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Why do we need a voltage regulator here?

  1. Nov 16, 2011 #1
    Hi all

    Here is the circuit that I will refer to: LINK : http://electronics-diy.com/lc_meter.php

    Ok my first question, Why do we need a voltage comparator (LM311) there? A voltage comparator compare the + and - voltages and output the difference??

    What I want to do is, measure the inductance of the L and then send that value to a computer for further analysis.

    Thanks o:)
  2. jcsd
  3. Nov 16, 2011 #2


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    The LM311 is working as an oscillator in this circuit and the frequency depends on the values of the inductor and capacitor in the circuit between pins 2 and 3.

    The output is taken to the microprocessor from pin 7, so you could take this output to your computer if you wished. You would have to calibrate the output yourself, but the component values may be directly related to the output frequency.

    The original kit has a programmed PIC 16F628 chip in it, so an unprogrammed one would not work.

    It does seem a bit overpriced, though.
    You could get one of these:
    for about $16.
    Last edited by a moderator: Apr 26, 2017
  4. Nov 16, 2011 #3
    Thanks for that vk6kro :smile:

    Im still a bit confused as to how that circuit works. Let me divide that circuit into 3 parts:

    1) Oscillator: Does this provide the alternating current the LC circuit needs to oscillate at resonant frequency? Does it also modify the frequency as the L in the LC circuit changes?

    2) Battery : Circuit is using a 5V DC battery I believe

    3) LC circuit
  5. Nov 17, 2011 #4


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    No its most likely using a 9V battery There's a 5V regulator there to provide a stable 5V for the osc cct and the PIC processor etc. The 7805 regulator needs at least 2V headroom for it to regulate correctly

  6. Nov 17, 2011 #5


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    The oscillator uses a LM311 with positive feedback, but the feedback happens at the resonant frequency of the tuned circuit formed by the external L or C and carefully chosen internal components.

    So, the oscillation happens only at the resonant frequency of the tuned circuit.

    The oscillator is the entire combined LC circuit and the comparator chip.

    The LM7805 chip produces 5 volts out but it must have about 7 volts minimum input to produce 5 volts out.
    So, the supply can be 7 volts, 8 volts, 9 volts or anything up to about 37 volts which is the maximum rating of the LM7805. The chip will waste more power and get hotter with higher voltages.

    There is a clearer diagram in this article and it may be easier to see how the oscillator works:
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