# question regarding the boundary behavior of electric current

L

#### laura

Jan 1, 1970
0
Hi,

I have a cable and I send electric current through it. I want to know
the moment when the current has arrived at the other end of the cable.

Can I visualize this moment by using an oscilloscope ?

If the answer is yes, I have another question for a more complex
experiment.

I have a very simple network with 2 nodes (A and B). There are 2
cables which are connecting these nodes. The cables have different
lengths L1 and L2. Assume that L1 is shorter than L2.

I have draw a small picture here:

--A*-------L1---------*B--
| |
| |
|______L2_____|

I apply electric power to this device. Because the cables have lengths
greater than zero I assume that it will take a while until the current
traverse the path from A to B.

More than that, in node A, the current is split in 2, because there
are 2 cables linking A with B. Because one of the cables is shorter I
assume that "a part" of the current arrives earlier (denote this by
moment M1) than the other "part" because it has to traverse a shorter
path. Am I correct ?

The other "part" of the current which has traversed cable L2 will
arrive later in B. Lets denote this by moment M2.

Can I measure these 2 moments by using an oscilloscope?
Or what other options for measurement I have ?

Are there some fluctuations of the electric current at moments M1 and
M2 ?

Are there high precision oscilloscopes for this experiment? Or should
I use very very long cables?

I'm not interested what happens after those moments.

Thanks,
Laura

A

#### [email protected]

Jan 1, 1970
0
Hi,

I have a cable and I send electric current through it. I want to know
the moment when the current has arrived at the other end of the cable.

Can I visualize this moment by using an oscilloscope ?

If the answer is yes, I have another question for a more complex
experiment.

I have a very simple network with 2 nodes (A and B). There are 2
cables which are connecting these nodes. The cables have different
lengths L1 and L2. Assume that L1 is shorter than L2.

I have draw a small picture here:

--A*-------L1---------*B--
| |
| |
|______L2_____|

I apply electric power to this device. Because the cables have lengths
greater than zero I assume that it will take a while until the current
traverse the path from A to B.

More than that, in node A, the current is split in 2, because there
are 2 cables linking A with B. Because one of the cables is shorter I
assume that "a part" of the current arrives earlier (denote this by
moment M1) than the other "part" because it has to traverse a shorter
path. Am I correct ?

The other "part" of the current which has traversed cable L2 will
arrive later in B. Lets denote this by moment M2.

Can I measure these 2 moments by using an oscilloscope?
Or what other options for measurement I have ?

Are there some fluctuations of the electric current at moments M1 and
M2 ?

Are there high precision oscilloscopes for this experiment? Or should
I use very very long cables?

I'm not interested what happens after those moments.

Thanks,
Laura

You are looking for a TDR.

T

#### Tom Bruhns

Jan 1, 1970
0
Hi,

I have a cable and I send electric current through it. I want to know
the moment when the current has arrived at the other end of the cable.

Can I visualize this moment by using an oscilloscope ?

If the answer is yes, I have another question for a more complex
experiment.

I have a very simple network with 2 nodes (A and B). There are 2
cables which are connecting these nodes. The cables have different
lengths L1 and L2. Assume that L1 is shorter than L2.

I have draw a small picture here:

--A*-------L1---------*B--
| |
| |
|______L2_____|

I apply electric power to this device. Because the cables have lengths
greater than zero I assume that it will take a while until the current
traverse the path from A to B.

More than that, in node A, the current is split in 2, because there
are 2 cables linking A with B. Because one of the cables is shorter I
assume that "a part" of the current arrives earlier (denote this by
moment M1) than the other "part" because it has to traverse a shorter
path. Am I correct ?

The other "part" of the current which has traversed cable L2 will
arrive later in B. Lets denote this by moment M2.

Can I measure these 2 moments by using an oscilloscope?
Or what other options for measurement I have ?

Are there some fluctuations of the electric current at moments M1 and
M2 ?

Are there high precision oscilloscopes for this experiment? Or should
I use very very long cables?

I'm not interested what happens after those moments.

