Need some simple help with a scope filter

Discussion in 'The Projects Forum' started by NashvilleFiero, Jun 28, 2008.

  1. NashvilleFiero

    Thread Starter New Member

    Jun 28, 2008
    3
    0
    Hey Guys!
    This is my first post.

    I know this circuit is fairly simple, but I need some help. I need a circuit to put in line with my test leads so that I can filter out the DC signal from a car alternator. I can look at the waveform on my Snap-On scope.

    Can anyone help?
    Thanks!
    Brit
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    All you really need is a small capacitor, perhaps 1uF. If you use an electrolytic, have the + side towards the alternator wiring. A poly cap would be much better. If your scope has extremely high input impedance, you may need a resistor to ground (say, 47k) on the output of the cap.

    Capacitors block DC, but allow the effects of AC to pass right through.

    See the attached for a simulation of a portion of an alternator, the bridge rectifier, outputs along the way, and the C1/R1 combination I've mentioned. Notice that the simulated O-scope trace from point (E), the green trace is virtually the same as the original output (D), the yellow trace, but is settling down to become centered around 0v; which you need to help diagnose problems with the rectifier bridge or the diode trio/exciter diode.
     
    Last edited: Jun 28, 2008
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    If your o'scope has an AC setting for the vertical input, it is capacitor coupled and will block the DC level.
     
  4. NashvilleFiero

    Thread Starter New Member

    Jun 28, 2008
    3
    0
    Unfortunately, this scope does not have that option :(
     
  5. NashvilleFiero

    Thread Starter New Member

    Jun 28, 2008
    3
    0
    Great info. Thanks!
     
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