High Voltage Wine

Applying a high voltage electric field can shorten the ageing process of wines. Research has been underway for ten years. Taste tests by a panel of experts indicate that this works.

The best results appear to occur with a field strength in the range from 300Vac/cm to 600Vac/cm. Details of the plug-in ageing process have been reported in New Scientist magazine. ''Not only can it shorten a wine's normal storage time, it can also improve some lower-quality wines,'' according to the New Scientist article.

Human Body Resistance and Capacitance

Here is an interesting piece written by Paul E. Schoen of P S Technology, Inc.

There is some more information [regarding the resistance of a human body] at http://van.physics.uiuc.edu/qa/listing.php?id=6793 , where it states that the external human body resistance is about 1k to 100k Ohms, and the internal resistance is 300 to 1000 ohms. Only a thin layer of dry skin separates the internal resistance from an external object.

The human body capacitance to a far ground is 100-200 pF, which is really a minimum value. This correlates to an impedance of about 13 megohms at 60 Hz, which corresponds to a minimum of 9 uA at 120 VAC to ground. This is enough to be sensed and used for capacitively operated light dimmers.






Here is a way to measure your body capacitance: http://web.mit.edu/Edgerton/www/Capacitance.html






The inside of your body can be considered a conductor, and thus if you place your hand flat on a metal plate, you will form a capacitor with an area of perhaps 15 square inches, with a thin (maybe 0.005”) insulating layer of dry skin, which will form a capacitor much higher in value than the 200 pF stated above. According to a formula in http://www.sayedsaad.com/fundmental/11_Capacitance.htm , this would be C = 0.2249 * k * A / d = 1350 pF, (assuming k for skin is 2, about like dry paper). This will be an impedance of about 2 megohms , and current of 60 uA. This is still below the normal threshold of sensation, and still far below the usual safe current levels of 1 to 5 mA.






The actual thickness of the epidermis (per http://dermatology.about.com/cs/skinanatomy/a/anatomy.htm ) varies from 0.05 mm (0.002”) for eyelids to 1.5 mm (0.06”) for palms and soles, but the actual outer layer of the epidermis that is a good insulator is composed of flat, dead cells, which is much thinner. So the capacitance could be much higher than the quick estimate above.






Probably the main reason for electrical current to reach levels high enough for electrocution to occur (6 to 200 mA for 3 seconds, according to http://www.codecheck.com/ecution.htm ), is when skin becomes sweaty or otherwise loses its dry protective layer, which quickly exposes the underlying 1000 ohms or less, which will conduct 120 mA at 120 VAC.






There are safe ways to measure the body’s resistance and capacitance using realistic higher voltages, skin conditions, and contact surfaces, but I’m not going to suggest anyone try it. Suffice it to say that ohmmeter readings are misleading, and any carelessness around any kind of voltage source can be dangerous.






For very high voltages, there are standard minimum distances that must be maintained between a worker and an energized line: http://www.dir.ca.gov/oshsb/rubberglove.html . I found this on a search for rubber glove testing.






The field intensity near high voltage lines is so great that it might be fatal to touch them even if you were suspended in free air. You may notice that birds can sit on lower voltage transmission lines which are 5kV to 50 kV or so, but not on 200kV+ lines.

Welcome Package for Opto Isolators

New high voltage opto isolators are always a welcome addition to a designers. Optek has a new configuration that you may find handy.

They have axial fiber optic isolators offered in isolation distances from 18mm to 80mm. Providing power electronics engineers with a means to isolate high voltage input/output circuits over varying distances, TT electronics OPTEK Technology has developed a series of axial optical isolators using plastic optical fiber. Designated the OPI1270 Series axial fiber optic isolators, the devices are constructed using a visible red LED emitter and a phototransistor detector in separate opaque molded plastic modules, linked by a shrouded optical cable that shields the optical signal from dust and other contaminations.

Your High Voltage Story

We are publishing your stories, comments or impressions of high
voltage. Tell us about design challenges, interesting applications,
shocks, problems, or solutions that relate to high voltage
electricity. We will gather and publish them.

Please email us at fred@highvoltageinfo.com with your story. Please
use "High Voltage Story" as the subject.

We reserve the right to edit all submissions, and all appropriate
legalese apply.

Precision High Voltage Resistors

When selecting and subsequently evaluating precision HV resistors, it is important to consider how rugged the parts are, with respect to surges and rapid voltage transitions. Both conditions can occur in typical applications.

Precision HV resistors are often used in high voltage divider applications. Thus, you are relying on the absolute resistance to remain constant over long term conditions.

If there is significance capacitance across the divider, then it is possible to have a large surge current if there is a momentary fault within the capacitor. THis is not unusaul with high voltage film capacitors, as they are self-repairing (if the internal fault is minor). High surge current can cause localized heating, resulting in permanent resistance change (as well as a temporary change).

