current waveform

Discussion in 'General Electronics Chat' started by ronn, Dec 21, 2007.

  1. ronn

    Thread Starter Member

    Nov 1, 2007
    is there a way how to make the oscilloscope display the current waveform?
    books said it only show the voltage waveform
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    If you place a resistance in series with the current, then you get a voltage waveform that is dependent on the current (E = IR).
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    This is the standard method, but there are a few wrinkles.

    You need a low value resistor, so as not to disturb the circuit too much. Look to make the ohms law voltage a few hundred millivolts. Make sure the resistor has enough power capacity for the expected current.

    If your ac signal has a frequency greater than a few hundred hertz then you must use a non inductive resistor. Since low value resistors are often wire wound it is worth checking the type. Non inductive wire wounds are made by winding either in a zig zag or by doubling the wiring and winding on a former.
  5. Externet

    AAC Fanatic!

    Nov 29, 2005
    Direct current would be better monitored with an ampermeter, so assuming you want to see alternating current on the oscilloscope; pass the current carrying conductor inside a coil and connect the oscilloscope to the coil terminals. The more coil turns, the more sensitive to small currents.
  6. Distort10n

    Active Member

    Dec 25, 2006
    What do you mean 'non-inductive'? There will always be parasitic inductance, albeit, it can be low: ~nH. Believe it or not, using a 0.1Ω current sense resistor with a current shunt monitor will have nH worth of inductance and will kill accuracy when the signal is a sine wave.
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
    I think we are talking powerline frequencies here or at highest a few hundred hertz.
    Parasitic inductance is a phenomenon important at multi - MHz to GHz range.

    If you are really interested, I first came across a description of non inductive emitter resistor construction in the article by Dr Bailey about his famous amplifier. If you double the resistance wire and wind the bifilar result around the former each half of the winding produces an equal and opposite self inductance, thus cancelling the overall inductance of the resistor.