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Struggling with my coil having very low impedance on the primary side...

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si2030

Sat Dec 24 2011, 08:00AM

Registered Member #1571 Joined: Wed Jul 02 2008, 03:26AM
Location: Bendigo Victoria Australia
Posts: 44

Hi there,

Been buggering around with a test rig for a Cockcroft Walton Multiplier. Here is a picture of the rig.

Power is supplied via an isolation transformer followed by a variac followed by a globe rig followed by a stepdown transformer and then the electronics. At the moment I am using a 4046 but am feeding the signal in it and sweeping the frequency to find resonance.

The air coil has 16 turns on the primary and 1300 turns on the secondary. Secondary Inductance measured at 2.148H with R=0.811ohm. Primary Inductance = 25mH

The aim was to get the coil to work in resonance and produce 100kV peak to peak.

I used a full bridge with IGBTs and for the single stage CW I made my own caps that come to 39pF.

I also included a globe rig in upstream of the variac.

My problem is that when I use the globes in series with the coil the coil has such a low impedance that I get nothing over the bridge and all the power dissipating in the globes. Take the globes out and, well the bridge is really seeing a dead short....

How do I approach this? How can I lower the impedance presented to the bridge and still get this to work. I have thought about building a ballast inductor but it will probably require ferrite as the frequency is running at 18.05kHz at resonance.

A friend has suggested using a different topology in which you feed the primary a sine wave rather than pulse... by using a transistor in a flyback configuration.

Right now I am open to ideas and suggestions re the impedance problem and the topology.

Kind regards
Simon

Steve Conner

Sat Dec 24 2011, 11:55AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, here's how I design a dual resonant system.

Start with an idea of what voltage, current and power (all real, not kVA) you want at the output. Let's say you want 100kV, 30mA, for 3kW of power.

You have a voltage and a current, so you can calculate an impedance: 3M ohms.

The secondary coil also has a characteristic impedance, the square root of (L/C). Any Tesla coil calculator should tell you C. The characteristic impedance should be one-tenth of the load impedance.

Now for the primary coil. Let's say you have a full bridge inverter running off 320V DC, and since you believe in conservation of energy, you designed it to supply 3kW, plus a bit for luck. So the inverter has an output impedance too. The output voltage is 320 times 4/pi as we are only interested in the fundamental of the square wave: about 400V peak, 280 RMS. Hence the output current is 3000/280 = about 11A. So the impedance is 280/11 = about 24 ohms.

You design your primary to have a characteristic impedance about 10x that, and the tuning capacitor needs a 4kV peak rating.

If you're using a single resonant system with an untuned primary, the secondary is designed the same way, but I always adjusted primary turns by trial and error.

si2030

Sat Dec 24 2011, 03:06PM

Registered Member #1571 Joined: Wed Jul 02 2008, 03:26AM
Location: Bendigo Victoria Australia
Posts: 44

Hi Dr Watt (Conner),

Thankyou for your reply.

My intention is to drive a CW at resonance.

As you have mentioned, my voltage should be 100kv, 30mA so the impedance at resonance will be 3.3M ohms.

This is where I get a little confused. I have trouble calculating the correct frequency for resonance where one side of the tank is the secondary but he other is a series of capacitors in CW tower... whats the equivalent capacitance of a CW tower?

Does Characteristic impedance matter for a CW tower?

Moving on to the Primary. How do you design a primary to be, in this instance, 240ohms? do I add a resistor? If so how do you construct a resistor that handles 3kW...

In this situation I had only planned for the secondary to be in resonance with the CW stack...

Some really basic questions but it all really helps.

Kind regards
Simon

Electra

Mon Dec 26 2011, 06:51PM

Registered Member #816 Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156

Impressive looking multiplier by the way. Though there are more knowledgeable people here with respect to this, Seeing as only the input capacitor seeâ€™s AC, I would have thought the input impedance would look like whatever value the first capacitor is, in series with a resistor equivalent to the transformed down output resistance, should this be like a transformer where you divide the output impedance by the square of the voltage ratio? (Or number of stages/2) (edit) or rather number of stagesx2.

I guess if the load on the output changes a lot or is unknown would this not shift the resonance frequency of the secondary by altering the complex reactance?

The Characteristic impedance of the primary is sqrt (L/C) as Steve said, so you would use this to select inductance, i.e. number of turns on primary.

Patrick

Mon Dec 26 2011, 07:06PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

that is a great oil tank, very nice. did i inspire you? Which oil did you use?

Antonio

Mon Dec 26 2011, 07:27PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

si2030 wrote ...

The air coil has 16 turns on the primary and 1300 turns on the secondary. Secondary Inductance measured at 2.148H with R=0.811ohm. Primary Inductance = 25mH
The aim was to get the coil to work in resonance and produce 100kV peak to peak.
I used a full bridge with IGBTs and for the single stage CW I made my own caps that come to 39pF.

The inductance and resistance values don't seem reasonable for an air coil. 25 mH is far too much for 16 turns. Maybe 25 uH (micro). 2 H is also too much. Maybe 2 mH for this resistance.
It's not clear if this setup can exhibit a resonance, with a diode-capacitor multiplier as load. You can probably make the transformer work as a transformer or in flyback mode but not at too high frequency.

Patrick

Mon Dec 26 2011, 07:32PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

antonio may be right. might we see a proper schematic?

Fisherdog3

Tue Dec 27 2011, 03:01AM

Registered Member #4290 Joined: Sat Dec 24 2011, 06:08PM
Location:
Posts: 5

Wow, if all else fails, it still looks great. Where did you get that acrylic, the only place i would be able to get that much it would cost around 200 USD

teravolt

Tue Dec 27 2011, 06:16AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

if you had a poly cap in parallel with your primary it would reduce the impedance that your bridge sees at resonance and increase your circulating current in the primary. have you built a ZVS before? You could put a group of ferrite U cores around your coil to concentrate your magnetic field. another option would to replace your coil with a custom fly-back and put it in the oil. If you had a big fly-back you could use your bridge or ZVS with a resonant primary.

Newton Brawn

Tue Dec 27 2011, 07:53PM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Hi Si 2030 !

Schematic with components specs may help answer your questions ...

Coil dimentions and winding data too

Regards

Newton

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