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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Hey everybody !
i am attempting to build my own high voltage capacitive voltage divider as suliaman et. al., have advised me in a previous thread. (instead of a resistive one)
A 1.8-2.0 pF 100Kv Capacitor....here are my first prototype pics for your amusement: Band sawed open for your viewing convienience.
ok good the pics finnally are upoaded.
ok now for the explanation: 1) Epoxy, 0.5 inches at least in all directions. @ 500 volts/mil 2) Single sided circuit board, 0.062 inches @ 700 volts/mil 3) Polycarbonate dielectric, 0.230 inches each times 2 pieces @ 400 volts/mil 4) Poly-methyl-meth-acrylate (PMMA) adhesive epoxy, [acrylic] 0.080 inches @ 500 volts/mil 5) PVC 2.5" dia. coupler as the body 3" tall, 0.155 inches single wall thick. @ 700 Volts per/mil
I calculate that the layered assembly is better then one consistant piece. i plan to change the 2 pieces of 0.230 inches of PC to six layers of 0.118 inches of PC. Also, i will increase the dielectric thickness from 0.520" to 0.730" of PMMA and PC. this is becuase insulating strength decreases as thickness INCREASES, I know you newbies read that a few times before commenting, as it is counter intuitive. (IE, 0.001 inch of Teflon is good for 1000 volts, BUT 1 inch will not nessicarilly insulate 1,000,000 volts, even though the D/V ratio is the same) for this reason i prefer the layered (for punch through resistence) topology especially above 50KV . I calculate the plastic in this radial design is good for 380-450Kv, but i dont expect it to survive more than 150KV , and i only expect to measure 70-100Kv on a contious basis. Only the FR-4, PMMA and PC are used as the dielectric for capacitence, the epoxy and PVC layers are only to protect the external devices/users.
Please tell me what you think, before i commit to a final design.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
The glued joints at the sides will fail quite easily. I would try a capacitor with air dielectric. Something as a ball for the HV terminal and a plate at some distance for the LV terminal. My attempts to use epoxy as insulator failed completely. I had some success with hot-melt glue.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Hey Antonio, Take a second look in detail of this next pic and tell me what your experience says...
pic:
And now a thought occurs! what if i suspend two ball bearings in oil that would yield a small predictible field intensity and shape, with no fringing at the edges as with plates, in the nuclear atom smashers, they use several 800 pound vacuum tubes to do something important or whatever, and requiered 380 to 450Kv to operate properly measureed in uSeconds, i have seen the CVD's from these and they all have scorch marks around the flat plate edges (where the field is always intense) oh CVD = capacitive voltage dividers. let me look for a pdf on that...
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
I dont think this is a double post as per the site rules...so heres the new idea.
I will attempt to use 1.5 or 2 inch Electrical Metal Tubing(EMT) which will have two plastic endcaps, then two ball bearings 0.430 inch dia. space 0.5 or 0.75 inches appart then filleed with clear epoxy, this should lessen the lichten burg problem as others have warned me. the flat capcitor plates have a bad tendency to concentrate the flux very near the periphery of the two plates thus causing high D/V intensity which breaksdown plastic like epoxy. with a sphere the field doesnt concentrate suddnelly, but predictablly and softer then the flat edges of plates. this would theoretically yield a better 100kv capable capcitor with out surface tracking or lichtenburg problems.
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