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Problems with building an ESR meter, part II

Discussion in 'Electronic Basics' started by Simoc, Jul 26, 2006.

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  1. Simoc

    Simoc Guest

    Sorry for reposting, so I posted earlier a thread "Problems with
    building an ESR meter",
    http://groups.google.fi/group/sci.e...ead/a2a2bc5b2520bed9/909e2ffec4e9986f?lnk=arm

    (Question is about a home-made ESR meter, documentation here:
    http://www.ludens.cl/Electron/esr/esr.html )
    Well, now I measured those and got:
    Output: 2.65 V
    - Input: 2.65 V
    + Input: 2.48 V
    Vcc: 4.95 V

    So it seems to be oscillating, although I'm a little confused about
    that all of them are a little above 1/2 Vcc...but might that just be an
    error of DMM in measuring average of pulsating DC?
    On DC range, DMM read just 0 mV...but is it AC or DC across the R6?
    On AC range, the DMM read 8 mV. Across the R5 I got the same readings.

    And btw, if it's AC voltage across the R6, does the DMM give any
    reasonable readings, as it's high frequncy square wave? (as AFAIK
    usually
    the AC range of DMM's is optimized for 50/60Hz sine wave)

    How could I continue troubleshooting now?

    Thanks in advance.
    Simoc
     

  2. Well,

    The readings of the oscillator fall in the expected range. You may try the
    to measure the AC-voltage at the C2/T1 junction. The reading will not be
    accurate due to both frequency and waveform but I expect you to find some
    2Vac. If you find only some mV at this point I'd become suspicious as to
    measure only 50/60Hz hum.

    You'll never find any DC across R6 (unless you made a grave wiring error,
    have a defective cap or some other serious howler). This part of the circuit
    only has very small AC voltages and the 8mV you found, is in the expected
    range.

    The amplifier, U1B, should have Vcc, so 4.95V as the op-amp share the power
    pins. Pin 5 should be at Vcc/2. Take or leave one, maybe two, tenth of a
    Volt depending on the accuracy of R9 and R10. For measuring the output
    voltage of this amplifier short the test leads and temporarely disconnect C6
    at the diode side. Measure between the loose end of the cap and ground.
    Should be some 300mV I guess. If no voltage, most likely your amplifier has
    gone, but there are still possibilities like bad solder joints, shorts or
    other defective components.

    After reconnecting C6, you can measure the DC across C7. Should be some
    hundreds of mV.

    petrus bitbyter
     
  3. Simoc

    Simoc Guest

    Thanks very much for your info. I measured there, and there wasn't any
    voltage.
    But one fault found...the opamp was bad now! As I took it apart and
    wired up for
    testing, I got readings of 2V - 3.3V on outputs and inputs when nothing
    of them
    was connected. So maybe they were just steady DC voltages that I
    measured on output
    and inputs of the oscillator section of the opamp. When I connected Vcc
    to plus
    input or gnd to minus input, the output went high, like it should, BUT
    when I
    connected Vcc to minus input or gnd to plus input, the output went high
    too!

    Btw, you advised to measure AC voltage at the C2/T1 junction, but isn't
    it just pulsating
    DC voltage there?

    I haven't more TL062 chips, nor I have a component store nearby, so
    it'd take less
    or more time to get a new one. But I have a working TLC272CP in my junk
    box. Would
    it work on this circuit? Although probably there is still some other
    mystic fault
    in the circuit as earlier when I was attempting to troubleshoot it, the
    opamp was ok
    when I tested it then.
    BTW, my DMM is true-RMS, so does that mean that it should give correct
    readings
    of the effective value regardlessly to the waveform? Although when I
    took a look on its manual,
    there was rated a frequency range on AC voltage measurement up to
    1kHz...but do you think
    that it's readings will be reasonable enough to measure these 50kHz
    voltages? (although
    it of course won't be as accurate)
    But like John said, there should be about 200mV according to the text
    on the Homo Ludens homepage.

