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Need help about RLC Meter

  1. Jul 24, 2012 #1
    Hello everyone,now i still working my final project for a graduation..
    and i must measuring my tools using RLC meter, but when i measure,my RLC meter showing a negative symbols (-) for the value of capacitance.. and when i check the manual book,there is'nt an explanation about it..

    did anybody know what's wrong there??

    thanks a lot..
     
  2. jcsd
  3. Jul 24, 2012 #2

    vk6kro

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    Yes, it could be an adjustment inside the meter. It is supposed to be balanced with no capacitance, but it is wrongly adjusted and giving negative output to the display.

    You might be able to fix it by putting new batteries in the meter. Or, you might be able to get information on which control to adjust.
    Don't try rotating controls unless you are sure what they do.

    A rough way of fixing it is to get a capacitor that just gives a zero reading and then put that across the input of the meter.
    Or, you can just add the negative value to the reading you get.

    If it always reads negative, even with any capacitor on it, then it needs to be fixed properly.
     
  4. Jul 24, 2012 #3
    try by changing mode of operation and operating frequency
    for higher capacitance prefer lower frequency while for lower capacitance prefer higher frequency..
     
  5. Jul 30, 2012 #4
    hmm.. but when i measure the capacitor with a low capacitance (nano Farad) with the same operating frequency,my RLC meter can showing the value of capacitor correctly.. but when i measure my tools,there's showing an negative value.. i don't know how to setting my RLC meter. i using GwINSTEK lCR 816/826. with maximum freq is 2kHz..
     
  6. Jul 30, 2012 #5

    vk6kro

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    I found an operation manual for that RLC meter.

    See if this makes sense. Press "menu" then F1 for offset menu then F1 for zeroing.

    4-3. Zeroing
    In order to eliminate strayed capacitance and impedance of test cable during the
    measurement, the LCR Meters should be zeroed to correct for test cable and/or
    test fixture errors before taking measurements. The corrections are calculated
    and stored in memory of the LCR Meters during the zeroing process. Open and
    short circuit zeroing should be done for test cable and/or test fixture. For the best
    accuracy, the test cable and/or test fixture should be zeroed once per day at least
    and each time test cable or test fixture is changed in general.
    The zeroing process of open and short circuits are following:
    Open Circuit
    l The test cable or test fixture should be open with no component connected.
    l Press MENU key.
    l Press F1 key to select “OFFSET” menu.
    l Press F1 key to select open circuit zeroing (the “CAP OFFSET” is indicated
    on the adjacent LCD monitor. ).
     
  7. Jul 30, 2012 #6

    The Electrician

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    RLC meters are intended to measure resistors (R), inductors (L) and capacitors (C), not tools.

    What are you talking about when you say you are measuring your "tools"? Is one of your "tools" a capacitor? If so, what is the expected value of the particular capacitor that is giving you a negative value?

    If you change the measurement frequency you are using for this particular capacitor, do you get the same result for a different measurement frequency? What value do you get for the lowest possible measurement frequency?
     
  8. Jul 31, 2012 #7
    i have low frequency antena,and i want to make an eqivalent circuit of that.. so i think i must find the capacitance,Resistance,and inductance that antena using an LRC right??
    please correct me if i'm wrong
     
  9. Jul 31, 2012 #8
    Usually negative values correspond to inductance or the meter is not calibrated. Typically you go through a shorted calibration and an open calibration.
     
  10. Jul 31, 2012 #9

    vk6kro

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    You need to make your measurements at the frequency of operation of the antenna.

    At this frequency, you will measure what appears to be an inductance or a capacitance, but it is really an effect of standing waves on the antenna causing these apparent components.

    The reflected wave from a previous input cycle can deliver power back to the input terminals of the antenna and depending on the phase of this power, it can seem like a capacitor or an inductor to the incoming signal.

    However, if you measured the antenna at a lower frequency, you would not find any trace of these components.

    There are antenna analysers available for adjusting antennas. Commercial devices are expensive, but there are kits and circuits available on Internet and you can build cheaper analysers yourself.
    These generate a signal on the operating frequency and measure the performance of the antenna at that frequency.
     
  11. Jul 31, 2012 #10

    The Electrician

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    You should be more specific in your requests for help.

    You say you have a low frequency antenna. Instead of saying "low frequency", tell us what the frequency is.
     
  12. Aug 1, 2012 #11
    ok thanx.. so i have an low frequency antena wich the function is to catch ligthing Elektromagnetric (impuls) so my range frekwency is between 20 khz-400 khz.
    now i must make an equicalent circuit of the antena,so what must i do for??
    i try to measuring capacitance,resistance,and inductance of antena with RLC meter on series and paralel mode..
    it's my work right??
     
  13. Aug 1, 2012 #12

    The Electrician

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    As vk6kro said, you need to make your measurements over the frequency range where the antenna will be used.

    What kind of antenna is it? Is it a loop, a long wire, a ferrite rod antenna or something else?

    I suspect that at 2 kHz, your antenna is inductive and that's why your meter indicated a negative value when you tried to measure capacitance.

    You need to use an LCR meter that can make measurements over the 20 kHz to 400 kHz range, and can measure the impedance (Z) rather than just capacitance or inductance.

    Once you have data showing the impedance, real and imaginary part, over your frequency range, you can work on the problem of deriving an equivalent circuit for that impedance.

    Check with your school's instrument inventory and see if you can find an impedance analyzer or a network analyzer to use.
     
  14. Aug 1, 2012 #13
    my antena type is loop antena..
    sorry,honestly i have no idea to make an equivalent circuit of this antena.. if i have an impedance of the antena,so how i must to set the equivalent circuit..

    must i set the circuit like an filter,or T mode?? or the there's any idea??
    cause i have test an antena, i gave an signal.. and i've got the antena's responce output is like bandpass filter.
    so my question,it's my equivalent circuit must have a responce result close to the antena responce??

    thanks..
     
  15. Aug 1, 2012 #14

    vk6kro

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    How big is your loop? That is, what shape is it and what is the diameter (if it is a circle) or how long are the sides if it is a square?

    What is the diameter of the wire?

    Is there a capacitor in series with, or in parallel with, the loop? If so how much capacitance does it have?

    How high above the ground is it?
     
  16. Aug 1, 2012 #15
    my antena shape is octahedron,each hand is 35 cm..
    i dont now exactly the diameter of wire.. it approximately 1 mm..

    there's no capacitor on antena..
    and the antenna is 1.5 m above the ground..

    that's all..
     
  17. Aug 1, 2012 #16

    vk6kro

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    Is it a single turn of wire, or many turns?
     
  18. Aug 1, 2012 #17

    vk6kro

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    I found a calculator for working out the inductance of a single turn octagonal loop.

    http://www.qsl.net/in3otd/ind1calc.html

    I fed in your values, 2.8 m perimeter and 1 mm wire, and it gives 3.79 μH.

    Since it is so small compared with a wavelength at those frequencies, using a value of inductance like that is probably OK.
    According to that, a capacitor of 0.27 μF should resonate at about 150 KHz with this coil.

    The output would be pretty low, though, and you might be able to increase it by adding extra turns.

    What sort of amplifier are you using after this loop?
     
  19. Aug 1, 2012 #18
    only single turn of wire..
     
  20. Aug 2, 2012 #19
    just a single turn
     
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