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08 Jun 14:01

High Voltage Please, But don’t Forget the Current

by Steven Dufresne

In high voltage applications involving tens of thousands of volts, too often people think about the high voltage needed but don’t consider the current. This is especially so when part of the circuit that the charge travels through is an air gap, and the charge is in the form of ions. That’s a far cry from electrons flowing in copper wire or moving through resistors.

Consider the lifter. The lifter is a fun, lightweight flying machine. It consists of a thin wire and an aluminum foil skirt separated by an air gap. Apply 25kV volts across that air gap and it lifts into the air.

Lifter flying with high voltage power supply Lifter parts showing the thin wire and aluminum foil

So you’d think that the small handheld Van de Graaff generator pictured below, that’s capable of 80kV, could power the lifter. However, like many high voltage applications, the lifter works by ionizing air, in this case ionizing air surrounding the thin wire resulting in a bluish corona. That sets off a chain of events that produces a downward flowing jet of air, commonly called ion wind, lifting the lifter upward.

Small Van de Graaff generator Lifter in the dark with bluish ionization

But that ionization of air requires current, electrons flowing, in the wires coming from a high voltage power supply. In fact, that ion-filled air gap is the equivalent of a high resistance wire in the circuit, along with some capacitance; it’s a part of the circuit.

Lifter circuit and equivalent circuit with resistor and capacitor instead of the lifter
Lifter circuit and equivalent circuit

A Van de Graaff generator, even a DIY 84kV one, is a low current power source and cannot supply a high enough current to ionize enough air fast enough to produce the necessary strong jet. However, the power supply powering the lifter above converts the energy from a wall socket using a flyback transformer and a Cockcroft-Walton voltage multiplier. That way it produces sufficient voltage and supplies more than enough current.

Another example is the corona motor, a type of electrostatic motor, that also works by producing a high voltage across an air gap, multiple air gaps in fact. The gaps are between sharp metal blades and a neutral plastic cylinder. The blade sprays ions across the gap onto the cylinder.

Corona motor turning fan Corona motor in the dark with ionization
Corona motor powered by the triboelectric effect
Corona motor powered by the triboelectric effect

However, the gap width is very short, requiring a lower voltage. And more importantly, the cylinder doesn’t have to be ionized much for the cylinder to start turning meaning that not as many ions are needed. i.e. the required current is lower than with the lifter. In this case perhaps the lowest current high voltage power source I’ve ever worked with, rubbing a PVC pipe with a cotton cloth, is sufficient. That utilizes the triboelectric effect.

And then there are applications where all that’s needed is to accumulate charge in a capacitor until there’s enough, taking as much time as necessary. An obvious example is to simply produce a big spark.

An example of that is a TEA laser. A TEA laser works by accumulating charge across a small spark gap and in two flat plate capacitors. When the voltage has built up sufficiently to breakdown the air in the spark gap, it fires, causing a subsequent sparking across the lasing channel, resulting in a laser beam. In the photos below you can see the laser being powered by a low current Wimshurst machine and by the same powerful Cockcroft-Walton power supply mentioned above for flying the lifter. Both produce an identical voltage, accumulate the same amount of charge and fire an identical laser beam. However, the Wimshurst machine requires around 12 seconds of hand-cranking to do so, resulting in the laser firing only every 12 seconds. The Cockcroft-Walton power supply fires the laser around every 1 second.

Tea laser powered by Wimshurst machine Tea laser powered by Cockcroft-Walton PSU

How do you choose a high voltage source with sufficient current to match your application? This is largely done by experience. The lifter needs to move a large mass of air, and to do so a large quantity of ions are required, and hence a high current to create those abundant ions. On the other hand, a corona motor works using the Coulomb force. Blades of one polarity repel areas of cylinders that are charged with the same polarity. With just a fan as the load on the cylinder, not much repulsion force is required. Naturally for a bigger load, more force would be required and hence a faster charging rate and so more current. Similarly with the TEA laser, more frequent laser beams require higher the current.

I’ve already taken you through a range of high voltage sources available to you. With the needed high voltage already available from any of those sources, we now need to look at the current available from them.

The flyback transformer and Cockcroft-Walton voltage multiplier power supply gets its power from the wall socket. Taking into account losses in the various resistors, transistors, capacitors, diodes and the flyback transformer itself, there’s still a relatively large amount of current available, even if it is in the single- or double-digit milliamps. For high voltage that’s considered quite a bit (remember that power is the product of voltage and current).

On the other end of the scale, the triboelectric effect works by a transfer of electrons when making contact between two specific materials, and the retention of those electrons when contact is broken; basically, it works by rubbing the materials together. In that case, very little charge is transferred compared to the current coming from a wall socket.

