How to step down voltage from 240VAC to 20VAC

Discussion in 'The Projects Forum' started by garch, Jan 30, 2010.

  1. garch

    Thread Starter Member

    Aug 23, 2008
    24
    0
    Hi,

    I want to buy one of those hobbiest PC Osilliscope / Logger / Frequeny Analyser etc. However the oscilliscope max voltage range is 0-20V. I want to use it to examine harmonics on the Mains AC (240VAC) line.

    Can anyone recommend a good "low noise" transformer that will not introduce harmonics into the very signal I am trying to measure?

    Also, How do I shift the the stepped down volatge into the 0-20V AC range ie not -10V / + 10V range that would come out the other end of the transformer?

    Many Thanks
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Preserving bandwidth with a transformer isn't easy.

    I'd suggest something more akin to a voltage isolation module such as the one shown in the attachment.

    You may prefer to do a more extensive search to find other options or manufacturers. Off-the-shelf availability may be an issue. I haven't investigated thoroughly.

    http://www.lem.com.cn/docs/products/cv%203-1000.pdf
     
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Your oscilloscope may be able to measure higher voltages with a x10 probe or more.
     
  4. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    Check here, amongst other places. I'm not associated with them; I've just used some of their products and they work fine.
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    How about using a voltage divider consisting of two 1% resistors....]

    Reference the reading to the neutral line (white) wire.
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    This is a cheap but not a good idea at all in your are not a proffesional because there is no isolation and a fault can be deadly.
     
  7. garch

    Thread Starter Member

    Aug 23, 2008
    24
    0
    Thanks everyone who replied. Very helpful.
     
  8. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    I would buy a x100 scope probe. that will make my 20V scale 2000 Volts/cm.
     
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