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Homebrew HV hiZ scope probe

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

  1. This HV load bank took a while to build.

    http://www.mediafire.com/view/?r7mxc48guymk9#o19a4alp8vk1kkb


    HV Probes are easy. A sealed dry AIR based (or gas) container for a
    precision HV resistor string. Potted versions lose accuracy and are
    harder to make calibration adjustments on.
     
  2. Robert Baer

    Robert Baer Guest

    See similar title in a.b.s.electronic for PDF.
    Almost took longer to document than to build..
     
  3. alt.binaries.schematics.electronic ?
    I get that group, but I don't see the post.

    Cheers
     
  4. Robert Baer

    Robert Baer Guest

    * Bet it took a fair amount of time for that labeling in the image.
    Would adding heat shrink on the 1% resistors mess them up at high (up
    to 20KV) voltage?
    As an alternate, what about wrapping them with Kapton(TM)?
     
  5. legg

    legg Guest

    Ditto

    RL
     
  6. legg

    legg Guest

    I believe its Baer's post that's gone astray.
    RL
     
  7. Jasen Betts

    Jasen Betts Guest

    freenews.netfront.net carries ABSE but only the smaller posts (not
    large binaries)

    For the binaries Astraweb.com 180GB $25 never expires. Since march
    ABSE has used aboiut 0.68GB, so at that rate I'll be putting it in
    my will.
     
  8. Robert Baer

    Robert Baer Guest

    Somehow the post vaporized, so i put it up on my corporate site at
    http://www.oil4lessllc.org/HV probes/ .
    That probe was a "quick and dirty"; rise time seems to be better than
    20nSec (my "pulse generator" is a HP3312A function generator).
    However, the output step only goes up 70% then a slow rise to max
    with a time constant of about 600nSec.
    *
    Am using SPICE to help design a 40KV probe and i seem to be homing
    into a design with values.
    The listing is below (and on that site).
    The attenuation ratio is 2500 and is "protected" to ground like the
    Tektronix probes.
    Why 2500? Because it takes two 1Gohm resistors to withstand the 40KV
    (target rating is 30KV with some "elbow room").
    Note the resistance ratio is exactly 2500:1 and (now) the capacitance
    ratio is close also.

    Physical layout is to have a floating ring around each resistor,to
    provide a controllable input coupling and capacitive divider (which is
    the secret of a (theoretical) infinite risetime.
    Now around this whole assembly will be a grounded shield (to isolate
    input from external bazzzz fazzzz).
    I think that five sections is a reasonable division of each resistor
    for emulation of the actual distributed part.

    Risetime seems to be infinite, but there is this slow "hump" that i
    am fighting.
    Any ideas as how to solve?

    Version 4
    SHEET 1 2064 680
    WIRE -352 -16 -448 -16
    WIRE -208 -16 -272 -16
    WIRE -64 -16 -128 -16
    WIRE 80 -16 16 -16
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    WINDOW 3 24 8 Left 2
    SYMATTR InstName C31
    SYMATTR Value 2478p
    SYMBOL res 1312 48 R0
    WINDOW 3 30 126 Left 2
    SYMATTR Value 4.008Meg
    SYMATTR InstName R31
    SYMATTR SpiceLine tol=0.1 pwr=1
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    SYMATTR Value 1Meg
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    SYMATTR Value {Cr}
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    SYMBOL cap 768 160 R0
    SYMATTR InstName C22
    SYMATTR Value {Cs}
    TEXT 264 256 Left 2 !.tran 0 10m 0 10n
    TEXT 264 -208 Left 4 ;40KV 2E9 ohms HV scope probe
    TEXT -144 -120 Left 2 ;Ohmite MOX2-131007FE\nin pi net distributed form
    TEXT 168 160 Left 2 !.PARAM Cr=10p, Cs=7.9p, Rp=200Meg
    TEXT 672 -120 Left 2 ;Ohmite MOX2-131007FE\nin pi net distributed form
     
  9. legg

    legg Guest

    On Mon, 22 Oct 2012 22:04:28 -0800, Robert Baer

    It's conventional to include a 'tip' resistor, that will absorb
    contact (arc) surges safely.

    The hardest thing about HV probe structure, is getting practical
    values that are physically possible, to do the job. Look at physical
    embodiments, make a few measurements, then go to the model.

    Not much point in speculating over something that you can't build.
    Sizing the capacitive divider is a real physical issue. Your Cr/Cs
    ratio may be impractical.

