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New guy with newbie questions.

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GrantX

Thu Sept 01 2011, 08:28AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Hey everyone,
First post here, been reading the forum for about a month. As a brief intro, I'm currently in my first year of Electrical Engineering and have started messing around with high voltage. At the moment I have 2 NSTs (6kV/30mA and 15kV/60mA) and a handful of high voltage capacitors and diodes.

For the first month all I did was set things on fire with the big NST before jamming together some magnet wire and pvc into the vague shape of a Tesla Coil (which has only given me a little corona glow so far).

Anyway I've decided I want a break from working on the SGTC and try something else. Is it possible to build a voltage multiplier for an NST? From what I understand I would need very large capacitors because of the low frequency, but what topography would I need? I believe it would need a capacitor leg from each output and ground (3 in total)?

On a similar note, I got a bag of ceramic doorknob capacitors and was testing them by attaching them to one output of the 15kV NST and drawing an arc. They all seemed fine, but one failed mid arc (short circuit). Capacitors were 30kV 1nF, so I thought they would be invincible to such abuse?

Enough of my rambling, and thanks in advance for any help.

Proud Mary

Thu Sept 01 2011, 09:26AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

This is a basic Cockcroft & Walton full-wave voltage tripler.

I use this circuit with a small 4kV-0-4kV continuous-duty ignition transformer to produce up to 30 kV @ 600Î¼A, but the circuit would be identical with a centre-tapped NST.

I use 10nF 20kV PTFE capacitors, and Chinese generic 30kV 10mA diodes. I control the output voltage by controlling the AC mains input with a miniature 1A variable voltage transformer. I protect the circuit against flash-over surge damage by having a 1MÎ© 40kV resistor immediately in series with the output.

This sort of simple circuit would not be popular with the arcs n sparks brigade, because of its low output current, but is excellent where you are bone idle, already have the parts knocking about, and want an HV PSU that will run continuously 24/7 without problems.

All C&W voltage multipliers suffer from voltage 'sag' and ripple when current is drawn, the amount of 'sag' depending on the size of the capacitors, the input frequency, and the number of stages.

Adam Munich

Thu Sept 01 2011, 05:00PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

And for the most part, that sag is unpredictable :p
CWs tend to put out a lot more voltage than expected unloaded (voltage that puts holes in my xray tubes!), and once you load them that voltage sags more than it is supposed to.

Here is some information on HV ravenor

GrantX

Fri Sept 02 2011, 04:36AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Thanks for the information, the teravolt website is very helpful. I think I'm sold on the CW, seems like it'll be a lot of fun.

If I wanted 100kV, what caps and diodes would be best? I have either a 15kV/60mA or 6kV/30mA NST for input (50hz)? I'm not too concerned about voltage sag and ripple, at this stage all I honestly want is a whole bunch of corona, ion wind and loud sparks :D.

Adam Munich

Fri Sept 02 2011, 04:47AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

100kv at what current?

GrantX

Fri Sept 02 2011, 05:27AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

A few hundred uA to a couple of mA, again I'm not fussy, as long as the sparks are visibly interesting I'll be more than happy with the final product.

I'm thinking of building it for the smaller NST, since then I can control the output with my 500VA variac, so I guess I'm going to need a lot more stages (but the smaller components will be cheaper.... hopefully).

Adam Munich

Fri Sept 02 2011, 11:06AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Edrop = I / ( f * C ) * (2/3 * nÂ³ + nÂ² / 2 - n / 6)

Edrop is the voltage drop
I is the current drawn in amperes
f is the frequency in hertz
n is the number of stages
C is the size of the capacitors used in farads

You're going to need some big caps at 60hz...

Proud Mary

Fri Sept 02 2011, 12:13PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

100kV @ 2mA = 200W. I can't see that happening with a C&W at 50Hz unless you have a very big budget.

Below is a giant 50Hz full-wave C&W multiplier built by Henning Umland of Denmark.

Henning says its no-load voltage is 160kV, which falls to 135kV when 10mA is drawn by an evil-looking X-ray tube.

The price of the monstrous 100nF 40kV capacitors is not of this world - you could buy a nice car for the cost of it all.

Make one mistake with this kind of open-plan high power machine, and you will be toast, my friend.

You can see more of Henning and his doomsday machine here:

Proud Mary

Fri Sept 02 2011, 12:17PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

posted in error and deleted

the mad scientist

Fri Sept 02 2011, 07:37PM

Registered Member #3768 Joined: Tue Mar 22 2011, 12:46AM
Location:
Posts: 107

CWs are tons of fun and easy to make i made 3 of them and i can use one to power the other, the initial power source is an ignition coil.

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