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Author Topic: Magnet Myths and Misconceptions  (Read 464327 times)

Offline MileHigh

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Re: Magnet Myths and Misconceptions
« Reply #1005 on: January 14, 2015, 04:27:35 AM »
So are you up to the question or not?

Offline EMJunkie

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Re: Magnet Myths and Misconceptions
« Reply #1006 on: January 14, 2015, 04:30:27 AM »
So are you up to the question or not?

Ask away, My electronics is OK, my Magnetics is much better.

Offline EMJunkie

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Re: Magnet Myths and Misconceptions
« Reply #1007 on: January 14, 2015, 04:33:19 AM »
Copper cables are not coaxial cables.  Every setup will have it's characteristic impedance and the formula for the speed is something like the square root of the permittivity divided by the permeability.   That's probably wrong but it is in the ballpark.  I would have to go look it up but I am not going to bother.  It's all part of transmission line theory.

Ok, so here you prove my point again. Look we cant all know everything all the time. I am not going to nail you to the wall on this! Because really I don't care if you don't know the real reason why.

Still, your demand is a ridiculous one and proves nothing! It is not constructive and helps no-one!

Offline MarkE

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Re: Magnet Myths and Misconceptions
« Reply #1008 on: January 14, 2015, 04:35:18 AM »
Copper cables are not coaxial cables.  Every setup will have it's characteristic impedance and the formula for the speed is something like the square root of the permittivity divided by the permeability.   That's probably wrong but it is in the ballpark.  I would have to go look it up but I am not going to bother.  It's all part of transmission line theory.
In a lossless transmission line the propagation velocity is:  c/(uR*eR)0.5.  Most of the time in cables and etch inside printed wiring boards, uR is close to 1.0 and it is the eR that sets the velocity.  Typical printed circuit boards have eR values of 4-5 so the velocity is half or less of c inside the board.  The speed actually varies depending on how much of the fiberglass is glass and how much is resin.  At the ferocious data rates that we have today that can be a big problem because how traces line up with the glass bundles changes their timing.  In a coaxial cable that uses very low density PTFE foam the average eR is only about  1.5 and the velocity is about 0.8 c.  Higher density of PTFE increases the eR and further reduces the propagation velocity.  Traces that run on the top side of the circuit board send some of the energy through air above the trace and other energy through the board material underneath the trace.  These components run at different speeds and that distorts the signals.  But what can really drive fast signals batty is the protective nickel plating that is often applied between outside traces and gold top finish.  Nickel has a high uR. 

Offline MileHigh

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Re: Magnet Myths and Misconceptions
« Reply #1009 on: January 14, 2015, 04:37:11 AM »
Okay, here we go.

The circuit is just a variable power supply connected to a coil - that's it, nothing else.   The coil is 2 Henries.   The coil is an ideal coil with zero wire resistance.  The power supply is an ideal power supply with zero output impedance.

At the start of the test (time zero), the power supply outputs 7 volts.   After five seconds, the power supply outputs 5 volts.

Please describe what happens with this circuit starting from time zero.

Offline EMJunkie

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Re: Magnet Myths and Misconceptions
« Reply #1010 on: January 14, 2015, 04:39:02 AM »
In a lossless transmission line the propagation velocity is:  c/(uR*eR)0.5.  Most of the time in cables and etch inside printed wiring boards, uR is close to 1.0 and it is the eR that sets the velocity.  Typical printed circuit boards have eR values of 4-5 so the velocity is half or less of c inside the board.  The speed actually varies depending on how much of the fiberglass is glass and how much is resin.  At the ferocious data rates that we have today that can be a big problem because how traces line up with the glass bundles changes their timing.  In a coaxial cable that uses low density PTFE foam the average eR is only about  1.2 and the velocity is about 0.9 c.  Traces that run on the top side of the circuit board send some of the energy through air above the trace and other energy through the board material underneath the trace.  These components run at different speeds and that distorts the signals.  But what can really drive fast signals batty is the protective nickel plating that is often applied between outside traces and gold top finish.  Nickel has a high uR.

Ok, not bad MarkE. We can markE this as an answer because its close enough to what I was looking for.

See My point MileHigh!

