Create a simple High pass filter to sense AC signal

Discussion in 'The Projects Forum' started by ac_guy, Sep 3, 2014.

  1. ac_guy

    Thread Starter New Member

    Sep 3, 2014
    3
    0
    Hi there,
    I am interested in measuring higher frequencies of the AC signal we get in our homes. I have figured out that in order to do so, I'll have to use a High pass filter (HPF) before connecting it to DSO/Spectrum analyzer which have lower input limits.

    I was wondering if anyone who has done this kind of thing before can share some thoughts. The circuit should be pretty simple. The procurement of parts won't be that simple. Would anyone where to get polypropylene capacitors (0.1 uf) or so.

    Also, would anyone know where to get off-the-shelf HPFs which can handle the 120 V RMS AC signal.
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    Its a serious shock hazard to connect any instrument directly to the power line and should never be done. You need to use an isolation transformer (any standard transformer will do, such as a 12V filament transformer).

    Once you have the transformer you can use an RC HPF to view the signal at its output. You don't need a polypropylene capacitor. Any film or ceramic capacitor should work fine since you don't need to particularly concerned about the exact corner frequency of the filter, just that it's well about 50/60 Hz..
     
  3. ac_guy

    Thread Starter New Member

    Sep 3, 2014
    3
    0
    Thanks a lot for your reply. While I always thought that an isolation transformer would be safer, I wonder how others can do this cheaply. For instance Powerline communication modems send and receive low amp signals at high frequency into the power line. These PLC modems are dead cheap. I would be curious to know how they end up being so cheap and still adhere to safety standards.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    The are isolated either by capacitive coupling and/or transformers from the line. And no external conductive surface is exposed to the power line voltage as would be the case in your measurement with no isolation transformer.

    Understand the danger is that one side of the power line is at earth ground and thus an exposure to the line can give you a shock to earth from just a single touch to the power. A transformer isolates you from that ground so, even if you had a 110V transformer you would have to touch both sides of the transformer output to get a shock, not just one.

    Another danger with external instruments is that their common connection is often connected to the earth safety ground through the power plug. Thus if you accidentally connect the hot line from the mains to the common of the instrument you will get a short with some fireworks, and likely do some damage to the instrument and/or yourself.

    None of that is worth the risk of working directly with the line voltage.
     
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