Wire Inductance & LCR Meter HELP!!

Discussion in 'General Electronics Chat' started by Management, Dec 4, 2007.

  1. Management

    Thread Starter Active Member

    Sep 18, 2007
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    I have a problem understand wire inductance. I know that the longer a wire gets the more inductance it has. Also, the longer a wire gets the more capacitance it will have.

    Now, I have a 20 meter long sensor and it is power by a drive signal (+/- 1.5V and sometimes +/- 3V). Now I found out that when I use a much longer sensor that the current draw increases a little. I was trying to figure why this is the case. Does anyone have any idea? EDIT: I must add that this drive signal is made by an opamp circuit that is power by the power supplies of +/- 4.2V. That is where I calculate my power. The +/- 1.5V and 3V stays virtually on changed.

    The main part of the sensor is a wrapped coil and it is connected inside the sensor to the 20 meter long cable of which is connect to the actual electronics (the electronics power the sensor). The coil is a core wrapped with wire and has its two ends attached to the 20 meter cable in series (wires 2 ends). I used an LCR meter to see the series inductance of the sensor. Then I added 30 more meters to the sensor and the LCR meter showed the series inductance become less. Is that suppose to happen? Should it not go up by added more wire? But if it is actually going down that should explain why more current is drawn (smaller inductance lets in more current right?) or am I just stupid? :confused:

    I have really no idea how to justify why this longer cable draws more current than the shorter cable. It is probably something real simple that is going over my head. If you could help me I appreciate it.

    Dru
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    What's the frequency range of your signal?

    Is your "cable" a coaxial cable? What kind?

    The increased capacitance caused by the increased length of the cable could explain your increased current draw. Your op amp has to charge/discharge that capacitance any time the signal level changes.

    The reduction in inductance could be explained by the increased resistance of the wire itself, along with the increased capacitance.

    The higher you go in frequency, the more impedance matching becomes necessary. RG6, RG11 and RG59 are right at 75 Ohms impedance (common for TV cable) and RG8, RG58 are 50 Ohms. If your op amp output isn't matched to your coax impedance at higher frequencies (MHz and up), you'll wind up with very strange readings.
     
  3. Management

    Thread Starter Active Member

    Sep 18, 2007
    306
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    Thank you for your reply but the cable is just a bunch of shield copper wire. The signal is 2kHz.

    So if I am getting you right. An increase of capacitance and an increase in resistance can cause an decrease in inductance? Therefore an increase in current for the longer wire? So with this long wire connected to the output of the opamp suppling a 2kHz +/- 1.5V I should see an increase in the supply currents of the opamp because of this?

    Sorry for so many questions but that is what is happening when I attached a longer cable, I see an increase in current in the supplies (no the output signal). I haven't really looked at the output current but I see that the supplies draw a little more current when a longer cable is attached. Just want to make sure that I understand why.

    Thank you.

    Dru
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    To elaborate on what SgtWookie said -Try looking at the current with nothing but the cable attached (leave the other end open-circuited). The longer the cable, the higher the current, because the capacitance of the cable goes up as it gets longer. Inductance does not change - it is just in parallel with a capacitance.
    BTW, your op amp may oscillate with a long cable attached to the output unless you have some series resistance between the op amp output and the cable.
     
  5. Management

    Thread Starter Active Member

    Sep 18, 2007
    306
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    Thank you for replying. I will do as you suggested. Also I do have a series resistor between the cable and the opamp output. So no oscillation. :)
     
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