    As to risetime, an intentional RCseries, paralleling C22 position can
    act on leading edges. Look at the current midpoint voltage (jn
    C10/C11). Also, run an ac sweep.......the LF and HF gains will meet at
    some point that will look like a can of worms, and is just as easily
    manipulated, given physical constraints.

    RL
     
  10. legg

    legg Guest

    Have you attempted a spice simulation of the part you've actually
    built and tested? This would be more than just instructive and assist
    in developing the newer iteration and an understanding of the spice
    derivative.

    RL
     
  11. Robert Baer

    Robert Baer Guest

    * The only guides i have so far,are the Tektronix P6015 probe and the
    "quick and dirty" 1Gohm probe i built.
    Unfortunately, i have no way to making reasonable guesses related to
    capacitances involved in their construction. The few P6015 drawings
    leave a lot to be desired, and measurements on my Q&D probe would be
    extremely difficult to do due to some of the very low values.
    * But i have one working one, and the model for it.
    And as far as sizing the capacitive divider, as long as there is a
    simple capacitor from tip to output, and its ratio to a total
    corresponding capacitance from there to ground, there is no problem;
    that is simple - for a simple model.
    * The Cs was tweaked for "best looks", and could be easily done
    mechanically. The Cr is done via floating coaxial "shield" around the
    resistor (see the aluminum foil wrap on the Q&D probe i built).
     
  12. Robert Baer

    Robert Baer Guest

    "non-linear" can mean many things..voltage coefficient, temperature
    coefficient, and resistance per unit across the length (i assume that is
    what you are referring to).
    Where is that "report"? What date?
    I do not think the resistance per unit length varies much, certainly
    not so much as to give the waveform seen in the simulation.
    That can be varied by quite a bit by changing Cs and/or Cr.
    However, it might be a GOOD THING (as Martha Stewart would say) to
    use ten pi pads instead of five for the resistor.
     
  13. Robert Baer

    Robert Baer Guest

    Yes, but i started with a very simple C across the 1G resistor and
    one to ground; it was plain that i needed (so to speak) more cable
    capacitance,and better adjust-ability of that.
    Made no refinements.
    Basically, that probe was an exercise of "can it be done"?
    And it has the noted deficiency of no ground return inside (hence the
    big fat warnings).
     
  14. Robert Baer

    Robert Baer Guest

    See http://www.oil4lessllc.org/HV probes/ for updated PDF.
    Have added two more scope pictures and modified closing notes.

    This shows that the fundamental risetime is faster than 20nSec.
     
  15. Fred Bartoli

    Fred Bartoli Guest

    Robert Baer a écrit :
    Wow, you're advertising the use of a mutilmeter (pun) probe for 7kV usage...
     
  16. MrTallyman

    MrTallyman Guest


    You're an idiot.

    First, you have no clue what "high power" is.
    You have no concept of stored energy.
     
  17. Robert Baer

    Robert Baer Guest

    Yep; cheap, small, does the job.
     
  18. legg

    legg Guest

    Does the model act as the physical part scopes out?

    RL
     
  19. Fred Abse

    Fred Abse Guest

    Takes forever to converge.

    I softened the analysis a bit by making the pulse risetime 20ns, similar
    to your HP3312A, but it's still excruciatingly slow.

    How sure are you of the resistor model? Have you considered making it a
    uniform RC line (U) model? I'll try that in a while, it might converge
    faster.

    There's a pole at around 67 Hz that needs addressing.

    I added 3 feet of RG179, which i have a model for, which is similar to
    RG175, for which I don't. The inevitable quarter-wave spikes appear at
    around 46MHz, et seq. 60 ohms in series with each end of the coax tames
    this, and we now have a 3dB rolloff around 70MHz, making risetime about
    5ns. That's why resistive cable is used in commercial probes.

    I'd do what Tektronix do, and do all compensation at the 'scope end.

    Bear in mind that Tek's 40kV probes used to run their HV resistor in an
    atmosphere of Fluorcarbon 114 vapor, which needed topping up from time to
    time. I don't know what they use these days in HV probes, but I doubt it's
    FC.

    Try this, it's your circuit with 3 feet of coax. Do an .ac analysis, with
    and without the 60 ohm resistors.

