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sampling 240VAC

Discussion in 'Electronic Basics' started by Thomas Magma, Sep 11, 2009.

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  1. Thomas Magma

    Thomas Magma Guest

    I'm looking for the best way to sample the 240VAC waveform from a powerline
    well staying within my design constraints. I would like to go
    transformerless because of size requirements and I would like to have a
    fairly high bandwidth, so I am looking into other options other than a high
    impedance resistive ladders. Does anyone have any suggestions on how to do
    this in a modern fashion? What about using a hall effect IC and a load
    resistor to convert the current back to a low enough sampling voltage?

    Thanks,
    Thomas
     
  2. Rich Grise

    Rich Grise Guest

    Hack a 'scope and use its attenuator?

    Good Luck!
    Rich
     
  3. Thomas Magma

    Thomas Magma Guest

    I looked into a few schematics of scopes and looked at their attenuators.
    The circuity looked a little tweeky with the use of communication capacitors
    to overcome the RC input capacitance. I'm kinda looking for an 'outside of
    the box' modern approach (if it exsists).

    Thomas
     
  4. Thomas Magma

    Thomas Magma Guest

    Thanks John,

    I hadn't really thought of capacitors as voltage dividers and you got me
    thinking, which lead me to determine that the reactance and frequencies
    would make it difficult to design anything that would be stable. First off
    you would be AC coupled which would prevent you from seeing any low
    frequency or DC offsets. Secondly, the large capacitor values needed to
    divide the low frequencies would be subject to inductive reactance at the
    higher frequencies and also subject to thermal tolerances.

    It would probably be wise to steer clear of any reactive components, when
    trying to design a higher bandwidth analyzer.

    Thomas
     
  5. Rich Grise

    Rich Grise Guest

    I once bypassed a large resistive divider to stop an HVPS "hunting". It
    didn't get approved, because every arc on the HV wrecked the input
    circuit, but if you can protect your input from LARGE glitches, and/or
    don't expect arcs, you might be able to get away with it.

    Good Luck!
    Rich
     
  6. You can get some farily small <1VA transformers, like this:
    http://australia.rs-online.com/web/...d=searchProducts&searchTerm= 310-1285&x=0&y=0

    22mm x 22mm x 15mm

    Dave.
     
  7. Phil Allison

    Phil Allison Guest

    "Thomas Magma is a Pisshead "

    ** Don't expect to see frequencies below 50Hz on the AC supply and there are
    no " DC offsets " as such - but the average value may sometimes become non
    zero due to uneven negative and positive peak values.

    There is nothing wrong with using a resistive divider to monitor the mains
    active relative to ground.


    ** ROTFL !!

    You are talking straight out your arse - fool.


    ** The caps would be at the same temp - fool.

    ** You have no idea how to design anything.

    There is STILL nothing wrong with using a resistive divider to monitor the
    mains active relative to ground.


    ...... Phil
     
  8. Phil Allison

    Phil Allison Guest

    "John Fields"


    ** The OP is a trolling fool with no clue about anything - consequently he
    has not considered the safety angle at all.

    But YOU should have !!!

    What you have drawn is potentially lethal.



    ..... Phil
     
  9. amdx

    amdx Guest

    I take it the risk is with the 26.8nF shorting and putting line voltage on
    the output?
    How about replacing the 26.8nF with 4 series 107.2nF 1000 Volt caps, with a
    5.1V
    zener on the output and a suitable fuse? If one 107.2nF shorts the output
    voltage
    goes high causing the zener to conduct and opening the "suitable fuse".
    Heck, let's put a crowbar on it.
    Running for cover :)
    Mike
     
  10. Phil Allison

    Phil Allison Guest

    "amdx"
    ** NO !


    ..... Phil
     
  11. Thomas Magma

    Thomas Magma Guest

    I'm starting to lean towards resistors. I guess the only problem with high Z
    resistors is that it will form a RC with the input capacitance of the ADC.
    In oscilloscopes they put coupling capacitors in parallel with the resistors
    to compensate for this effect. I haven't done the math to see were the
    roll-off is though for my design and bandwidth.

