Saturday, July 28, 2007

Detect Flaws in Coatings

The presence of flaws in coatings, sometimes referred to as porosity
or holidays, can be detected with a DC high voltage spark tester.

This technique is good for finding flaws such as pinholes or very
thin sections in non-conductive coatings that have been applied to a
conductive substrate. The ground wire from the tester is first
connected to the substrate, and then the voltage is set so that if
there is a flaw, the voltage from the tip of the high voltage probe
will cause an arc, or current flow to occur. The high voltage probe
is moved along the coating surface to look for defects.

Depending on the type of coating and thickness, the voltage setting
can range between 500V and 30kV for most cases. Of course, proper
safety precautions must be followed.

Thursday, July 26, 2007

High Voltage Batteries

While the terms "high voltage" and "batteries" don't seem to go
together, there are at least two places where they do.

In hybrid vehicles, battery voltages can be more than 300V. Thus,
the same care must be given as when working on any high voltage system.

Second, some batteries, such as 3.6V batteries in AA size, have been
called "high voltage batteries." I think a better description would
be "higher voltage batteries."

Wednesday, July 25, 2007

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.

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Tuesday, July 24, 2007

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:

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Monday, July 23, 2007

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 with your story. Please use "High Voltage Story" as the subject.

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

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Friday, July 20, 2007

Biodegradable Insulating Liquid

Another high voltage insulating liquid is Envirotemp® FR3® manufactured by Cooper Power Systems. It is a transformer fluid made from soybeans, and is biodegradable. It does not contain any petroleum, halogens or silicones. It is fire resistant, and is currently used in both indoor and outdoor transformers. Its dielectric strength is rated at 56kV for a 2.0mm gap.

Environmentally friendly insulators such as this could be good for products that are WEEE compliant. (Waste Electrical and Electronic Equipment directive 2002/96/EC.)

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Thursday, July 19, 2007

Upgrade for Planet Analog

One of my favorite resources on electronics has been Planet Analog because of its focus on analog and power electronics. Edited by long time analog writer and editor, Bill Schweber, it includes news, product announcements and articles on analog circuit design. High voltage related articles and products show up regularly.

In the July 16, 2007 edition of EE Times, Bill announced a "New, improved and vital (really)!" Planet Analog. In an era where digital takes the spotlight, those of us in the (real) analog world have this to look forward to, in every issue of EE Times.

You can find the online edition at


Wednesday, July 18, 2007

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?

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Tuesday, July 17, 2007

High Voltage Insulators: Solids

As you no doubt expected, here is the final installment in this brief introduction to high voltage insulators, solids, of course. However, this is limited to common encapsulating solids.

For best results, insulating materials should be homogeneous, and should not contain air spaces or moisture. This is especially critical with solids, as corona and arcing can occur in air voids.

Epoxy is best when you need excellent adhesion. It has good thermal conductivity for a solid.

Silicones come in two basic types. The curing process of additive cure silicones can be accelerated with the addition of heat, so that it can complete in an hour. Condensation cure silicones can take 24 hours or more to cure, however, the adhesion properties are better. For both types, adhesion can be an issue.

Other potting materials include tar (low cost) and polyurethane.

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Monday, July 16, 2007

HIgh Voltage Insulators: Liquids

Oil is commonly used in high voltage transformers and high power systems due to its excellent thermal conductivity, which can be improved further by circulation. Handling oil can be messy, and there can be environmental regulations to deal with if large quantities are used. One common type is manufactured by Shell.

Silicone oil is another good liquid insulator. In addition, silicone heat transfer fluids such as Dow Corning Syltherm™ can also be used as high voltage insulators.

Perfluorinated liquids such as Fluorinert™ are clean and easy to use. They are great insulators and can be used for testing high voltage circuits before they are potted. However, they are expensive, some types evaporate quickly.

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Sunday, July 15, 2007

High Voltage Insulators: Gases

Air is the most common insulator for high voltage. Cost is certainly a big advantage. It can contain a variable amount of moisture. I would consider it to be “dynamic” in that there is movement such as convection currents, and can sometimes contain unwanted particles or ions. Thus, its dielectric strength varies with time.

Sulphur Hexafluoride (SF6) is a great gaseous insulator, however, it is extremely hazardous, so it must be handled carefully, and used in a sealed chamber. One hazard is that poisonous gases are generated if there is an arc. Second, it is denser than air. Thus, if it is breathed in, it will settle in the bottom of the lungs, and displace the air, causing suffocation.

