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Homebrew HV hiZ scope probe

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

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  1. josephkk

    josephkk Guest


  2. josephkk

    josephkk Guest

    How squirrely is this going to get?

  3. Robert Baer

    Robert Baer Guest

    Correct, except it is 1000:1; all else is OK.

    I am purposely using a larger capacitance as i am not mechanically
    inclined; easy to put plastic tube around resistors, centered with thin
    sheet, and wrap adhesive copper foil on outside cut for same length as
    Easy to calculate capacitance of the resulting coax (resistor / air /
    inner tube surface / acrylic / outer tube surface = = floating shield.
  4. Robert Baer

    Robert Baer Guest

    Thought of a way to (perhaps) make resistive coax with element in the
    50 ohm per foot region.
    Find some Nichrome or other resistive (heater) wire,and use Teflon
    sleeving in successive layers or use shrink tubing in successive layers.
    Slip braid around that.
  5. Yew guys are knot getting board with these jokes?

    ("My Tree Puns"... Fred MacMurray?)

    Best regards,
    Spehro Pefhany
  6. Fred Abse

    Fred Abse Guest

    The discussion is about oscilloscope probes. Thousand meg. DC HV probes
    are a dime a dozen. Most for use with 10M DMMs, so there's a 1M load
    resistor in those. I have one. Don't use it much.
    Could be. I just looked up the CFC dielectric properties table in Kaye and
    Laby. All there are around 2. It doesn't specifically mention 1.1.4.
    I don't do Yahoo groups.
  7. Fred Abse

    Fred Abse Guest


    My usual convention for transfer functions is Vout/Vin.

    Equal to 1/1000.
    Or 1:1000

    BTW, I would never use a high-voltage probe that didn't have an internal
    load .

    Consider the situation where there is HV applied, and the scope amplifier
    is inadvertently switched to GND (which means the input socket will be
    open circuit).
    Practically the full HV will appear at the amplifier input socket.
    The insulation won't stand that.

    Even worse, a 7A13, switched to "near infinite impedance".

    Relying on the scope's input resistance is dangerous. We all make mistakes.

    I'm sure that UL, CSA, VDE, TüV, et al. would agree.
  8. Robert Baer

    Robert Baer Guest

    I vaguely one of them can be soldered; which one?
  9. Robert Baer

    Robert Baer Guest

    I vaguely remember that one can be soldered; which one is that?
  10. Robert Baer

    Robert Baer Guest

    Look at it this way; two coax capacitors in parallel one inside the
    First one from resistor as center conductor and inner surface of
    plastic tube for "outer" coax and air being the dielectric.
    Second one has its center conductor as the inner surface of the
    plastic tube, and the outer coax is the outside that has the conductive
    foil; the plastic being the dielectric.

    In physics, that inner surface would be called a Gaussian surface.
  11. Constantan. Type T (Cu-Constantan) can have junctions soft-soldered
    without any kind of special flux. Some J (Fe-Constantan) wire has the
    iron copper-clad so the same is true.


    Best regards,
    Spehro Pefhany
  12. Robert Baer

    Robert Baer Guest

    I knew one of them could be that is the wire to use if
    the right resistivity can be has with a "reasonable" wire gauge (for the
    sleeving and outer braid shield).
  13. Fred Abse

    Fred Abse Guest

    Have you forgotten the stated end-to-end capacitance per Ohmite data?

    In the case of MOX-2-13XXXX it's 0.6pF. I suspect that this will be the
    dominant parameter.
  14. Robert Baer

    Robert Baer Guest

    Tried the BabyBird (GooGull); most hits were Chinese sellers, many do
    not say how to buy, those that do say how much do not give resistivity
    of the wire (ohms per unit length).
    So still have zero idea if any of the wire that might be available is
    in the right resistance range.
    Nobody seems to have a range of wire sizes.
  15. Fred Abse

    Fred Abse Guest

    The sample was too long to get meaningful open and shorted measurements,
    but the TDR showed some unexpected results.

    On a 1.2 meter length of cable. transit time was 9.5 nanoseconds, giving
    a velocity of 42.11% of c. That corresponds to a dielectric permittivity
    of 5.64, which is too high for any flexible dielectric I know of. That
    suggests that the inner conductor is a helix. Resistance is 186.66 ohms
    per meter. Inductance calculates (from rho at the sending end, and
    velocity), to be 1.07 uH per meter, and capacitance 58.6 pF per meter.