    Version 4
    SHEET 1 2064 680
    WIRE -352 -16 -448 -16
    WIRE -208 -16 -272 -16
    WIRE -64 -16 -128 -16
    WIRE 80 -16 16 -16
    WIRE 224 -16 160 -16
    WIRE 448 -16 304 -16
    WIRE 592 -16 528 -16
    WIRE 736 -16 672 -16
    WIRE 880 -16 816 -16
    WIRE 1024 -16 960 -16
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    WIRE -272 16 -272 -16
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    WIRE -128 16 -128 -16
    WIRE -64 16 -64 -16
    WIRE 16 16 16 -16
    WIRE 80 16 80 -16
    WIRE 160 16 160 -16
    WIRE 224 16 224 -16
    WIRE 304 16 304 -16
    WIRE 448 16 448 -16
    WIRE 528 16 528 -16
    WIRE 592 16 592 -16
    WIRE 672 16 672 -16
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    WINDOW 0 -24 71 VTop 2
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    SYMATTR Value {Rp}
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    SYMATTR Value {Cs}
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    SYMATTR InstName C22
    SYMATTR Value {Cs}
    SYMBOL ltline 1616 0 R0
    SYMATTR InstName O1
    SYMATTR Value RG179
    SYMBOL res 1504 0 M270
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    SYMATTR InstName R11
    SYMATTR Value {R}
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    WINDOW 3 0 56 VBottom 2
    SYMATTR InstName R12
    SYMATTR Value {R}
    TEXT 264 256 Left 2 !.ac dec 1000 1 200meg
    TEXT 264 -208 Left 4 ;40KV 2E9 ohms HV scope probe
    TEXT -144 -120 Left 2 ;Ohmite MOX2-131007FE\nin pi net distributed form
    TEXT 168 160 Left 2 !.PARAM Cr=10p, Cs=7.9p, Rp=200Meg
    TEXT 672 -120 Left 2 ;Ohmite MOX2-131007FE\nin pi net distributed form
    TEXT 272 384 Left 2 !.model RG179 LTRA (\n+ len=3\n+ L=.1u\n+ C=19.5p\n+ R=252.5e-3\n+)
    TEXT 272 328 Left 2 !;.step param R 10 100 10m \n.param R = 60
    TEXT 272 -176 Left 2 ;3 feet RG179 (similar to RG175) added to model\nBelden 83265 data used.
     
  20. Robert Baer

    Robert Baer Guest

    ** This is what i have, before i saw your response; not too bad
    considering simple compensation and implicit coax of indeterminate (but
    limited) length to scope end.
    Also posted at: http://www.oil4lessllc.org/HV probes/ .