A good compromise may be nitrogen. It is not as hazardous as SF6, does not contain moisture, and is homogeneous.

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Friday, July 13, 2007

What is Medium Voltage?

Various definitions exist. In general, medium voltage is a term used by the electrical power distribution industry. Here are a few ways medium voltage is defined:

ANSI/IEEE 1585-2002 refers to: Medium Voltage (1 - 35 kV). [It is assumed that this is ac.]

IEEE Std 1623-2004 refers to: Devices rated to medium voltage (1 kV-35 kV). [It is assumed that this is ac.]

NECA/NEMA 600-2003 refers to "medium voltage cables rated from 600 volts to 69,000 volts AC"

Littlefuse says: "The terms “Medium Voltage” and “High Voltage” have been used interchangeably by many people to describe fuses operating above 600 volts. Technically speaking, “medium voltage” fuses are those intended for the voltage range from 2,400 to 38,000 VAC."

Siemens says, "Medium voltage metal-clad switchgear (above 1000Vac up to 38kVac)"
This last one covers most of the definitions. Thus, probably a good working definition of medium voltage is from 1kVac to 38kVac.

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Thursday, July 12, 2007

Medium Voltage Definition

1kVac to 38kVac seems to be the consensus. More details to follow.


Tuesday, July 10, 2007

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.

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Monday, July 9, 2007

Arc-Fault Circuit Interrupters

Arc-fault circuit interrupters (AFCI) will be required in new homes staring in 2008. This is a requirement in the National Electrical Code, and is in addition to GFI (ground fault interrupters). The AFCI is supposed to trip whenever eight cycles of arcing occur with a period of 0.5 seconds.

With mass production of arc detection circuitry, there is an opportunity to utilize these circuits and IC's, dedicated to arc detection, in high voltage power supplies and systems to detect high voltage arcs.

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Sunday, July 8, 2007

Arc Flash Clothing

Selecting arc flash protective clothing

1. See NFPA 70E 2004 Edition table 130.7 (C)(9)(a) for hazard risk category.

2. See Table 130.7 (C)(10) to determine what type of clothes and equipment are required based on the hazard risk category

3. See Table 130.7 (C)(11) to determine the ATPV (arc thermal performance value) rating needed.

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Friday, July 6, 2007

High Voltage Connectors

There are very few industry standard high voltage connectors. About the only HV connectors manufactured by multiple companies are the Tyco LGH series, and the so-called federal standard connectors, used in x-ray systems. In addition, there are a few connectors that are second sourced by other manufacturers.

LGH high voltage connector

Often the connector is proprietary to the HV power supply manufacturer. At voltages above a few kV, special processes are necessary to ensure that the connector is reliable. Encapsulation, or potting, may be required when attaching the connector to the HV wire. The high voltage wire itself may need to be treated, and any semiconductive layer will need to be removed.

Federal Standard HV connector


High Voltage Diodes


  • Avalanche type is preferable, especially when using hv diodes in series, or if it is possible for a reverse voltage transient to appear across the diodes, such as resulting from an arc or flashover.
  • Greater forward voltage drop because a high voltage diode is comprised of multiple diodes in series, which enables the part to achieve a higher voltage rating.
  • Pay particular attention to creepage distance, and adhesion to whatever potting material is used.
  • Quality of the manufacturer is most important when evaluating diodes.

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Thursday, July 5, 2007

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.

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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.

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Wednesday, July 4, 2007

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

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Tuesday, July 3, 2007

High Voltage Power Supplies

A high voltage power supply consists of the following sections:
  • ac to dc converter (for ac input types)
  • dc to ac converter which generally operates at 20kHz or above
  • high voltage transformer
  • voltage multiplier, consisting of hv capacitors and hv diodes in a configuration that, as the name implies, multiplies its input voltage by an integral amount (neglecting losses and inefficincies). This also serves as the rectifier, converting the ac back to dc.
  • output filter (to reduce the ripple to the desired amount)
  • hv divider to measure the output voltage
  • control and monitoring circuits
  • output hv connector

Sometimes known as high voltage generators, high voltage power supplies are available in pc board modules, larger modules and as rack mount high voltage power supplies. Very high voltage and very high power supplies can encompass an entire rack, and/or have an external high voltage section.

Both linear and switching mode configurations are available.

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Monday, July 2, 2007

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.

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