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Voltage Measurement on a Capcitor

Discussion in 'Electronic Basics' started by James Howe, Dec 12, 2004.

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  1. James Howe

    James Howe Guest

    I'm analyzing a circuit which contains a capacitor. The capacitor is fed
    a constant current and is discharged periodically. The capacitor is a
    non-polarized .01uf capacitor connected on one side to a -12v supply and
    the other side to the constant current source. If I measure voltage on
    one pin, I get approximately -7.5v, the other pin measures around -11.94.
    When I hook an oscilloscope up to the pin with the -11.94, I see a flat
    line which I presume would be the -11.94 v source voltage. When I hook up
    to the -7.5v side, I see a sawtooth ramp which is 8 volts high. I
    expected to see both of these outcomes. What I'm trying to figure out,
    being new to both electronics and using oscilloscopes, is what the -7.5v
    volt meter value represents.

    I'm sure this is a really dumb question and when I see the answer I'll
    probably go 'Doh!'

  2. Brian

    Brian Guest

    Your volt meter is probably reading the RMS voltage, which is the peak
    voltage (of the sawtooth), divided by the square root of 3 (with a little
    bit if DC voltage added in too).
  3. Bob

    Bob Guest

    If you're using a voltmeter when you get the -7.5V reading then you're
    probably seeing either the average or rms value (depending on the type of
    voltmeter you have). The meter's response is too slow for you to see the
    variations in the voltage -- whereas the scope is fast enough.

    Also, be aware that the meter and/or scope can affect the reading if the
    current source is small (in value). With low-amperage current sources the
    current drawn by the measuring equipment can have an effect on the actual
    voltage in the circuit. For example, I recently was measuring the voltage on
    a capacitor that was tied to a non-varying current source. When I hooked up
    a voltmeter, I saw the capacitor's voltage changing -- even though I knew
    that its voltage had stopped changing (before I connect the meter). The
    voltmeter was drawing current from the capacitor and thus caused its voltage
    to change.

  4. The average voltage at that point, relative to whatever point the other
    meter lead is probing.
    Well, you'd better! ;-)

  5. John Larkin

    John Larkin Guest

    -7.5 is an interesting "RMS" voltage!

  6. John Larkin

    John Larkin Guest

    A DC voltmeter reads the average voltage. The average voltage of an 8
    volts p-p sawtooth, riding on a base of -12, should be

    -12 + (0.5 * 8) = -8,

    close to what you're seeing.

  8. Brian

    Brian Guest

    If you subtract the effective RMS voltage of a sawtooth wave that has a
    peak voltage of about 8 volts, from the - 12 volts DC, you come out with an
    effective RMS voltage of 7.38 volts. While this is not an AC voltage, it
    would take an AC voltage of this value to get the same heating effect. Since
    his meter is set to read DC voltage, he will get a negative reading.
    12V - 4.62V = 7.38V
  9. John Larkin

    John Larkin Guest

    But DC voltmeters (usually) read average, not RMS.

    The RMS value of this waveform (an 8v p-p sawtooth whose max negative
    excursion is -12) must be greater than 8, because the average is 8.
    The RMS is actually 8.33.

  10. James Howe

    James Howe Guest

    Ok, that's what I suspected. There was some doubt about whether the cap
    was really discharging completely back to -12 before ramping again, but
    given the wave is 8v p-p and the -7.x v is close to the average if the
    swing was from -12 to -4 I feel pretty comfortable that the cap is
    draining to -12 before recharging.

  11. Brian

    Brian Guest

    Your Right.
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