Homebrew HV hiZ scope probe

[Sjouke Burry]

CUT

Any car repair shop.
Used to hook up spark plugs.

 

[Sum Ting Wong]

Personally for a one case, I would take normal coax,
pull the center conductor out,
and insert thinner resistance wire.
Should work for length of 1m or there about.

Not sure if that helps for your case.

Yeah, like dragging the center conductor out of a meter long piece of
coax is an easy task, much less feeding another back through.

Standard RG-8 center conductor dielectric strength is 1.5kV.and they
are not easy to get out. Fairly tightly bound to the core media.

Probably cheaper to find and get the right stuff.

 

[Fred Abse]

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.

Not any more. It has derated itself to about 12kV. It won't do 40kV
anymore without the fluorocarbon.

If you can get the 114 from somewhere, you can recharge it. See the manual.

 

[Fred Abse]

Perhaps, but us mortals with a limited budget cannot order umpteen
thousand feet of coax with a resistive center.

Then do what I did, resistors at each end.

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).

Wrong. The value of 60 ohms was empirically determined from an .ac
analysis, with the resistor values stepped. See my LTspice listing, all
the necessary information is there, just uncomment the .step param card,
and comment (or delete) the 60 ohm R parameter. Pick the value which gives
the smoothest, flattest rolloff, at the HF end. Try without any resistors,
too, you'll see what I mean about resonance.

The value of R is *not* the Zo of the cable (which is complex,anyway).

 

[TheGlimmerMan]

Not any more. It has derated itself to about 12kV. It won't do 40kV
anymore without the fluorocarbon.

If you can get the 114 from somewhere, you can recharge it. See the manual.

DuPont (Ithought)(apperently 3M now) FC-40 also works. "Fluorinert"

http://multimedia.3m.com/mws/mediawebserver?66666UuZjcFSLXTtnxTEo8s6EVuQEcuZgVs6EVs6E666666—

 

[TheGlimmerMan]

It's not nice to needle people like that.

Is "people like that" referring to the needling method or the "way" the
person is?

 

[amdx]

Root around long enough, and we'll find a pun.
Mikek

 

[JW]

Do you think we should just leaf them alone?

I Woodn't.

 

[[email protected]]

Yew guys conifer control yourselves.

Deciduously not.

 

[[email protected]]

Don't be such a conehead.

I think you've found the root of the problem.

 

[josephkk]

That stems from our basic need for humor.

'It is upon the trunk that a gentleman works. When that is firmly set up,
the Way grows. And surely proper behavior to parents and elder brothers is
the trunk of goodness.' (Ancient Chinese. Analects, i. 2)

 

[Robert Baer]

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).

Sorry...mistype... cable is about 50 ohms per foot.

 

[Robert Baer]

CUT

Any car repair shop.
Used to hook up spark plugs.

Resistance is about an order of magnitude higher than desired.

 

[Robert Baer]

He's just another sap...

OOooo!

 

[Robert Baer]

Personally for a one case, I would take normal coax,
pull the center conductor out,
and insert thinner resistance wire.
Should work for length of 1m or there about.

Not sure if that helps for your case.

That sounds like it may be do-able.
BUT where does that resistance wire come from? Most heaters have a
much larger diameter nichrome wire, and i do not know where one can buy
"plain" resistive wire like that.
Suggestions?
Oh, "insert" might be a bitch..one might be forced to spot-weld the
resistive wire to the existing internal wire.
And it ain't easy to pull out....

 

[Robert Baer]

Yeah, like dragging the center conductor out of a meter long piece of
coax is an easy task, much less feeding another back through.

Standard RG-8 center conductor dielectric strength is 1.5kV.and they
are not easy to get out. Fairly tightly bound to the core media.

Probably cheaper to find and get the right stuff.

Reason why center conductors are virtually impossible to pull out,is
that the insulation is hot-cast onto the wire..

 

[Robert Baer]

Not any more. It has derated itself to about 12kV. It won't do 40kV
anymore without the fluorocarbon.

If you can get the 114 from somewhere, you can recharge it. See the manual.

Know where that rainbow is----^ ?

 

[Robert Baer]

It's not nice to needle people like that.

Excellent!! You are on a roll!

 

[tm]

Reason why center conductors are virtually impossible to pull out,is
that the insulation is hot-cast onto the wire..

Why not make an active HV probe and have it drive a standard cable?
Many scopes have a probe power source available with +/- 15 volts or so.
There is plenty of room in the handle of a 1 Gohm to 10 meg 1000X probe to
put an opamp and other parts.

 

[Sum Ting Wong]

Sum Ting Wong wrote:
Reason why center conductors are virtually impossible to pull out,is
that the insulation is hot-cast onto the wire..

Then wrapped even tighter by a Chinese handcuff overbraid of silver
plated copper shielding. So tightly it makes a deep fingerprint
impression of the overbraid onto the core media. It doesn't just slide
off either.