Thanks,
Laura

When you're dealing with the transmission time along a cable, it's
good to realize that controlled cables are important. Specifically,
it's very helpful to use transmission lines with controlled impedance.

Yes, you can measure such things with oscilloscopes. It doesn't even
take a particularly fast 'scope. You can use, say, 100 feet of
coaxial cable for your line L1, and 150 feet for your line L2. The
line will be much cheaper than a high speed 'scope, or very much
cheaper than the very high speed 'scope you'd need if you tried to use
lines, say, 1 inch and 1.5 inches long. With 100 feet of common RG-6
coaxial cable like is used to wire houses for television, you'll get
about 125 nanoseconds of delay, and with 150 feet, you'll get 1.5
times as much. Even a 'scope with 5MHz bandwidth should be able to
easily resolve that sort of delay. You also need a signal generator
that can generate a pulse that transitions quickly from one state to
another, but that's pretty easy with modern digital ICs, if you don't
already have an appropriate signal or pulse generator available. Use
shorter cables, maybe 10 feet and 20 feet, if you have a scope that
can resolve shorter times--maybe one with 50MHz or 100MHz bandwidth.

Technology web site, and you'll have a simulation tool that can easily
show you what happens. Once you learn to run the program (which isn't
difficult), you can set up and simulate just what you've proposed in a
very few minutes. If/when you do, try some different things: Make
the lines both 100 ohms impedance. At B, try it with no load, and try
it with a load of 200 ohms across the line, and 100 ohms across the
line, and 50 ohms across the line.

Cheers,
Tom

N

#### [email protected]

Jan 1, 1970
0
Hi,

I have a cable and I send electric current through it. I want to know
the moment when the current has arrived at the other end of the cable.

Direct current (battery) or alternating current (wall plug)?

Mark L. Fergerson

R

#### Robert Baer

Jan 1, 1970
0
laura said:
Hi,

I have a cable and I send electric current through it. I want to know
the moment when the current has arrived at the other end of the cable.

Can I visualize this moment by using an oscilloscope ?

If the answer is yes, I have another question for a more complex
experiment.

I have a very simple network with 2 nodes (A and B). There are 2
cables which are connecting these nodes. The cables have different
lengths L1 and L2. Assume that L1 is shorter than L2.

I have draw a small picture here:

--A*-------L1---------*B--
| |
| |
|______L2_____|

I apply electric power to this device. Because the cables have lengths
greater than zero I assume that it will take a while until the current
traverse the path from A to B.

More than that, in node A, the current is split in 2, because there
are 2 cables linking A with B. Because one of the cables is shorter I
assume that "a part" of the current arrives earlier (denote this by
moment M1) than the other "part" because it has to traverse a shorter
path. Am I correct ?

The other "part" of the current which has traversed cable L2 will
arrive later in B. Lets denote this by moment M2.

Can I measure these 2 moments by using an oscilloscope?
Or what other options for measurement I have ?

Are there some fluctuations of the electric current at moments M1 and
M2 ?

Are there high precision oscilloscopes for this experiment? Or should
I use very very long cables?

I'm not interested what happens after those moments.

Thanks,
Laura
Methinks you are going to have a lot of fun making measurements
related to speed-of-light travel.

R

#### Robert Baer

Jan 1, 1970
0
Tom said:
When you're dealing with the transmission time along a cable, it's
good to realize that controlled cables are important. Specifically,
it's very helpful to use transmission lines with controlled impedance.

Yes, you can measure such things with oscilloscopes. It doesn't even
take a particularly fast 'scope. You can use, say, 100 feet of
coaxial cable for your line L1, and 150 feet for your line L2. The
line will be much cheaper than a high speed 'scope, or very much
cheaper than the very high speed 'scope you'd need if you tried to use
lines, say, 1 inch and 1.5 inches long. With 100 feet of common RG-6
coaxial cable like is used to wire houses for television, you'll get
about 125 nanoseconds of delay, and with 150 feet, you'll get 1.5
times as much. Even a 'scope with 5MHz bandwidth should be able to
easily resolve that sort of delay. You also need a signal generator
that can generate a pulse that transitions quickly from one state to
another, but that's pretty easy with modern digital ICs, if you don't
already have an appropriate signal or pulse generator available. Use
shorter cables, maybe 10 feet and 20 feet, if you have a scope that
can resolve shorter times--maybe one with 50MHz or 100MHz bandwidth.

