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Registered Member #2423
Joined: Tue Oct 06 2009, 02:49AM
Location: Oklahoma City
Posts: 10
I've looked a bit more into corona leakage, and it sounds like when a conductor hits a certain minimum potential gradient it will start to leak energy in the form of ionised particles. I know this is very elementary, but I just want to be sure about what I am understanding. Now, the Potential gradient is determined by the shape of the conductor, the voltage applied to that conductor, and the dielectric constant (permitivity) of the conductor's surroundings. ( Is this last logical leap correct? ) So if my logic stands, there is a maximum "stable" voltage for every system, and that depends on conductor shape (curvature), voltage, and dielectric constant. Assuming it is potted in a uniform material, the time it takes to "leak" down to this voltage / charge will vary depending on how many sharp edges there are. This is starting to make a little more sense to me. The bottom line is that you are only as strong as your weakest link, but the more weak links you have the faster you break.
Antonio, I am starting to like your point about Air having better resistivity than oil. Even if our resistance between the plates around the capacitor is in the teraohms we will have an RC time constant in the matter of a couple of hours completely discounting corona leakage. I guess I could double the capacitance to double the RC Time Constant...
I guess the purpose is to play around with high voltage at (relatively) low powers before I REALLY start trying to fry myself. I want to create a large static electric field and measure what kind of oscillations I'm getting from the environment. I also might try to drive it with a low voltage AC form on top of the HV DC signal. We'll see what happens. I would like to be able to take the measurements for some time without having to hit the entire system with a virtual HV sledge hammer to charge it back up. I am correct in assuming that If I produce a large electric field, I should get amplified noise coming back in from any local electric fields, right?
The ball electroscope seems like the way to go for measuring the Charge / Voltage. I think the key here is High Resistivity, and High Dielectric constant as well as minimizing ALL sharp edges even if they are immersed in wax or oil. A couple more questions: Does anyone know a ballpark value for the resistivity of vegetable oil? If the electroscope is immersed in oil and the oil has a higher dielectric constant than air this should produce less corona leakage, shouldn't it? I just noticed this in Wikipedia - "corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor". If it is surrounded by a solid, can there be no Corona Discharge? Corona Discharge
I really appreciate the responses I'm getting from you all, and your knowledge and wisdom is truly invaluable.
EDIT: Wow, more posts. OK. I think that sounds like a very good way to measure the leakage, but as you can see, I'm not trying to measure the leakage, but eliminate it. ( reduce - maybe? ) Right now the cap is just in air. I am probably going do something else for the cap to help reduce / eliminate the sharp edges. Then, depending on whether or not everyone rips my above logic to shreds, I will pot it in wax with the entire system. I already have 12 lbs.
Registered Member #2478
Joined: Mon Nov 23 2009, 03:24AM
Location: Texas A&M University
Posts: 47
It is. I started my post, went to dinner, and came back. You must have posted while I was away.
On further thought, the op-amp ammeter wouldn't help much, as a resistor small enough to avoid saturation could be connected directly in series with the capacitor without significantly disturbing the system. The large voltage in series with the capacitor is a pretty good current source already.
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
Dielectric constant is how easily a dielectric is polarized, not ionized. I think dielectric strength is the term you are looking for. That's what determines when a material breaks down/ionizes.
How do you plan to double the RC time constant? Doubling the capacitance is done by either halving the space between plates, which is halving the resistance (half the length of a resistor has half the resistance), OR doubling your capacitor plates, which implies doubling your amount of dielectric in a way that would also halve the resistance (adding the dielectric in this way is equivalent to placing two of the original resistances in parallel). Either way you slice it, by doubling C you half R and do not change T.
A solid would help prevent corona TO AN EXTENT. Corona can still form on the other side of the solid if depending on the electrical field's strength and the insulator's width (I've had glowing wires... fun!) Also, even solids will break down over a long time without the influence of air, I think. Long before that happens, though, the trace bits of air between the conductor and insulator will form corona and eat through the insulator! And not to mention, cause corona losses (duh).
Commercial capacitors are vacuum sealed to minimize this effect. You may want to consider buying a couple instead of getting messy with wax and oil and still coming out with subpar capacitors.
One time I almost got into the rolled poly cap craze, but when I broke open my $4 Walmart-bought Avery Sheet Protectors and saw all the little imperfections in the plastic and realized how much room I'd have to clear off on my desk to roll it, and that I'd have to cut my aluminum foil to the right size, and that it would probably fail in a shorter amount of time than it would take to make... I went back to online capacitor hunting.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
ThereIsNoSky wrote ...
I am correct in assuming that If I produce a large electric field, I should get amplified noise coming back in from any local electric fields, right?
if you look at my thread " " youll see the trouble ive had to deal with from field-field interactions so yes, and for your purpose of real time field evaluation, i would recomend the Electrometer circuit i mentioned previously.
ThereIsNoSky wrote ...
The ball electroscope seems like the way to go for measuring the Charge / Voltage. I think the key here is High Resistivity, and High Dielectric constant as well as minimizing ALL sharp edges even if they are immersed in wax or oil.
yes, the ball electroscope is easy to use however field, and i would buy real well made caps too.
Registered Member #2478
Joined: Mon Nov 23 2009, 03:24AM
Location: Texas A&M University
Posts: 47
I think you might be looking for a coulomb balance, or a similar apparatus, as depicted here. I would not trust the stability of the spring constant of the string. If I were a stickler for precision, I would null the electrostatic force with a solenoid in the field of a Helmholtz coil. The voltage between the spheres could then be calculated from the Helmholtz coil current, the solenoid current, and first principles.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Russell Haley wrote ...
