingers, and can be held down with tape.

http://www1.mscdirect.com/CGI/NNSRIT...MT4NO=16067219

The Stewmac stuff has conductive adhesive.

I recall that. If one makes a ring of such foil tape, it will be a shorted turn. How much effect trhis will have on the pickup will depend on how much of the field is intercepted by this turn. If the turn is far enough out, the effect may be minimal.

Another reason to space the shield foil out from the coil is to reduce the added capacitance. This is true if the foil forms a shorted turn, or not.

[Joe Gwinn]

How close are the leads? Inch or 2? I just want to make sure Im understanding correctly. Thanks for the responses too guys

It doesn't much matter, anything from a half inch to a few inches will do, as this is a qualitative measurement only.

Making an accurate measurement is a bit more complex, but still not that hard. The key observation is that a square piece of resistive sheet measured between high-conductivity strips on two parallel edges will yield the same resistance regardless of the size of the square. This is both why the resistance of films is specified in ohms per square, and a description of one way to do the measurement.

Here is a technical discussion: http://www.ece.gatech.edu/research/l.../sheetRes.html.

In practice, sheet resistance is most often measured using a four-point probe, but this is far more complex than needed here. One can also use eddy current probes to measure sheet resistance.

[David Schwab]

Oof. That's pretty thick stuff. I bet it was a bit awkward to use.

It was a little tricky. I used it because I found a big roll of it in my basement that had belonged to my dad. It wasn't too thick though. I had a big roll of aluminum flashing too. Quite handy for shielding, and you could solder to it! That always puzzled me... Maybe it was tin?

What might be easier to handle is brass shimstock. Electrically, any thickness will work. Mechanically, the stuff that's ~0.003" thick is convenient. This stuff can be cut with ordinary paper-cutting scissors, is easy to solder to, is easily formed to shape with the fingers, and can be held down with tape.

That's a good idea.

If one makes a ring of such foil tape, it will be a shorted turn. How much effect trhis will have on the pickup will depend on how much of the field is intercepted by this turn. If the turn is far enough out, the effect may be minimal.

Bill Lawrence has a patent on a pickup that has a closed loop coil as the outer wrap. His claim is that it flattens the frequency response by smoothing out resonant peaks. He even had a tuning potentiometer! I've been dying to try it myself. Might be one of those things that doesn't quite sound good in this iteration.

Another reason to space the shield foil out from the coil is to reduce the added capacitance. This is true if the foil forms a shorted turn, or not.

Good stuff Joe! Thanks. I knew about the distance, but had never thought of the shorted turn... which makes perfect sense now.

[Joe Gwinn]

It was a little tricky. I used it because I found a big roll of it in my basement that had belonged to my dad. It wasn't too thick though. I had a big roll of aluminum flashing too. Quite handy for shielding, and you could solder to it! That always puzzled me... Maybe it was tin?

It had to be tin, or soldering would have been just about impossible. Is it magnetic? If yes, it's tin. Or ternplate, which is solder-coated mild steel foil.

That's [using shimstock] a good idea.

Comes from poring over 3000-page catalogs, a sickness to be sure. Iron fever?

Bill Lawrence has a patent on a pickup that has a closed loop coil as the outer wrap. His claim is that it flattens the frequency response by smoothing out resonant peaks. He even had a tuning potentiometer! I've been dying to try it myself. Might be one of those things that doesn't quite sound good in this iteration.

What's the patent number? I'll look it up.

Good stuff Joe! Thanks. I knew about the distance, but had never thought of the shorted turn... which makes perfect sense now.[/QUOTE]Welcome. Always remember that all effects operate at once, varying only in degree from situation to situation.

[David Schwab]

What's the patent number? I'll look it up.

5376754

And also check out:

3711619, and 5789691.

[Joe Gwinn]

5376754

And also check out:

3711619, and 5789691.

Thanks. It turns out that I had seen these a few years ago, but I'll reread them. I think that the underlying issue was the control of coil inductance.