Homebrew HV hiZ scope probe

[Spehro Pefhany]

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.

It comes on spools- manganin is better than nichrome, but nichrome
isn't all that bad. In a pinch, brass or stainless are pretty awful
tempco-wise, but relatively high resistance.

These guys have 25m lengths of it with Kapton insulation:-

http://www.iceoxford.com/Cryogenic-spares/Wiring.htm

If you need many pounds/kg of it, it's easy to buy.

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....

Except for cable TV coax where it seems pretty easy to pull out (esp
accidentally).. the dielectric is slippery PE, I think, and the center
wire a thick smooth solid wire. But it's awfully stiff for test prod
wire.


Best regards,
Spehro Pefhany

 

[Jeroen Belleman]

Does Tektronix or HP list the vendor in their parts list? They do
for most parts.

Good suggestiin the P6106 manualon, but no luck. Tektronix lists
itself as manufacturer of most parts, cable included. Interestingly,
it lists the cable as '39 ohm' cable. The resistance of the central
conductor of a 1m P6106 cable is about 130 ohms. The geometry of the
cable suggests a Z0 much higher than 39 ohms, so I don't know what
this is supposed to mean.

Jeroen Belleman

 

[Frank Miles]

Good suggestiin the P6106 manualon, but no luck. Tektronix lists itself
as manufacturer of most parts, cable included. Interestingly, it lists
the cable as '39 ohm' cable. The resistance of the central conductor of
a 1m P6106 cable is about 130 ohms. The geometry of the cable suggests a
Z0 much higher than 39 ohms, so I don't know what this is supposed to
mean.

Jeroen Belleman

Hmmn. Maybe 39ohms/ft (series resistance)?

 

[Fred Abse]

Good suggestiin the P6106 manualon, but no luck. Tektronix lists itself as
manufacturer of most parts, cable included. Interestingly, it lists the
cable as '39 ohm' cable. The resistance of the central conductor of a 1m
P6106 cable is about 130 ohms. The geometry of the cable suggests a Z0
much higher than 39 ohms, so I don't know what this is supposed to mean.

P6015 manual describes the cable as "50 ohms per foot".

 

[Fred Abse]

Does Tektronix or HP list the vendor in their parts list? They do
for most parts.

Not in the P6015 manual.

 

[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.

The Tek probes used 100meg resistors in the probe head, with a 100K
effective load in the termination unit. Much more tractable to compensate.
Any reason for going to 1000meg in your design?

 

[tm]

The Tek probes used 100meg resistors in the probe head, with a 100K
effective load in the termination unit. Much more tractable to compensate.
Any reason for going to 1000meg in your design?

1/10 th the circuit loading.

 

[josephkk]

Well I just order it from conrad.nl,
several types at several Ohm/meter, some VERY thin.
I have used it to make thermocouples.
I am sure US has many places that sell that wire.
Yes the inner coax does not always come out easy,
I have done it, put one end in the vice, and take an hour to pull it out.
Use gloves, hurt my hands doing it...

Gloves? Like hell. Use a mandrel (or a dowel rod).

?-)

 

[josephkk]

It comes on spools- manganin is better than nichrome, but nichrome
isn't all that bad. In a pinch, brass or stainless are pretty awful
tempco-wise, but relatively high resistance.

These guys have 25m lengths of it with Kapton insulation:-

http://www.iceoxford.com/Cryogenic-spares/Wiring.htm

If you need many pounds/kg of it, it's easy to buy.


Except for cable TV coax where it seems pretty easy to pull out (esp
accidentally).. the dielectric is slippery PE, I think, and the center
wire a thick smooth solid wire. But it's awfully stiff for test prod
wire.


Best regards,
Spehro Pefhany

Copper clad steel rather frequently, no good for broadband resistivety.
But that is what you pull out.

I would likely as not try to pull in constantan (perhaps nicked from
thermocouple wire).

?-)

 

[Ralph Barone]

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.

What was the code name for the coax with the helically wound insulation,
leaving mostly air as the dielectric? You might stand a chance replacing
the center conductor on that stuff.

 

[[email protected]]

What was the code name for the coax with the helically wound insulation,
leaving mostly air as the dielectric? You might stand a chance replacing
the center conductor on that stuff.

I've known it as "heliax".

 

[Jeroen Belleman]

Hmmn. Maybe 39ohms/ft (series resistance)?