The Van de Graaff generator actually starts with the triboelectric effect. It’s the making and breaking of contact between the rollers and the belt that is the rubbing together of two materials. However, unlike rubbing a cotton cloth against a PVC pipe by hand, in a Van de Graaff generator the rollers can be rotating at hundreds of RPM, generating charge more rapidly. But the amount is somewhere in the low microamps for a tabletop Van de Graff, small compared to wall socket current (that’s assuming you’re taking charge directly from the Van de Graaff’s dome and not waiting for sparks.)

And a Wimshurst machine generates its charge by induction when sectors on the opposing disks pass a neutralizer bar. Surprisingly, it’s possible to dimly light a small 20mA LED, where the LED is placed in series with the neutralizer bar. However, by the time the charge is inefficiently removed at the collectors, it’s reduced substantially. The current from the output of a Wimshurst machine is usually in the single digit microamps (again, we’re not waiting for sparks.)

Those are some of the examples I can recall where people, myself included, have forgotten that just because high voltage is involved, that doesn’t mean that basic electronics no longer applies. I’m curious what examples you’ve encountered, either where you or others have forgotten about the current or maybe even some other electrical property. Let us know in the comments below.

Filed under: classic hacks, Engineering, Featured, Original Art
18 Sep 06:30

Learn All the Windows 10 Keyboard Shortcuts with This Cheat Sheet

by Melanie Pinola

Windows 10 comes with a few new keyboard shortcuts to take advantage of new features like the virtual desktop and the action center . Here’s a handy table of shortcuts you might not know about yet.


08 Jun 01:20

Oh my glob.

10 Jul 11:00

Open an Incognito Window and Look at the Highlights of Last Night's Game



Open an Incognito Window and Look at the Highlights of Last Night's Game

Submitted by: (via PHKatie)

Tagged: pr0n , twitter , world cup , failbook
11 Jun 05:45

All of These Years Wasted Wearing Purple, Charles

02 May 04:00

xkcd Phone


So much want :D

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16 Apr 17:40

Really have to try one of these someday

18 Mar 15:40

Fill'er up, human

17 Mar 10:15

He is everywhere!



11 Mar 04:38

Dear people who appreciate jokes,

Dear people who appreciate jokes,

What do you call it when Batman skips church?

Sincerely, a Christian Bale

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03 Jan 05:00


The problem with Haskell is that it's a language built on lazy evaluation and nobody's actually called for it.
07 Mar 09:55

Amazing dark souls 2 full scale statue.

05 Mar 22:10

20 Terrifying Two-Sentence Horror Stories

03 Mar 20:30

This one is called "The debt".


ahh, the wonders of uni

20 Feb 21:30

Parents Are Getting Really Protective

Parents Are Getting Really Protective

Submitted by: Unknown

Tagged: highschool , dad , prom , parents , g rated , dating
20 Feb 22:00

Eye See You!

Eye See You!

Submitted by: Unknown

Tagged: box , spy , if i fits i sits , Cats , funny
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My Friday night

03 Mar 10:25

Scumbag Brain...seriously...

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My favorite Tennant moment.

28 Feb 08:25

As a college student, it's true.

28 Feb 10:45

Arab pickup line



27 Feb 21:46

well SHIT says FML

by well SHIT

Today, while having a sneak through my brother's browser, I found a bookmark for a Google Docs file. It was a short story involving him horrifically killing our entire family. It ended with the words: "And that is what happens when people don't respect the author's privacy." FML

15 Feb 23:14

Anonymous says FML

by Anonymous

Today, I proudly informed my grandma that I now have a girlfriend. My grandpa overheard and said how surprising that was, given how expensive blowup dolls are. He and my grandma then both laughed out loud. FML

30 Dec 07:09

Michael Schumacher's former crew went to visit him.

by Joe Robbins
Michael Schumacher's former crew went to visit him.

They changed the wheels on his bed and his drip in 11.4 secs.
15 Nov 08:00


22 Nov 05:00

Telescope Names

The Thirty Meter Telescope will be renamed The Flesh-Searing Eye on the Volcano.
06 Dec 05:00

Galilean Moons


Makes me lol every time :D

I'm SO glad I escaped. They almost had me caught in their weird ... thing.
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Feel the White Hot Rage of a Scientist

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Submitted by: Unknown

21 Dec 13:30

Scientifically Accurate Twinkle Twinkle Little Star

09 Dec 13:00

Yeah, No Thanks...

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Submitted by: Unknown