Making HV Equipotential Resistors

[George Schmermund]

The beam on target project will require an accelerator tube to deliver the positive ions from an ion source (yet to be designed) to the target. Previous posts have described the process of making and testing an electrode/insulator combination. There is still much design and test work to be done in this area, but it's always interesting to find the limits of reasonable compaction and materials.

One of the requirements of the tube is that the electrodes need to be connected to each other electrically in order to create an equipotential voltage gradient. This can be accomplished by using a series of high voltage resistors, each having very high resistance. Such resistors can be purchased commercially, but many will be needed and they ain't cheap. My initial goal is to have a tube with 10 stages.

In an effort to exercise my Scroogian approach to building projects, it became necessary to surreptitiously pilfer my wife's sewing supplies. The booty included an assortment of threads and ribbons of various sizes and composition.

The purpose of all this was to test how well the different bits would wick-up and uniformly distribute India ink into the weave. Previous efforts using Aquadag as a high resistance material have been posted here sometime back. Aquadag works well on hard and non-flexible substrates, but it's very friable when mechanically stressed. What is needed for the current project is a liquid resistance medium that can survive some minimum amount of handling when dried onto/into a flexible substrate . Therefor, India ink became the liquid of choice because of its ready availability and extraordinarily fine and uniform grains of carbon. The Aquadag looks like large lumps of coal compared to the ink when viewed under a good microscope.

The initial spec for a test resistor was 1 Gohm in an 8" length of support material. The first choice was cotton but the wicking tests showed that polyester worked dramatically better. Then there was the choice between waterproof and non-waterproof ink. The waterproof variety turned out to be much more stable when dry. It still has its foibles, but they may be manageable in the final versions.

I thought that plain thread would be the best choice for the substrate, but after trying very narrow ribbon there was no turning back. The complexity of the weave and surface condition of each fiber within the weave was remarkable. I'm always astonished by the revelation of detail that appears when viewing things through a microscope.

A piece of PVC pipe was used to make a form to wind the resistor on. The dimensions of the PVC parts are a close match to the Delrin insulators that will be used on the finished tube. The thread grooves evenly space the resistor turns and the depth of the thread also forms a corona barrier between turns. This is about as low a profile for an equipotential resistor string on an accelerator tube as I've seen.

This resistor boondoggle is an 'on-again, off-again' project, but the photos indicate some of the efforts to this point.

[prestonbarrows]

What sort of resistance, voltage, and current capability are you looking for?

For very low currents, static dissapative materials can work for high voltage grading. Depending on the exact material, values in the ballpark of Gohm/cm is fairly common. Of course, being plastic means they can not handle much heat. But, it is a volumetric dissipation which helps. This is workable in the sub-miliamp or microamp type current range. Given that you are planning to use PVC as a substrate anyways, I am assuming the planned heat load is negligible.

There are also dissapative paints cheaply available for ESD control on electronics.

Commercial resistors in the Gohm, 20kV, 1-10W type range are only around $10 each. Not too terrible for 10 stages given all the other shiny kit you seem to have. At some point, it's not worth your time trying to cobble together a reinvention of the wheel when someone else has done all the hard work already for you.

http://www.mouser.com/Passive-Component ... =Pricing|0

Example:
http://www.mouser.com/ProductDetail/Ohm ... Az3rLrg%3d

[George Schmermund]

Prestonbarrows - Do you have a hobby? I have several of them. One of my favorites is to re-invent all sorts of wheels! You're rather late to the party when it comes to pointing out my eccentricities.

[prestonbarrows]

Haha, then have at it Quixote.

Another thing to keep in mind is wound resistors will have high series inductance. When a column inevitably arcs, the huge dI/dt can lead to all sorts of problems with inductive loads. Again, not exactly sure what sort of voltage/current you plan for these but just a warning. This is the reason most bigger HV resistors come with a non-inductive 'zebra stripes' option.

[Dennis P Brown]

This exact topic has been covered before and that post might be of some interest to you.

[John Futter]

Preston
Give George a break
Take a look at how many turns he is putting on the former
Go and download a program called coilcal --used by many Rf engineers like myself guess the diameter and length
work out the inductance then calculate dI/dt and see how many volts are dropped across the resistor resistive component compared to the inductive component
and report back

We are waiting for the apology

[Dennis P Brown]

I too am building a VDG powered linear accelerator and have addressed many of these issues he is asking about - some have already been answered in the past; I have had to address many other issues concerning the construction and optimization of such a device but I have never posted these details since this is a fusor forum and I did not consider them appropriate. This exact issue he is asking I have discovered has a very simple solution that has worked great on my device but again, fixing VDG supplies isn't a fusor topic and I haven't shared this and other designs.