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Homebrew HV hiZ scope probe

Discussion in 'Electronic Design' started by TheGlimmerMan, Oct 20, 2012.

  1. Robert Baer

    Robert Baer Guest

    I see you made Cs, capacitance from floating shields, a lot larger =
    8pf instead of my wild guesstimate of 0.1pf.
    Those floating shields allow defined and predictable capacitive
    coupling across the resistors; the capacitance from them to "outer
    space" ground seems to be undefinable: coax capacitance runs
    (log(D/d))^-1 and with a theoretically infinite D (or very large D in
    reality),the capacitance is rather close to zero.
    I picked 0.1pf as an estimate to that "zero"; seems you picked a huge
    8pf.
    *
    I did not know of that uniform RC line (U) model; it would be an
    excellent choice; better than the pi-pad scheme.
    Values can easily be calculated on basis of resistor diameter,
    dielectric material and floating shield diameter.
    Seems that an eXplicit shield around the floating shield makes for
    more trouble.
     
  2. Fred Abse

    Fred Abse Guest

    No, I refer you to your previous post,<Tzphs.1182$>:


    "TEXT 168 160 Left 2 !.PARAM Cr=10p, Cs=7.9p, Rp=200Meg"

    Cs=7.9pF

    Exact copy of your published schematic, with the addition of 3 feet of
    cable, and two resistors.

    I wondered about that.

    The same value occurs in the listing on your website.

    I didn't pick it, you did ;-(
     
  3. Fred Abse

    Fred Abse Guest

    You can't ignore the effect of what is, in effect, an open-circuit quarter
    wave line, at a frequency within the intended bandwidth of the probe.

    3 feet of 0,66 velocity coax, terminated in a large resistance will
    resonate at about 54MHz, and ring like a bell. That's why resistive cables
    are used.
     
  4. Do you have any idea where one can buy resistive cable?

    Jeroen Belleman
     
  5. Modern automotive "spark plug wire". Has a "graphite core". Specific
    R per foot value. Meant to be an emission suppressive passage for HV
    pulses being fed to the plugs, something which generally causes a pretty
    big magnetic spike to emanate.

    There were folks in the '50s and '60s (hams) who even went so far as to
    fully ground shield each of their spark plug wires to reduce the problem.
    Then they (those 'they' people) came out with the better, resistive wire
    and the problem sufficiently subsided..

    May not be exactly what you seek, but that is the only version I am
    aware of.
     

  6. Probably easiest to buy a scope probe and cut the ends off!

    See: Patent number: 2883619
    Issue date: Apr 21, 1959
     
  7. Jamie

    Jamie Guest

    We had an old car here once that I decided to tackle a blower problem
    in the heating and ventilation system. THe problem, no blower operation.

    After tearing out the blower box under the glove compartment, which
    was a big job getting to, I found there was nothing wrong with the
    blower motor. The plug was too hard to get to so it wasn't an easy
    option to simply test it before hand. So I went to the control switch,
    that worked ok. So I then started ripping apart the harness, they had
    put a resistor wire in the harness for the blower to reduce the brush
    noise in the electrical system.

    At that point, I really didn't care much for the car, it was getting
    old, so I just hung a copper wire outside of the harness.. I can say
    this, not only was that wire there to reduce noise, it also served as a
    voltage drop to the blower, that thing was like a wind tunnel afterwards :)

    Jamie
     
  8. Jamie

    Jamie Guest

  9. Jamie

    Jamie Guest

  10. Fred Abse

    Fred Abse Guest

    I suspect that probe manufacturers get their resistive cables made to
    order. That implies a few thousand feet to get a cable manufacturer
    interested.

    *Almost* the same effect can be got by putting a small (tens to hundreds
    of ohms) in series with each end, if all you want is to damp resonances in
    a probe.

    You might try a few of the "usual suspect" cable manufacturers. There are
    a few specialist cable companies in Germany who claim to make small
    quantities of specials to order.
     
  11. Jeroen

    Jeroen Guest

    It's the scope probe type cable I'm after indeed. I contacted Draka
    to see if they could deliver any. They said they were willing to make
    it to my specs, provided I'd buy at least 1km. I'd need a dozen or so
    five-meter pieces. Oh well. Maybe Spehro was right: Just chop of the
    unwanted ends off a scope probe.
    I tried that. Not good enough.
    Thanks, I'll shop around.

    Jeroen Belleman
     
  12. Robert Baer

    Robert Baer Guest

    Oops..now i see an unreasonable Cr of 10pf; that would total to 50pf
    on a resistor; VERY hard to implement.
    Dimensions:

    +++ +++
    | |++++++++++++++++++++++++++| |
    ----| |----
    | |++++++++++++++++++++++++++| |
    +++ +++
    ^-------metal end caps-------^

    Metal end caps 0.285 dia, body 0.265 dia, end cap width 0.180, and
    end-to-end width 2.085 inches
    Most plastics (for insulation and support of the floating shield)
    have roughly 3 for dielectric constant, and a tube minimum diameter
    would contact only the end caps at best; 1/16 wall thickness at smallest.
     
  13. Robert Baer

    Robert Baer Guest

    It has been a loooong time; Tektronix made two high voltage probes -
    one with a significantly higher voltage rating.
    The one i have _did_ have the liquid inside but that has slowly
    leaked out over the ages; it is the P6015 rated at 20KV, 40KV peak.
    I think that is the top end in voltage.
     
  14. Robert Baer

    Robert Baer Guest

    Perhaps, but us mortals with a limited budget cannot order umpteen
    thousand feet of coax with a resistive center.
    The P6015 used ten feet of coax with resistive center (direct quote
    from manual).
    Cannot say what its resistance is..i read 657 ohms end-to-end on the
    cable, and the schematic states 150 ohms 10% at probe end, implying the
    coax center is about 500 ohms (~5 ohms per foot).

    Since you specified 75 ohm cable, the R should then be 75 ohms
    instead of 60 ohms (pickie).
     
  15. Robert Baer

    Robert Baer Guest

    Yes...all it takes is a lot of $$ for the special order.
    Unless one is VERY fortunate to find it in a surplus house or willing
    to rat out the cable from a P6015..
     
  16. Robert Baer

    Robert Baer Guest

    Did a Baby Bird (GooGull) snoop..NGK resistive wires "utilize a
    construction method known as "variable pitch" wire winding to create
    resistance to radio frequency interference. NGK wires have a lower
    resistance than conventional carbon core wires (8k ohm/meter vs. 16k
    ohm/meter)."
    So these "better" spark plug wires have a "mere" 2.4K/foot
    resistance, which exceeds a desirable amount by about an order of magnitude.
    Excellent suggestion, however..
     
  17. Robert Baer

    Robert Baer Guest

    SACRILEGE!!!
     
  18. Robert Baer

    Robert Baer Guest

    If you were a true tree huger, you _would_ "pine" for them..
     
  19. Robert Baer

    Robert Baer Guest

    * Check.
     
  20. Robert Baer

    Robert Baer Guest

    That is exactly what was used as a reference by Fred Abse in his
    Spice listing (Belden 83265 is RG-178B/U 50 ohms) .. but data used was
    for the RG179 which is 75 ohms but otherwise rather similar.
     
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