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help using scope for ESR

Discussion in 'Electronic Basics' started by tempus fugit, Jun 21, 2004.

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  1. tempus fugit

    tempus fugit Guest

    Hey all;

    I'm trying to adapt my scope to measure ESR using the info on these pages:

    I'm not sure I'm hooking things up correctly (although I think I am
    following the schem correctly). When I hook my scope up as shown (hot lead
    on junction of the 2 resistors, ground lead to ground), the screen blanks. I
    start out with a 2V p.p. square wave, which I can view from the output of my
    signal generator, but as soon as I move the probe to the other end of the
    higher value resistor, I get nothing on the screen.

    Also, if I connect a cap in parallel as shown, I get nothing on the screen
    again. The resistor they have in the schem I'm assuming is not an actual
    resistor, but the ESR of the cap. Is that right, or should I be putting a
    resistor in series there?

  2. This method produces a very low amplitude wave (a small fraction of
    the oscillator wave amplitude). You must have the triggering set to
    normal mode, which holds the trace till a trigger event is detected.
    If the amplitude of the signal does not cross the trigger threshold,
    you get no trace. Try turning the vertical amplifier voltage range
    down to a smaller volts per division and/or tweak the trigger level
    knob. If you switch the trigger mode to auto (wait a while for a
    trigger and then release the trace, so you can see what is going on,
    even if it is not synced). This will make it easier to see how big
    the wave is, even if it is rolling across the display. Once you get
    the vertical amplifier set to give you a few division signal, you can
    readjust the trigger level and make the waveform hold still.
  3. Jamie

    Jamie Guest

    i do some side work for a capacitor company locally that make mica caps.
    the instruments that are used there for ESR/Q testing use a method that
    has been ok by the Mil. for years.
    what is done is a Tuned Tank and Air capacitor from a fixed 1 Mhz osc
    with regulated output to keep the level steady. this tank is use to
    null balance a meter. also in this circuit is a POT that adds resistance
    when you connect the Cap under test you need to move the Air Cap to
    get resonance gain or should i say find the dip! you will then notice
    that the output to center the meter will not be on center any more due
    to ESR. then we move the POT control wich has a vernier dial on it until
    we get to peak at the center point.
    now since the POT control is no longer on zero we now know the ESR of
    the cap under test.
    becuase there is such a slite offset in many cases this equipment i
    refer to uses a Crystal pass amp to get very sensitive readings on the
    i suppose with todays Hi-Gain Opamps it can be done also..
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