If there is a rapid dV/dt voltage change, such as when an external arc occurs, it is possible for the resistor to suffer a permanent change in resistance value. In addition, depending on parasitic capacitances, voltage during a transient will most likely not distribute evenly over the total divider resistance. Thus, the localized dV/dt may be very high, and the absolute voltage across some portions of the resistor(s) can be higher than its ratings.

Thus, it can be worthwhile to run surge and dV/dt tests on precision resistors that are being considered for your applications.

How to Repair Live High Voltage Transmission Lines(!)

Generally voltages above 110kV are used for transmission lines. While voltages can be as high as 1200kV, the bulk of energy transmission occurs between 138kV and 765kVac. So, repairing live transmission lines is a bit difficult, to say the least!

Here's an excellent video on how high voltage power lines are inspected prior to repair:
http://www.youtube.com/watch?v=9tzga6qAaBA

Tell us your high voltage story!

We will publish your stories, comments or impressions of high voltage. Tell us about design challenges, interesting applications, shocks, problems, or solutions that relate to high voltage electricity. We will gather and publish them.

Please email us at fred@highvoltageinfo.com with your story. Please use "High Voltage Story" as the subject.

We reserve the right to edit all submissions, and all appropriate legalese apply.

Cockcroft-Walton Voltage Multiplier

In addition to using a transformer to multiply a voltage, another useful technique is the voltage multiplier. This technique uses an arrangement of capacitors and diodes to multiply an ac voltage and rectify it.

The circuit was named after James Cockcroft and Ernest Walton, who developed it in 1932 at Brookhaven National Laboratories. They needed a high voltage for an accelerator they were building. They received the Nobel Prize in 1951 for their work.

Some people like to refer to this circuit as Greinacher multiplier, since it was developed by Heinrich Greinacher in 1919. So, the voltage multiplier may have been independently developed twice?

High Voltage Safety

There are many safety procedures and safety equipment that must be followed to maximize high voltage safety. Sometimes it is easy to forget one of the most obvious aspects of high voltage--you cannot see it. Thus, it is easy to be complacent while working around high voltage and forget that it is there. Even in the presence of a "Danger High Voltage" sign or while wearing protective clothing, the danger is not always apparent.

High Voltage Resistors

Once the resistor voltage rating goes above a few hundred volts, there are very few manufacturers to choose from. The main types of high voltage resistors are:

• Surge high voltage resistors, which are used in series with circuit elements that might experience arcs or intentional transients. Examples include resistors in series with an arc lamp or the resistor in an RC filter since they will need to carry current due to arcs.
  
• Precision resistors (used for high voltage dividers). Typically these are thin film resistors.
  
• General purpose high voltage resistors, generally thick film. Compared to precision high voltage resistors, these are not as stable, have worse voltage coefficients, worse temperature coefficients, and worse tolerances. However, they are lower cost and can be rated at higher power. Applications include high voltage dividers where precision is not important, load resistors and bleeder resistors.

More than with low voltage resistors, it is important to select a high quality manufacturer to obtain a part with good performance and reliability.

High Voltage Capacitors

What makes a capacitor identifies as "high voltage" is a rating above 2kV. That's because there are many manufacturers that build capacitors rated up to 1600V or 2000V. (This is in accordance with my definition of high voltage, "High voltage starts at the point where designers have to consider additional technical issues, and where there are significantly fewer component suppliers to choose from. " Typical dielectrics used for high voltage capacitors include ceramic, polyester and mica. Issues that become more important at higher voltages that affect performance and reliability include capacitor construction (and how it affects the electric field), voltage coefficient, creepage distance along the surface, surge current capability (during an arc) and compatibility with potting compounds. See this list of high voltage capacitor manufacturers.

High Voltage Transformers

High voltage transformers are used for isolating two sections of a circuit or system, and along with voltage multipliers, generate the output of high voltage power supplies. Some HV related issues that need to be addressed in the design include:

• layer winding or universal winding (also know as pi winding)
• volts per layer
• compatibility with subsequent potting or encapsulation. (Incompatibility can cause failure of the potting material to cure, subsequent failure due to dielectrics in series, and other problems.)
• clearance and creepage distances

What is High Voltage?

A good place to start is to define high voltage. There is no universally accepted definition. So, here's a list of high voltage definitions to select from!

Related is this high voltage glossary, which can come in handy now and then.

Introduction to High Voltage

Welcome to this blog on high voltage. We'll discuss all aspects of high-voltage, including what it is, what makes it different from low voltage, and what precautions are necessary. We'll comment on high voltage components and high voltage equipment. We'll address high voltage design issues, and applications that use high-voltage. Also included will be medium voltage and issues useful to electrical utility service. For an excellent resource on high voltage information, please see http://www.highvoltageinfo.com/

If there are specific issues that you would like to see covered here, please let us know!