    Version 4
    SHEET 1 2064 680
    WIRE 1488 -64 1488 -240
    WIRE -352 -16 -448 -16
    WIRE -208 -16 -272 -16
    WIRE -64 -16 -128 -16
    WIRE 80 -16 16 -16
    WIRE 224 -16 160 -16
    WIRE 448 -16 304 -16
    WIRE 592 -16 528 -16
    WIRE 736 -16 672 -16
    WIRE 880 -16 816 -16
    WIRE 1024 -16 960 -16
    WIRE 1168 -16 1104 -16
    WIRE 1200 -16 1168 -16
    WIRE 1312 -16 1280 -16
    WIRE 1440 -16 1312 -16
    WIRE 1568 -16 1440 -16
    WIRE 1600 -16 1568 -16
    WIRE -352 16 -352 -16
    WIRE -272 16 -272 -16
    WIRE -208 16 -208 -16
    WIRE -128 16 -128 -16
    WIRE -64 16 -64 -16
    WIRE 16 16 16 -16
    WIRE 80 16 80 -16
    WIRE 160 16 160 -16
    WIRE 224 16 224 -16
    WIRE 304 16 304 -16
    WIRE 448 16 448 -16
    WIRE 528 16 528 -16
    WIRE 592 16 592 -16
    WIRE 672 16 672 -16
    WIRE 736 16 736 -16
    WIRE 816 16 816 -16
    WIRE 880 16 880 -16
    WIRE 960 16 960 -16
    WIRE 1024 16 1024 -16
    WIRE 1104 16 1104 -16
    WIRE 1440 64 1440 -16
    WIRE 1568 64 1568 -16
    WIRE 1616 64 1568 64
    WIRE 1312 80 1312 -16
    WIRE 1616 80 1616 64
    WIRE 1168 96 1168 -16
    WIRE -352 112 -352 80
    WIRE -272 112 -272 80
    WIRE -272 112 -352 112
    WIRE -208 112 -208 80
    WIRE -208 112 -272 112
    WIRE -128 112 -128 80
    WIRE -128 112 -208 112
    WIRE -64 112 -64 80
    WIRE -64 112 -128 112
    WIRE -32 112 -64 112
    WIRE 16 112 16 80
    WIRE 16 112 -32 112
    WIRE 80 112 80 80
    WIRE 80 112 16 112
    WIRE 160 112 160 80
    WIRE 160 112 80 112
    WIRE 224 112 224 80
    WIRE 224 112 160 112
    WIRE 304 112 304 80
    WIRE 304 112 224 112
    WIRE 448 112 448 80
    WIRE 528 112 528 80
    WIRE 528 112 448 112
    WIRE 592 112 592 80
    WIRE 592 112 528 112
    WIRE 672 112 672 80
    WIRE 672 112 592 112
    WIRE 736 112 736 80
    WIRE 736 112 672 112
    WIRE 784 112 736 112
    WIRE 816 112 816 80
    WIRE 816 112 784 112
    WIRE 880 112 880 80
    WIRE 880 112 816 112
    WIRE 960 112 960 80
    WIRE 960 112 880 112
    WIRE 1024 112 1024 80
    WIRE 1024 112 960 112
    WIRE 1104 112 1104 80
    WIRE 1104 112 1024 112
    WIRE -448 160 -448 -16
    WIRE -32 160 -32 112
    WIRE 784 160 784 112
    WIRE 1568 176 1568 144
    WIRE 1616 176 1616 144
    WIRE 1616 176 1568 176
    WIRE 1616 192 1616 176
    WIRE 1312 208 1312 160
    WIRE 1168 224 1168 160
    WIRE 1440 224 1440 144
    WIRE -448 272 -448 240
    WIRE -32 272 -32 224
    WIRE 784 272 784 224
    WIRE 1312 336 1312 272
    FLAG 1440 224 0
    FLAG 1616 192 0
    FLAG 1600 -16 scope
    FLAG 1168 224 0
    FLAG -448 272 0
    FLAG 784 272 0
    FLAG -32 272 0
    FLAG 1312 336 0
    SYMBOL voltage -448 144 R0
    WINDOW 0 8 7 Left 2
    WINDOW 3 11 105 Left 2
    WINDOW 123 0 0 Left 2
    WINDOW 39 0 0 Left 2
    SYMATTR InstName V2
    SYMATTR Value PULSE(0 250 0 1p 1p 10 20 1)
    SYMBOL cap 1184 160 R180
    WINDOW 0 -37 59 Left 2
    WINDOW 3 28 4 Left 2
    SYMATTR InstName C91
    SYMATTR Value 1170p
    SYMBOL res 1424 48 R0
    WINDOW 3 -89 120 Left 2
    WINDOW 0 -49 70 Left 2
    SYMATTR Value 4.008Meg
    SYMATTR InstName R33
    SYMATTR SpiceLine tol=0.1 pwr=1
    SYMBOL cap 1600 80 R0
    SYMATTR InstName C32
    SYMATTR Value 22p
    SYMBOL res 1584 160 R180
    WINDOW 0 36 76 Left 2
    WINDOW 3 36 40 Left 2
    SYMATTR InstName R32
    SYMATTR Value 1Meg
    SYMBOL cap -368 16 R0
    WINDOW 0 20 11 Left 2
    SYMATTR InstName C1
    SYMATTR Value {Cr}
    SYMBOL cap -288 16 R0
    SYMATTR InstName C2
    SYMATTR Value {Cr}
    SYMBOL res -368 0 R270
    WINDOW 0 -24 71 VTop 2
    WINDOW 3 68 60 VBottom 2
    SYMATTR InstName R1
    SYMATTR Value {Rp}
    SYMBOL cap -224 16 R0
    WINDOW 0 20 11 Left 2