    Transformers are too big for the design and don't have the required
    bandwidth. Also I haven't found one with a <10mm profile for that frequency.

    I'm really just fishing to see if there was any alternative designs.

    I'm not sure what is 'deadly' about using capacitors. Phil must be worried
    that I'm designing him a new ass dildo.

    Thomas
     
  12. Thomas Magma

    Thomas Magma Guest

    The use of an opamp is definitely a good idea. A good way to set gain and
    frequency response.



    I never said Phil was wrong...perhaps a little vague. I do apologize for my
    moment of weakness. I do expect an earful.

    Thomas
     
  13. Phil Allison

    Phil Allison Guest

    "George Herold"


    Phil, If I ask nicely will you tell us what is the problem with the
    series capacitors?


    ** Can a 2.65uF cap connected to a 240 volt AC supply pass enough current
    to kill ??

    Whenever you see some circuit powered directly by the AC supply - ask these
    two what ifs:

    1. What if the phase and neutral connections are reversed ?

    2. What if the neutral is not connected but the phase is ?

    Consider if the circuit breaks safety standards in either of the above
    scenarios.

    Is it still safe to use, handle and connect to other devices ?



    ..... Phil
     
  14. Jasen Betts

    Jasen Betts Guest

    the two methods you have rejected are the best that I know of.


    perhaps you can make a capacitative divider work for you?
     
  15. Phil Allison

    Phil Allison Guest

    "George Herold"
    "Phil Allison"
    "George Herold"
    " ** Can a 2.65uF cap connected to a 240 volt AC supply pass enough
    current to kill ??"

    Call it an impedance of a few k ohms. (at 50/60 Hz) that'll be 100mA
    or so.... sure enough to kill me!

    Thanks for the pointers.

    Isn't this still going to be a problem even with resistors?


    ** Not if the resistance in series with phase and neutral is 100kohms or
    more.

    The resistors need to be adequately voltage rated too.


    ....... Phil
     
  16. Phil Allison

    Phil Allison Guest

    "whit3rd is a witless turd "

    3. The circuit breaker protects this circuit, right?

    ** Not relevant to the electrocution hazard of an appliance that plugs in.


    4. In case of (for instance) lightning, it isn't going to become a
    bomb, is it?

    ** Wot insane crap.


    In some cases, a capacitive load will contribute to arcing that can
    kill the circuit breaker.

    ** Totally irrelevant to a load of 26nF - fuckwit.


    the capacitor
    divider (and the resistor input circuit, too) will take current from
    the AC and divert it to GND. That will trip GCFI protection ..


    ** Bullshit.

    The AC current is only a mA or two in any sane design.

    Piss off - you RABID NUTTER !!!



    ..... Phil
     
  17. Phil Allison

    Phil Allison Guest

    "George Herold"


    "The AC current is only a mA or two in any sane design."
    ** Then apply ohms law - dickhead.


    ** The OP has never designed, made or sold anything.

    Get real.


    ..... Phil
     
  18. Phil Allison

    Phil Allison Guest

    "George Hair Oil"

    " ** The OP has never designed, made or sold anything."

    Really, how do you know that?



    ** Cos he is VERY obviously a totally clueless TROLL

    You pathetic dope !!!
     
  19. Thomas Magma

    Thomas Magma Guest

    Bandwidth limits are not really specified by IEC 61000-4-30 or IEEE C37.118.
    DC to 10kHz for sure, but we would like to design in the ability to go up to
    several hundred kHz for the "who knows" factor.

    (Yes very active in R&D Phil...r u)

    Thomas
     
  20. Phil Allison

    Phil Allison Guest

    "whit3rd is a witless TURD "


    the capacitor
    divider (and the resistor input circuit, too) will take current from
    the AC and divert it to GND. That will trip GCFI protection .


    ** Bullshit.

    The AC current is only a mA or two in any sane design.

    Piss off - you RABID NUTTER !!!



    ..... Phil
     
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