Offline MileHigh

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Re: Magnet Myths and Misconceptions
« Reply #1011 on: January 14, 2015, 04:40:54 AM »
In a lossless transmission line the propagation velocity is:  c/(uR*eR)0.5.  Most of the time in cables and etch inside printed wiring boards, uR is close to 1.0 and it is the eR that sets the velocity.  Typical printed circuit boards have eR values of 4-5 so the velocity is half or less of c inside the board.  The speed actually varies depending on how much of the fiberglass is glass and how much is resin.  At the ferocious data rates that we have today that can be a big problem because how traces line up with the glass bundles changes their timing.  In a coaxial cable that uses very low density PTFE foam the average eR is only about  1.5 and the velocity is about 0.8 c.  Higher density of PTFE increases the eR and further reduces the propagation velocity.  Traces that run on the top side of the circuit board send some of the energy through air above the trace and other energy through the board material underneath the trace.  These components run at different speeds and that distorts the signals.  But what can really drive fast signals batty is the protective nickel plating that is often applied between outside traces and gold top finish.  Nickel has a high uR.

You are amazing Mark.  As you know, 25 years ago, clock frequencies were much lower and you could pretty much ignore this stuff.

Offline MileHigh

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Re: Magnet Myths and Misconceptions
« Reply #1012 on: January 14, 2015, 04:42:03 AM »
Ok, not bad MarkE. We can markE this as an answer because its close enough to what I was looking for.

See My point MileHigh!

I gave you the real short answer - the impedance of the medium determines the velocity.

Offline MarkE

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Re: Magnet Myths and Misconceptions
« Reply #1013 on: January 14, 2015, 04:42:45 AM »
You are amazing Mark.  As you know, 25 years ago, clock frequencies were much lower and you could pretty much ignore this stuff.
We are approaching 48Gbps over a single pair with goals to get to 100Gbps before the end of the decade.  We are talking very wicked fast.

Offline MileHigh

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Re: Magnet Myths and Misconceptions
« Reply #1014 on: January 14, 2015, 04:45:02 AM »
We are approaching 48Gbps over a single pair with goals to get to 100Gbps before the end of the decade.  We are talking very wicked fast.

The entire Library of Congress in 17.9 seconds!  lol

Offline EMJunkie

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Re: Magnet Myths and Misconceptions
« Reply #1015 on: January 14, 2015, 04:47:31 AM »
Okay, here we go.

The circuit is just a variable power supply connected to a coil - that's it, nothing else.   The coil is 2 Henries.   The coil is an ideal coil with zero wire resistance.  The power supply is an ideal power supply with zero output impedance.

At the start of the test (time zero), the power supply outputs 7 volts.   After five seconds, the power supply outputs 5 volts.

Please describe what happens with this circuit starting from time zero.

MileHigh - Please describe what conditions the Power Supply is under during the Test, you said Variable! What is the variance due to the test and applied conditions - Not enough Information Provided.

Any coil that has a Voltage applied to it will see a Charging Time Constant!!! this is T=L/R - 5 Time Constants charge the Coil to 99.3% of its total applied charge vs the Voltage Applied. The Magnetic Field stores the Energy in the following formula: 1/2 LI^2

Current I would normally lag the Voltage by a phase angle. This would then catch up as the Time Constants move from 0.

Again you have not provided enough information as to why the Power supply shows a lower Voltage. I can only assume that its source impedance is to low for the current drawn.


Offline EMJunkie

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Re: Magnet Myths and Misconceptions
« Reply #1016 on: January 14, 2015, 04:50:29 AM »
I gave you the real short answer - the impedance of the medium determines the velocity.

No you did not give me the answer as to why. You gave me a verification of the drop not why!

Offline MileHigh

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Re: Magnet Myths and Misconceptions
« Reply #1017 on: January 14, 2015, 04:51:06 AM »
Before time zero the output from the power supply is zero volts.  Then for 5 seconds the output from the power supply is 7 volts.  After 5 seconds the output from the power supply is 5 volts.  This is a given.

There is no resistance anywhere in the circuit.

Offline EMJunkie

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Re: Magnet Myths and Misconceptions
« Reply #1018 on: January 14, 2015, 05:01:07 AM »
Before time zero the output from the power supply is zero volts.  Then for 5 seconds the output from the power supply is 7 volts.  After 5 seconds the output from the power supply is 5 volts.  This is a given.

There is no resistance anywhere in the circuit.

So if I understand you correctly, then I have answered your question!

Lets hear your desired answer?

Offline MileHigh

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Re: Magnet Myths and Misconceptions
« Reply #1019 on: January 14, 2015, 05:06:35 AM »
So if I understand you correctly, then I have answered your question!

Lets hear your desired answer?

C'mon Chris, get real please.  Please post your answer.