HV oil probe details
Sent: Thursday, September 25, 1997 12:31 AM
To: Tesla List
Subject: Re: HV oil probe details
Tesla List wrote:
> From: terryf-at-verinet-dot-com[SMTP:terryf-at-verinet-dot-com]
> Sent: Tuesday, September 23, 1997 11:03 PM
> To: tesla-at-pupman-dot-com
> Subject: Re: HV oil probe details
> Hi Tom,
> First of all, let me make it clear that I have not actually built
> and tested this type of probe. It has to be categorized as theoretical and
> experimental. I don't assume any responsibility as to this design working,
> being safe, etc. Obviously, this should only be attempted by experience
> high voltage experts who can handle the known and unknown risks and problems
> that may be encountered.
> The probe would consist of a 4 inch diameter PVC pipe that would be
> placed up the center of the Tesla coil (probably a six or eight inch
> diameter coil). The top end of the pipe would be in the center of the
> toroid assembly where it would be surrounded by the electrostatic fields of
> the toroid. The bottom of the PVC pipe would extend down past the bottom of
> the coil and extend a few inches lower to safely clear the primary coil.
> Inside the pipe is a 1/4 copper tube that would travel through the
> center to a point level with the top turn of the secondary coil. It would
> have the needed supports. The bottom of this copper tube would exit the PVC
> pipe at the bottom and would travel a safe distance (maybe 10 feet) from the
> Tesla coil. This end would be connected to a very good ground independent
> of the Tesla coil's ground.
> This 1/4 inch tube contains a thin coax cable. The top of this
> cable goes up the 1/4 inch tube and has its bare center conductor protruding
> about 3 inches at the top. This would act much like an antenna that would
> in effect be capacitively coupled to the intense fields at the top terminal.
> The other end of the coax would extend a distance to where the test
> equipment would be located (maybe 40 feet). This coax would be grounded to
> the test equipment grounding at this end. This ground would be separated
> from the Tesla coil's ground and the 1/4 inch tube's grounds.
> The PVC pipe would be filled with oil to insulate the grounded
> conductor's inside it from the high voltages. This layer would be about
> 1.75 inches minimum at any point in the PCV (except the bottom). The top
> antenna would be covered with about three more inches of oil to prevent
> arcing to it. The PVC tube is capped at both ends and the exiting copper
> tube and coax is sealed to prevent leaks.
> So we now have an oil insulated antenna at the top of the coil that
> is connected through a double shielded path back to the instruments. The
> 1/4 inch copper tube absorbs the vast majority of the "noise" from the coil
> and carries the capacitively coupled charges on it to ground. The coax
> cable's shield can protect the signal from the relatively low-level noise
> At the instrument end of the coax are two 0.01uF poly caps (the
> common 50 volt types) connected to ground. The two poly caps are in
> parallel (fail safe) along with a 10M ohm resistor to bleed off any DC
> voltages (a high value inductor may be a better choice?). The antenna end
> inside the coil will act as an ~0.5 pF capacitor to the high voltage
> terminal. The two 0.1uF caps will act as the lower end of the capacitive
> voltage divider at a value of 0.005uF. This gives about a 10000:1 AC
> attenuation ratio. This divided signal is sent though a 1k ohm resistor and
> clamped by transorbs (just in case) to the equipment.
> There are two known dangers that would need to be addressed:
> 1. There is a possibility that the oil insulation layer could
> be punctured by too high of voltage. This would send heavy currents and
> voltages to the test equipment (if the copper tube and coax shield didn't
> take the hit). The capacitors would have to short to ground and the
> transorbes would need to be able to clamp this event. This would be tested.
> 2. A voltage puncture could break the PVC and possibly ignite
> the oil causing a fire. None flammable oil would prevent fire but there
> could be a mess. The oil would be tested to insure it's insulation
> properties and determine a safe operating voltage. (400kV??)
> 3. The losses in the probe may cause heating of the probe.
> This would have to be monitored. The probe needs a tiny vent hole at the top.
> There are four known errors that would need to be addressed:
> 1. The actual attenuation ratio would need to be measured. The
> primary voltage could be used to charge top conductor to a know and
> measurable voltage for calibration (with a typical high voltage divider).
> You would need detune the coil to prevent any resonate amplification. This
> needs more thought. I have equipment that can do this so I didn't give it much.
> 2. The grounded center conductors and the oil insulated column
> would require retuning the coil substantially. It would form about a 15pF
> capacitor to the secondary coil. The 1/4 inch copper tube would act as a
> shorted winding. This would mean that the coils true best output would not
> be measured due to these losses.
> 3. The high voltage would cause corona between the secondary
> coil and the 4 inch PCV. This may cause noise to be picked up by the
> antenna. The top portion could be covered by metal connected to the top
> terminal to prevent this and increase coupling.
> 4. The frequency response of the probe would need to be tested
> to insure it is reasonable. I never got to the point of carefully
> calculating this out. It would work at the 200KHz range.
> This probe would eliminate the capacitive coupling of the signal
> from areas other than the actual terminal area (this may be a problem with
> your resistive probe? Can the resistors pick up stray E-fields? They
> should be shielded but those shields attract arcs and filter high
> frequencies out.) It would also load the coil reactively so that there
> would not be much resistive loss. If it worked as planned the measurements
> would be defendable as to their accuracy and others could verify the
> results. It is based on sound theory and proven principles. I assume
> nobody else has a 2 megavolt X-ray tube and the associated calibrated
> standards laying around. I don't know what kind of losses the X-ray system
> may have had. However, the reported measurements are fascinating and the
> method is cleaver!
> I was planning on making a special coil to accommodate this
> assembly. It would have low turn-to-turn voltages to prevent arcing at the
> coil or probe. The initial testing would be done using single pulses
> without test equipment connected just to see if everything was ok. I would
> start out at very low voltages and work my way up. My cheap scope ($50 easy
> to fix Heathkit from the 1950's) I had one of those too! Smoked it, I forget how. RF, probably...would a
resistive spark plug wire be a
useful probe here... part of the voltage divider circuit somehow...
lesser effect on the TC's qualities.....?
. There is always
> still some risk!
> You can see why I stopped this project to pursue a fiber-optic
> <snip> There were very few responses to
> the original post about this and the fiber-optic probes (I was glad to hear
> that the fiber should not fluoresce! Thanks Sulaiman :-)) Maybe this is at
> the "bleeding edge" of Tesla coil research! :-))
> Terry (waiting for mail-order parts to get started!)
> >Hi Terry
> >All I have at present is a high value resistive divider that is a little
> >less than linear ;)
> >Any info would be appreciated
> >Tom Weagant
> ><From: terryf-at-verinet-dot-com[SMTP:terryf-at-verinet-dot-com]
> ><Sent: Friday, September 19, 1997 2:46 PM
> ><Subject: Secondary voltage measurement.
> ><I was working on a more conventional voltage probe but it would add about
> ><15pf to the secondarys internal capacitance. This would require retuning
> ><and would materially change the coil's true output. <snip>
Ferrous cores increase inductance, as we all know. Less advertised
is the fact that non ferrous cores DECREASE inductance, rather than
being non reactive. Which would also alter the coils parameters.
> ><If these do end up working I will post the plans and results. It all costs
> ><about $75 (not including the test equipment it plugs into). If anyone is
> ><intersted in the old voltage probe idea, I can also provide details on it.
> ><Someone may want to play with this. If the fiber-optic probes fry, it will
> ><be me :-)