Plasma Sterilization

In still another use for high voltage, researchers have developed a plasma that operates at room temperature. The plasma is generated between a high voltage probe and the surrounding room air. One use envisioned is to sterilize medical instruments. A second use is to kill bacteria, specifically dental.

Generally, plasmas operate at high temperature, and thus are impractical to use on humans. However, these develpments appear promising.

The research paper is in Applied Physics Letters.

A Few Low Cost High Voltage Power Supply Circuits

Here's a circuit that generates 100V from 25V. Output current is 25mA. You can find it at http://www.edn.com/index.asp?layout=article&articleid=CA46343

Another high voltage power supply circuit is at http://www.instructables.com/id/Cheap-Semi-High-Voltage-Power-Supply/ It is actually just a high voltage multiplier circuit, and is open loop. In other words, you put in an ac signal, and you get the high voltage output.

This is a low cost 50kV power supply. Like the one above, it is open loop, however in this case it includes a way to vary the output voltage. It uses parts that are available at low cost. See http://www.instructables.com/id/Build-a-Variable-High-Voltage-Power-Supply/

Alternating Current – Minimum Approach Distance

The following table is from Occupational Safety & Health Standards Board of the California Cal/OSHA program website. http://www.dir.ca.gov/oshsb/rubberglove.html Please see this website for more information.

It is related to conductors and equipment energized over 7,500 volts

Alternating Current – Minimum Approach Distance

Normal Voltage Range (Phase to Phase)Kilovolt	Approach Distance Phase to Ground Exposure
Above 0.6 to 15	2 ft 1 in.
Above 15 to 36	2 ft. 4 in.
Above 36 to 46	2 ft 7 in.
Above 46 to 72.5	3 ft 0 in.
Above 72.5 to 121	3 ft 4 in.
Above 121 to 145	3 ft 7 in.
Above 145 to 169	4 ft 0 in.
Above 169 to 242	5 ft. 3 in.
Above 242 to 362	8 ft 6 in.
Above 362 to 552	11 ft. 3in.
Above 552 to 765	15 ft 0 in.

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.

Extremely low cost high voltage power supplies

If you are looking for an extremely low cost high voltage power supply, a good place to look is consumer products that incorporate high voltage. For example there are electronic fly swatters that claim to produce 4kV! Of course the current is very low, and performance in terms of ripple, regulation and stability is not all that good. The electronic fly swatter may not even put out nearly the voltage claimed. However, if these performance related issues are not important in your application, or just need to generate a spark, you're in luck. Besides, you can find them for under $10, and sometimes even under $5. Other products to consider as a source of low cost high voltage are air purifiers, mosquito zappers, LCD display backlights, piezoelectric spark generators, and vehicle electronic ignitions.

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.

High Voltage Comments

You may noticed a recent change in our high voltage blog. You now have an opportunity to leave a comment on any of the posts. Feel free to indicate what you like/dislike, what you agree/disagree with, or you may submit ideas for future posts. Questions are always welcomed. Of course we expect all comments to be in keeping with the professional nature of the blog.

We look forward to your comments!