her
price. Labor time is about the same with Plexiglas, Lexan, and glass sheet capacitors. Exotic plastics such as
Teflon are not needed unless your application demands extreme chemical and thermal deterioration resistance.
Polyethelene has excellent chemical resistance, but breaks down gradually upon exposure to ozone gas (always present
around high voltage) becoming brittle and less resistant to arc puncture.
That brings us to another important consideration: the capacitor's useful life. To enhance a capacitor's life keep
the working voltage at or below the rated specification in both DC and AC applications. We discovered that charging
at no more than 70% of a capacitor's working voltage resulted in an amazing 10-fold increase in life time for one type
of commercial capacitor. Also, for DC capacitors, watch out for voltage reversals. If your system has a lot of
inductance, reverse voltage swings are always produced. Increase the safety margin if a lot of inductance is in the
circuit. Furthermore, the temperature should be kept below 120°F. As mentioned earlier, watch out for
superimposed AC, voltage spikes, and ringing. These types of AC waves can drastically shorten lifetime. Tesla coils
have notorious ringing. To repeat: if feasible, use an oscilloscope to visually analyze your circuit. Often a power
resistor inserted in the current path to the capacitor quenches ringing. With this criteria under our belts, let's
look at some problems your design and construction methods should prevent.
Signs of Trouble:
Your assembly techniques should seek to minimize the likelihood of a few possible problems. Luckily, all of them can
be prevent3ed at least in part by using ample amounts of insulating material such as No-arc or Corona Dope and/or high
voltage putty on all exposed areas. A plastic case to enclose the apparatus is also recommended (more on that later).
Still and all, you should know what problem insulation is preventing. The first problem insulation relieves is the
possibility of electrical shock.
Insulation also minimizes the production of ozone--a gas created when high voltage causes three oxygen atoms to join
together. Ozone has a tart, sweet "electrical" smell, and is 100 times as poisonous as carbon monoxide. Beware: it
quickly causes headache, nausea, vomiting, and respiratory irritation. In addition to insulating all the exposed HV
areas, you should also operate your equipment with good ventilation if it produces any ozone.
Closely linked to ozone generation is corona leakage. It is produced by a charge being leeched off a highly charge
object by the air. That typically produces ozone. However, sometimes a device (such as a Van de Graaff generator) is
constructed specifically to display corona discharge, and insulating it would defeat that purpose. In such cases,
good ventilation is the only practical means of hazard prevention.
Ozone can also be created by arcing, which can occur anywhere. However, ozone production is not the greatest hazard
arcing presents. At 50Kv, a spark can arc between an uninsulated contact and your body if you come within 2 inches of
the contact. Arcing commonly takes two forms; directly through a capacitor's dielectric (as mentioned earlier), or
across the edges of a capacitor's plates to an adjacent plate. A snapping sound indicates the presence of arcing, so
keep your ears open.
Arching from the edges of a capacitor plate, or anywhere the shape of a conductor changes abruptly (such as the tip of
a nail) is called point discharge. It can be readily observed in a dark room at very high voltages,. Small, bright
blue pinpoint(s) are seen leaking electrons into the air, accompanied by a hissing sound and copious ozone production.
Once again, insulation and proper ventilation are the proper solutions to all these problems, and there are some
specialized techniques to insulate your capacitors and otherwise improve the safety of your high voltage projects.
Let's get to those now.
A key ingredient in a good assembly is a proper case. Your capacitor's housing must protect it against moisture, 