Ah yes, exactly. I still get caught sometimes when some
engineering outfit doesn't use metric throughout.

Jeroen Belleman

 

[Sum Ting Wong]

I've known it as "heliax".

Yes, and it would be even harder to do that job as the elements that
suspend the center conductor would get trashed on the removal, and then
the replacement would no longer be "at the center", especially when you
bend, form, or use the cable.


The 'heliax' is an integrated structure, and breaking the bonds holding
the center will mean the replacement can move about however it gets
pushed. Not good.

 

[Jasen Betts]

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

Use coax with a high tensile steel core weld a piece of
nichrome wire to the other end first

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.

you may have to ripple it off, like how a caterpillar walks.
the outer is elastic, but the core is not,

Probably cheaper to find and get the right stuff.

probably a few hour's work plus gettign the welds made (perhaps by a
jeweler?)

 

[Fred Abse]

1/10 th the circuit loading.

There's a problem with that. The proposed 1:1000 single-stage probe
requires a parallel compensating capacitance 1/1000 of the total
scope-plus-cable capacitance, which is likely to be in the order of 50pF,
which implies a compensating capacitor of 50 fF, or 0.05 pF. across the
probe series resistor. This is probably impossible to realize.

The Tektronix 1:100 HV probe used a special "leaf-and-collar" capacitor of
a few pF, across the 100M probe resistor, which is taking things about as
far as they can go, and still withstand the voltage gradient across the
resistor. The capacitor dielectric was Freon, rather than air, giving a
higher dielectric strength. Freons have a permittivity of around 2, which
helps as well. The probe actually had a load of 100K at the scope end of
the cable, not just the 1meg/22pF scope input, giving 1:1000 ratio.
Compensation adjustment was done at the scope end of the cable, as is
ubiquitous in higher-end probes today.

 

[Fred Abse]

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

Says so on the schematic in the probe manual.

Probe cables, at least modern ones, use foam dielectric, hence lower
capacitance per foot.

I've got some probes with removable cables, I'll TDR one when I get the
time. I'll have a go at open and shorted measurements, too, and calculate
the complex Zo. With a resistive center, Zo will be significantly complex
at higher frequencies than "regular" coax.

 

[Fred Abse]

Are you fir real?

He's just taking the pith.

 

[tm]

There's a problem with that. The proposed 1:1000 single-stage probe
requires a parallel compensating capacitance 1/1000 of the total
scope-plus-cable capacitance, which is likely to be in the order of 50pF,
which implies a compensating capacitor of 50 fF, or 0.05 pF. across the
probe series resistor. This is probably impossible to realize.

The Tektronix 1:100 HV probe used a special "leaf-and-collar" capacitor of
a few pF, across the 100M probe resistor, which is taking things about as
far as they can go, and still withstand the voltage gradient across the
resistor. The capacitor dielectric was Freon, rather than air, giving a
higher dielectric strength. Freons have a permittivity of around 2, which
helps as well. The probe actually had a load of 100K at the scope end of
the cable, not just the 1meg/22pF scope input, giving 1:1000 ratio.
Compensation adjustment was done at the scope end of the cable, as is
ubiquitous in higher-end probes today.

Certainly not disagreeing with you Fred. In fact I don't know of any 1000:1
probes that are good at high frequency measurements. But for DC
measurements, you would want a 1Gohm probe for best accuracy measurements.

Also, I think I read somewhere that the Freon 114 used in the 6015 probe had
a permittivity near to 1. There was some discussion of using F-11 as a
replacement but its permittivity is up there and the probe does not work
well with it.

There is some good information on the yahoo tekscopes group archives
concerning this probe.

Regards

 

[tm]

Certainly not disagreeing with you Fred. In fact I don't know of any
1000:1 probes that are good at high frequency measurements. But for DC
measurements, you would want a 1Gohm probe for best accuracy measurements.

Also, I think I read somewhere that the Freon 114 used in the 6015 probe
had a permittivity near to 1. There was some discussion of using F-11 as a
replacement but its permittivity is up there and the probe does not work
well with it.

There is some good information on the yahoo tekscopes group archives
concerning this probe.

Regards

I meant to say any 1 G ohm, 1k:1 probes.

 

[josephkk]

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.

Nicrhome is ok for heater uses, but i prefer constantan for electronic
purposes. Check out the properties differences, it will be worth it.

?-)