    Thanks.
    Simoc
     
  4. No. Have a look at the schematic. A DC current, pulsed or not, should come
    from some place and go to another. Neither the capacitor nor the tranformer
    will let through DC. (That's to say you can't get DC from primary to
    secundary in the transformer.)
    Don't know. Download the datasheets of both op-amps and compare. You can
    give it a try anyway.
    Can't be sure. The true-RMS may be a disadvantage in this case as it may
    contain electronics that does not work at 50kHz at all. A dumb analog meter
    using only a rectifier and adjusted for 50-60Hz sine wave will give some
    reading up to in the MHz range. (At least mine does.) Totaly inaccurate but
    enough to find out whether or not a 4MHz X-tal oscillator works.
    200mVdc across R6? I'll eat my hat if someone can make me clear how that has
    been done. More likely you'll find it across C7 although I'd expect somewhat
    more. Look at the schematic again. Test leads shorted, no battery involved.
    Where should the DC current come from to cause that voltage? Capacitors will
    not conduct DC so as far as DC is concerned, the circuit ends at C2 resp.
    C3.
    petrus bitbyter
     
  5. Simoc

    Simoc Guest

    Pulsed DC goes through a transformer. Just steady doesn't. In some
    simple SMPSs there is just a pulsed DC fed to the primary. As the opamp
    oscillator outputs just pulsed dc (although it gets connected to gnd
    when not voltage there), is it the cap which makes it ac, by
    discharging itself through the opamp output when it's low? Took a time
    to realize that :) Am I right now?
    Yes, but I don't know (I haven't skills to tell from the schematic :)
    that what are the requirements of the opamp in this case. Do you know
    how to tell? Then I could check them from the datasheets. Those opamps
    have, of course, many differences.
    Yes, but then I'm unable to tell if the opamp is incompatible or if
    there is other fault in the circuit :-( Is there some way then to find
    out by measurements if the inopereability is due to an unsuitable
    opamp?
    Thanks for the info. I haven't an analog multimeter, but maybe I could
    build an ACV meter from a panel meter and rectifier, sometime when i've
    time.
    I didn't say DC! :) (Sorry I quoted too much, that messed up on you)
    It's AC, of course, but I just meant to say that it should be 200mV, so
    8mV is far too low, unlike you stated...

    Thanks for the reply.
    Simoc
     

  6. Yes and no. Pulsed DC is not only steady DC but can be considered to have a
    "pure" DC part and an AC part. The AC part can pass through a capacitor or a
    transformer, the DC part can't. So you'll never find a DC voltage on the
    C2/T1 junction. Only AC.

    At first glance the TLC looks like good enough though I can't go too deep
    into the datasheets at the moment. But you can try it out without problems I
    think. Best instrument for looking at these circuits is an o'scope, alas I
    guess you have none available :) But... You can listen! Replace C1 by a 10nF
    cap and you can hear the 5kHz by an ordinary cheap earphone. You only may
    need to use a series resistor of somewhere between 100 Ohm and 10k to avoid
    blowing it. If everything works fine, you can go ultrasone again. Remind,
    haste is an enemy. Make sure the power supply is ok. Then, build only the
    oscillator, including C2. If you can hear your oscillator, build the Test
    leads part, including C3. Sound will be week but usless your earphone is
    very insensitive, you will hear it on C3 (test leads shorted.) Next part is
    the amplifier followed by the rectifier. Don't forget to replace C1 before
    you start ESR measurements.
    The oscillator produces some 5Vpp/50kHz triangle. It has an RMS value of
    about 1.5V (AC part only, so at the C2/T1 junction.) The transformer ratio
    is 20:1 so at the T1/R5 junction I'd expect a 75mVrms which will become
    lower when loaded with a 5 Ohm resistance. Measured with a multimeter that's
    usually adjusted for 50/60Hz sine, an 8mV looked like acceptable. But I can
    of course be mistaken. I guess the 200mV mentioned in the Homo Ludens
    homepage is the peak-peak value as seen on an o'scope.
    petrus bitbyter
     
  7. Simoc

    Simoc Guest

    Yes, I meant that in flyback-type switchings PSs and regulators, pulsed
    DC is used there, having "only the AC-component" being still DC as its
    polarity never gets reversed. But that's off-topic since I realized
    that the cap, by charging and discharging in turns, makes it "true" AC
    (going in reverse direction, too). Was I right?
    Agreed, if my cap-making-it-AC -theory was right :) (So was it?)
    I could look the datasheet myself, but the problem is just that the
    requirements of this circuit aren't told there ;-)
    Correct guess :)
    Oh, I'm impressed! Very nice trick, I have to try it, thanks.
    Isn't it square?
    Maybe...

    Thanks for the reply.
    Simoc
     
  8. You're right for the AC going through the capacitor.

    You're right for the square as well. The triangle is on the minus input of
    U1A. Which makes my calculations worthless. Hope you nevertheless got the
    idea.

    petrus bitbyter
     
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