    The following model corresponds quite closely with measured data:

    ..model scopecbl ltra (
    + len=1.2
    + R=186.666
    + L=1.07E-006
    + C=5.86E-011)

    The following is a good approximation to what the TDR shows. Change the
    time (X) axis to "time/2" to show one-way time.

    Version 4
    SHEET 1 880 680
    WIRE -160 128 -320 128
    WIRE -16 128 -64 128
    WIRE -320 272 -320 208
    WIRE -160 272 -160 160
    WIRE -160 272 -320 272
    WIRE -64 272 -64 160
    WIRE -64 272 -160 272
    WIRE -16 272 -16 160
    WIRE -16 272 -64 272
    WIRE 32 272 -16 272
    WIRE 80 272 80 160
    WIRE 80 272 32 272
    FLAG 32 272 0
    SYMBOL ltline 32 144 R0
    SYMATTR InstName O1
    SYMATTR Value scopecbl
    SYMBOL voltage -320 112 R0
    WINDOW 3 -159 -8 Left 2
    WINDOW 123 24 132 Left 2
    WINDOW 39 24 28 Left 2
    SYMATTR Value PULSE(0 1 0 22p 22p 1u 2u 1)
    SYMATTR SpiceLine Rser=50
    SYMATTR InstName V1
    SYMBOL tline -112 144 R0
    SYMATTR InstName T1
    SYMATTR Value Td=1n Z0=50
    TEXT -312 384 Left 2 !.tran 0 100n 0 1p
    TEXT -312 336 Left 2 !.opt plotwinsize=0
    TEXT -40 336 Left 2 !.model scopecbl ltra (\n+ len=1.2\n+ R=186.666\n+ L=1.07E-006\n+ C=5.86E-011)
    TEXT -312 360 Left 2 !.plot v(n001)
    TEXT -176 72 Left 2 ;TDR Simulation
  16. Robert Baer

    Robert Baer Guest

    Speak of variable results..
    Concerning the original Q&D probe where the scope must be DC coupled
    ONLY, i have determined that pickup of external signals (hum and related
    bazz-fazz), having the floating shield is a bit better than not having
    it at all.
    The negative of that is, getting pinkies close to that shield does
    severely compromise the risetime & undershoot of the probe.

    Concerning the "development" of the two 1G internally terminated
    probe, each of those resistors seem to be too long and pickup becomes
    intolerable, and a floating shield might have to be referred to ground
    (maybe a resistor).
    It also looks like a multiple pi-pad model is not the best for
    modelling these resistors..
  17. Fred Abse

    Fred Abse Guest

    I wouldn't want fingers anywhere near an energized HV probe. The right
    place to put compensation adjustment it at the scope end, like most good
    commercial probes do.
    Use a fixed shield, referred to ground, and work with whatever capacitance
    you're left with. Minimize capacitance to ground as much as possible, it's
    the end-to-end capacitance that *really* matters for compensation.
    According to Ohmite, that's 0.6pF. For a 1:1000 probe, that means 600pF at
    the scope input.
    multiple pi-pad model is not the best for
    I'd agree with that. You need to do some work to determine what they
    really look like.

    As an aside about modeling, I discovered that LTSpice supports the TXL (Y)
    transmission line model, though it's undocumented. It runs faster than the
    LTRA model, and also supports full RLCG, whereas LTRA only supports
    RLC. G can be a parameter, which suggests that may be a way of modeling
    frequency-dependent dielectric loss.
  18. Robert Baer

    Robert Baer Guest

    I could find no end-to-end capacitance spec for these resistors; i
    calculate 0.125pF end-to-end on the basis of endcap spacing and aluminum
    oxide insulator.
    See probes/1Gresistor.pdf for
    dimensions and calculations.
    The cylindrical capacitance info at the bottom of the second page is
    reference only for now.
  19. Fred Abse

    Fred Abse Guest

    MOX-2-12 series 0.6pF end to end.

    Ohmite PDF catalog October 26 2011. "Power resistors, Component Selector

    Page 93 in PDF version, labeled 85. "Maxi-Mox".
  20. Robert Baer

    Robert Baer Guest

    Well,i snooped around their site and could find no way to get or find
    that catalog.
    No way to get that description, etc.
    Dead end.
    The best i could find, using MOX as a 3-character selector for part
    number, thengetting the PDF for the Max-Mox series,was a one page PDF
    named res_maximox.pdf; page labeled 85.
    Now i see (only since you pointed it out) the 0.60pF .
    On the basis of my calculations, i do not see how it can be that large.
    I guess i will have to make a rather sensitive capacitance
    measurement device, as DVMs tend to imply the low value i calculated.
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