    SYMATTR InstName C3
    SYMATTR Value {Cr}
    SYMBOL cap -144 16 R0
    SYMATTR InstName C4
    SYMATTR Value {Cr}
    SYMBOL res -224 0 R270
    WINDOW 0 -24 71 VTop 2
    WINDOW 3 68 60 VBottom 2
    SYMATTR InstName R2
    SYMATTR Value {Rp}
    SYMBOL cap -80 16 R0
    WINDOW 0 20 11 Left 2
    SYMATTR InstName C5
    SYMATTR Value {Cr}
    SYMBOL cap 0 16 R0
    SYMATTR InstName C6
    SYMATTR Value {Cr}
    SYMBOL res -80 0 R270
    WINDOW 0 -24 71 VTop 2
    WINDOW 3 68 60 VBottom 2
    SYMATTR InstName R3
    SYMATTR Value {Rp}
    SYMBOL cap 64 16 R0
    WINDOW 0 20 11 Left 2
    SYMATTR InstName C7
    SYMATTR Value {Cr}
    SYMBOL cap 144 16 R0
    SYMATTR InstName C8
    SYMATTR Value {Cr}
    SYMBOL res 64 0 R270
    WINDOW 0 -24 71 VTop 2
    WINDOW 3 68 60 VBottom 2
    SYMATTR InstName R4
    SYMATTR Value {Rp}
    SYMBOL cap 208 16 R0
    WINDOW 0 20 11 Left 2
    SYMATTR InstName C9
    SYMATTR Value {Cr}
    SYMBOL cap 288 16 R0
    SYMATTR InstName C10
    SYMATTR Value {Cr}
    SYMBOL res 208 0 R270
    WINDOW 0 -24 71 VTop 2
    WINDOW 3 68 60 VBottom 2
    SYMATTR InstName R5
    SYMATTR Value {Rp}
    SYMBOL cap 432 16 R0
    WINDOW 0 20 11 Left 2
    SYMATTR InstName C11
    SYMATTR Value {Cr}
    SYMBOL cap 512 16 R0
    SYMATTR InstName C12
    SYMATTR Value {Cr}
    SYMBOL res 432 0 R270
    WINDOW 0 -24 71 VTop 2
    WINDOW 3 68 60 VBottom 2
    SYMATTR InstName R6
    SYMATTR Value {Rp}
    SYMBOL cap 576 16 R0
    WINDOW 0 20 11 Left 2
    SYMATTR InstName C13
    SYMATTR Value {Cr}
    SYMBOL cap 656 16 R0
    SYMATTR InstName C14
    SYMATTR Value {Cr}
    SYMBOL res 576 0 R270
    WINDOW 0 -24 71 VTop 2
    WINDOW 3 68 60 VBottom 2
    SYMATTR InstName R7
    SYMATTR Value {Rp}
    SYMBOL cap 720 16 R0
    WINDOW 0 20 11 Left 2
    SYMATTR InstName C15
    SYMATTR Value {Cr}
    SYMBOL cap 800 16 R0
    SYMATTR InstName C16
    SYMATTR Value {Cr}
    SYMBOL res 720 0 R270
    WINDOW 0 -24 71 VTop 2
    WINDOW 3 68 60 VBottom 2
    SYMATTR InstName R8
    SYMATTR Value {Rp}
    SYMBOL cap 864 16 R0
    WINDOW 0 20 11 Left 2
    SYMATTR InstName C17
    SYMATTR Value {Cr}
    SYMBOL cap 944 16 R0
    SYMATTR InstName C18
    SYMATTR Value {Cr}
    SYMBOL res 864 0 R270
    WINDOW 0 -24 71 VTop 2
    WINDOW 3 68 60 VBottom 2
    SYMATTR InstName R9
    SYMATTR Value {Rp}
    SYMBOL cap 1008 16 R0
    WINDOW 0 20 11 Left 2
    SYMATTR InstName C19
    SYMATTR Value {Cr}
    SYMBOL cap 1088 16 R0
    SYMATTR InstName C20
    SYMATTR Value {Cr}
    SYMBOL res 1008 0 R270
    WINDOW 0 -24 71 VTop 2
    WINDOW 3 68 60 VBottom 2
    SYMATTR InstName R10
    SYMATTR Value {Rp}
    SYMBOL cap -48 160 R0
    SYMATTR InstName C21
    SYMATTR Value {Cs}
    SYMBOL cap 768 160 R0
    SYMATTR InstName C22
    SYMATTR Value {Cs}
    SYMBOL res 1184 0 R270
    WINDOW 0 32 56 VTop 2
    WINDOW 3 0 56 VBottom 2
    SYMATTR InstName R91
    SYMATTR Value 1200
    SYMBOL cap 1328 272 R180
    WINDOW 0 24 56 Left 2
    WINDOW 3 24 8 Left 2
    SYMATTR InstName C92
    SYMATTR Value 900p
    SYMBOL res 1296 64 R0
    WINDOW 3 -56 49 Left 2
    WINDOW 0 -40 8 Left 2
    SYMATTR Value 590K
    SYMATTR InstName R92
    TEXT 264 256 Left 2 !.tran 0 10m 0 10u
    TEXT 264 -232 Left 4 ;40KV 2E9 ohms HV scope probe 2500:1
    TEXT -144 -120 Left 2 ;Ohmite MOX2-131007FE\nin pi pad distributed form
    TEXT 168 160 Left 2 !.PARAM Cr=2.048p, Cs=0.1p, Rp=200Meg
    TEXT 672 -120 Left 2 ;Ohmite MOX2-131007FE\nin pi pad distributed form
    TEXT 1536 -152 Left 2 ;[SCOPE]
    TEXT 1320 -152 Left 2 ;[PROBE]
    TEXT 336 -184 Left 3 ;Tr~27nSec; 2% overshoot at 1mSec
     
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