# Arduino Forum

## Topics => Science and Measurement => Topic started by: brkblbn on Mar 14, 2019, 10:08 pm

Title: Measuring capacitance of a dual cable with arduino
Post by: brkblbn on Mar 14, 2019, 10:08 pm
Can anyone help me???

How can I measure capacitance of a dual cable or any cable with arduino. I found this
https://circuits4you.com/2016/05/13/capacitance-measurement-arduino/ but this is just measuring the unknown capacitor.

Thanks :)
Title: Re: Measuring capacitance of a dual cable with arduino
Post by: jremington on Mar 14, 2019, 10:15 pm
If the cable has two leads, or an inner and outer conductor, use those two in the same circuit. Makle sure the wires are not connected at the far end.

You may have to change the resistor values, as the cable capacitance will usually be in the pF to nF range (unless it is quite long).

Hint: you can look up the capacitance per unit length for most cables and wiring. For example, 12 AWG "zip" cable for household lamps and audio speakers has capacitance of about 60 pF/meter.
Title: Re: Measuring capacitance of a dual cable with arduino
Post by: brkblbn on Mar 14, 2019, 10:49 pm
First, Thank you for your message.

So, you say I keep the same circuit and source code but change capacitor with dual cables. Does it work

like that? I don't think that the measuring will be accurate.

Title: Re: Measuring capacitance of a dual cable with arduino
Post by: jremington on Mar 15, 2019, 12:01 am
Quote
I don't think that the measuring will be accurate.
Obviously, you have no basis for thinking that, until you have actually tried the experiment. Use a known capacitance for tests.
Title: Re: Measuring capacitance of a dual cable with arduino
Post by: brkblbn on Mar 16, 2019, 02:10 pm
I tried to do the experiment but I do not understand how to implement the cables to the circuit. Connections may be wrong.
Title: Re: Measuring capacitance of a dual cable with arduino
Post by: jremington on Mar 16, 2019, 11:41 pm
Quote
If the cable has two leads, or an inner and outer conductor, use two AT THE SAME END in the same circuit.
Make sure the wires are not connected at the far end.
https://learn.sparkfun.com/tutorials/capacitors/all (https://learn.sparkfun.com/tutorials/capacitors/all)
Title: Re: Measuring capacitance of a dual cable with arduino
Post by: brkblbn on Mar 17, 2019, 10:25 pm

the design using arduino :(
Title: Re: Measuring capacitance of a dual cable with arduino
Post by: jremington on Mar 17, 2019, 11:06 pm
Quote
I already know how capacitor works
No, you don't, because you don't understand that a cable with two or more leads IS a capacitor.

What is so difficult to understand about connecting two leads of a two wire cable to a circuit?

Maybe this picture will help:

(https://forum.arduino.cc/index.php?action=dlattach;topic=603679.0;attach=299766)
Title: Re: Measuring capacitance of a dual cable with arduino
Post by: brkblbn on Apr 13, 2019, 05:01 pm
My supervisor did not like the method he told me to do with TDR(Time Domain Reflectometry) so my new concern is:

I need a send a pulse to a cable(coax or simple copper wire) either sin/cos wawe or square. Then that

signal goes to end of the cable and reflected back to first position. After I understand the

exact(disconnection) open circuit location of the cable. For generating the signal can my oscillator in

arduino generate that kind of signal or I need to buy a new oscillator? And can I see the reflected signal

waweform from arduino software without an oscilloscope.

Title: Re: Measuring capacitance of a dual cable with arduino
Post by: MarkT on Apr 13, 2019, 05:12 pm
My supervisor did not like the method he told me to do with TDR(Time Domain Reflectometry) so my new concern is:
TDR doesn't measure the cable capacitance, it measures its length (assuming the dielectric constant
is known, or vice versa).

You measure the cable's characteristic impedance, but that depends on the capacitance and
inductance per unit length.

Quote
I need a send a pulse to a cable(coax or simple copper wire) either sin/cos wawe or square.
[/qoute]
Rectangular pulse with very fast edges, the rise and falltimes limit the resolution.
Quote
Then that signal goes to end of the cable and reflected back to first position. After I understand the

exact(disconnection) open circuit location of the cable. For generating the signal can my oscillator in

arduino generate that kind of signal or I need to buy a new oscillator? And can I see the reflected signal

waweform from arduino software without an oscilloscope.
The fastest edges an Arduino generate is perhaps 3 to 5ns risetime, good enough for a resolution of
a few feet, but it has no way to measure arrival time to better than the system clock period, and the
complication is that the return voltage can be 0V, 5V or 10V for a 5V pulse going in.

A TDR needs some high speed auxiliary circuitry, these days most 'scopes are fast enough to do the
measurement.
Normally you can just look up the cable capacitance in the specs for the cable...
Title: Re: Measuring capacitance of a dual cable with arduino
Post by: brkblbn on Apr 13, 2019, 06:27 pm
I don't measure capacitance with TDR. I 'm gonna measure length of the cable then I put it into test. In

test I measure the "known" length of a cable then I'll check the length from the TDR, if lengths are not

match so I understand that there is a open circuit fault and tell the exact location. So according to you I

can't make that circuitry with arduino. How can I do that? Do I need osciloscope?
Title: Re: Measuring capacitance of a dual cable with arduino
Post by: Idahowalker on Apr 14, 2019, 04:54 am
Have 'you' tried to do a search, using your favorite search engine with the words "arduino tdr"?

If you did you'd find this https://forum.arduino.cc/index.php?topic=183770.0 (https://forum.arduino.cc/index.php?topic=183770.0) which is quite informative.
Title: Re: Measuring capacitance of a dual cable with arduino
Post by: MarkT on Apr 14, 2019, 04:46 pm
Whoops, I got the quoting nesting wrong in my posting - I say that most 'scopes these days are fast
enough to do this (proper scopes, not toys).