Technology web site, and you'll have a simulation tool that can easily
show you what happens. Once you learn to run the program (which isn't
difficult), you can set up and simulate just what you've proposed in a
very few minutes. If/when you do, try some different things: Make
the lines both 100 ohms impedance. At B, try it with no load, and try
it with a load of 200 ohms across the line, and 100 ohms across the
line, and 50 ohms across the line.

Cheers,
Tom
Well.. the OP mentioned wires, and the diagram implied that the
longer one wiggled around some to get from A to B even if the shorter
one was in a straight lin.
No specs on the wires (shape,size, insulation, etc) and the rest too
fuzzy.
Hence the "fun" comment...

L

#### laura

Jan 1, 1970
0
Dear Tom,

I have downloaded the LTSpice software, but I dont know how create 2
cables having different lengths. I'm not very sure, but I think that
the software assumes that the length of cables are not important.

Can you help me with this example?

thank you very much,
Laura

T

#### Tom Bruhns

Jan 1, 1970
0
Dear Tom,

I have downloaded the LTSpice software, but I dont know how create 2
cables having different lengths. I'm not very sure, but I think that
the software assumes that the length of cables are not important.

Can you help me with this example?

thank you very much,
Laura

You need to place two "tline" components on a schematic (F2; type
tline). You can then place the cursor over the "Td=50n Z0=50" text,
right-click, and change it to a new time and new impedance. I suggest
making one of the times 100n (100 nanoseconds) and one of the times
150n. Connect the bottom left and bottom right terminals to ground,
for both lines. Place a source (F2; type vo) to the left of the
lines. Connect the source "-" terminal to ground. Connect the "+"
terminal through a 50 ohm resistor to the upper left terminal of one
of the lines. Connect the "+" terminal through another 50 ohm
resistor to the upper left terminal of the other line. Change the
source to a pulse; let's say delay 10n, rise time 0.5n, fall time
0.5n, initial voltage 0, pulse voltage 10, pulse time 1u, period 10u.
Connect the upper right terminal of each line together, and optionally
to a load resistor to ground. Label that net with some name like
"out". Label the upper left terminals of the line something like
"in1" and "in2". Do a time simulation for, say, 500n or 1u.

Cheers,
Tom

L

#### laura

Jan 1, 1970
0
Dear Tom,

Many, many thanks.

I did created the design as you have suggested. But I think that I've

Could you help me to correct them? I've sent you the design by email.

Thanks again,
Laura

I

#### Ian

Jan 1, 1970
0
laura said:
Dear Tom,

I have downloaded the LTSpice software, but I dont know how create 2
cables having different lengths. I'm not very sure, but I think that
the software assumes that the length of cables are not important.

Can you help me with this example?

thank you very much,
Laura
Insert "tline" components (enter tline in the component selection field,
left click at a couple of positions on your schematic to place them).
These are transmission lines (cables).

Note they arrive with default Td (electrical length in nsec) of 50n
and Z0 (characteristic impedance) of 50 ohms.

If you right click on the symbols you can edit these parameters and change
them to Tom's suggestions.

Add a voltage source and the resistors, label the inputs and outputs you
want to look at with names you will recognise. Right click on the voltage
source, select "Advanced", select "PULSE" and set up the parameters
you want. Right click on the schematic and run the simulation.

Regards
Ian

L

#### laura

Jan 1, 1970
0
Hi Ian,

I have created the example ... but is not working. I'm sure that I've

Could you help me to debug it if I'll send it by email?

thanks,
Laura

Replies
3
Views
496
Replies
4
Views
1K
Replies
1
Views
617
Replies
6
Views
968
Replies
10
Views
893