It is. I started my post, went to dinner, and came back. You must have posted while I was away.
On further thought, the op-amp ammeter wouldn't help much, as a resistor small enough to avoid saturation could be connected directly in series with the capacitor without significantly disturbing the system. The large voltage in series with the capacitor is a pretty good current source already.
Registered Member #2423
Joined: Tue Oct 06 2009, 02:49AM
Location: Oklahoma City
Posts: 10
ScotchTapeLord wrote ...
Dielectric constant is how easily a dielectric is polarized, not ionized. I think dielectric strength is the term you are looking for. That's what determines when a material breaks down/ionizes.
I disagree. If you look at Peek's Law, it is based basically on the "disruptive critical potential gradient". If you dig into where on God's green Earth that came from, I believe you will find that it is based principally on permittivity. Directly proportional to Reletive pernitivity or Dielectric Constant. It wasn't immediately obvious to me, but I think this is correct.
ScotchTapeLord wrote ...
How do you plan to double the RC time constant? Doubling the capacitance is done by either halving the space between plates, which is halving the resistance (half the length of a resistor has half the resistance), OR doubling your capacitor plates, which implies doubling your amount of dielectric in a way that would also halve the resistance (adding the dielectric in this way is equivalent to placing two of the original resistances in parallel). Either way you slice it, by doubling C you half R and do not change T.
I agree, good catch. I was thinking of just adding a second capacitor, but alas nothing is free. Suggestions for a good HV cap which would hold a charge for quite some time? The resistivity of paraffin is probably somewhere in the range of 100 - 100,000 times that of air. This should increase my Time Constant shouldn't it? I'm also thinking concentric cylinders or spheres for my caps to increase the "disruptive critical voltage" or max bleed down to voltage.
@Patrick - I would prefer to make direct measurements on the circuit. My current plan is to put an inductor or resistor in series between the cap and ground. This will give me a very good low resistance Earth ground for my HV DC coupling on the electrostatic field. Then I would like to measure the voltage across the inductor or resistor to see any E-Field interactions. Do you think this will work? As my E-field is affected by other E-fields this should induce a small(?) voltage rise or dip across the capacitor. This change will then be measured by a scope on the inductor / (small value) resistor without draining any of the charge off of the HV side of the capacitor.
One of the other reasons I like this project is because I am truly a novice with even moderately complex AC circuits, and it lets me use my DC strengths while learning more about AC.
EDIT: For clarification, the only reason for the electroscope is to verify roughly the HV in the system. I won't be using it to make any precision measurements.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
ThereIsNoSky wrote ...
I disagree. If you look at Peek's Law, it is based basically on the "disruptive critical potential gradient". If you dig into where on God's green Earth that came from, I believe you will find that it is based principally on permittivity. Directly proportional to Reletive pernitivity or Dielectric Constant. It wasn't immediately obvious to me, but I think this is correct.
Permittivity is the ease with wich the flux/field can penetrate a medium, dielectric constant is how much the dielectric is polarized by the field. Dielectric Permitivity and dielectric constant are different. Dielectric Breakdown is the punch through voltage which is fatal for an insulator.
ThereIsNoSky wrote ...
@Patrick - I would prefer to make direct measurements on the circuit. My current plan is to put an inductor or resistor in series between the cap and ground. This will give me a very good low resistance Earth ground for my HV DC coupling on the electrostatic field. Then I would like to measure the voltage across the inductor or resistor to see any E-Field interactions. Do you think this will work? As my E-field is affected by other E-fields this should induce a small(?) voltage rise or dip across the capacitor. This change will then be measured by a scope on the inductor / (small value) resistor without draining any of the charge off of the HV side of the capacitor.
oh, man you've asked a very complicated question. i think you will have problems using a 1M/or 10M ohms scope impedenace. the resistor or inductor you describe should really be considered a antenna and its physical shape may matter a whole lot for intercepting any givin signal/field. and close, low field changes will be detected as the same as a far, High field. at least for a detector single antenna intercept type devcie.
i think you should experiment with this transistor circuit first so you can see all this for yourself.
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
Patrick wrote ...
ThereIsNoSky wrote ...
I am correct in assuming that If I produce a large electric field, I should get amplified noise coming back in from any local electric fields, right?
if you look at my thread " " youll see the trouble ive had to deal with from field-field interactions so yes, and for your purpose of real time field evaluation, i would recomend the Electrometer circuit i mentioned previously.
Patrick: I'm not sure that's what ThereIsNoSky means:
ThereIsNoSky: I'm no expert on this, and the details in your question are vague, but why do you expect your field to produce 'amplified noise' from other smaller fields? I'm concerned because it sounds like the kind of pseudo scientific claim I can imagine being made for some 'quack medical' electric field detector and perhaps you've been mislead? Or I've misunderstood?
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
i took his implication to mean that he wants to investigate e-field interactions and related math and instrumentation, but i dont know he hasnt been clear as to his intent.
his use of the word amplifier may be in error, i think he wants to measure field influence. obviously when chagre cloud or e-fields meet they distort and interfere with each other and this can be measured, but "field amplification" is not the right way to say such a thing.
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Charge Leakage and measuring Voltage with an Electroscope
