Stefan's Tesla-Pages

My Cockroft-Walton-Multiplier  (cascade)
again using a fly-back transformer


Table of contents:

specs of the circuit
small model (testbed, only 4 stages)
future experiments

The specs of the planned Cockroft-Walton multiplier are:


I'll use my first AC-Flyback or my ignition circuit or even build a special (more current) driver for it somewhere in the future.
I will NOT use oil for insulation after the experiences with my homemade capacitor. Instead, I'll "simply" use air as an insulator. Most of the people who build "hobby multipliers" write of lots of corona and flash-overs when using no oil or other potting (wax etc.). But until I have the same problems I blame this for a bad circuit layout, because I know it can be done by professionals like the company Glassman. They use potential rings (screening electrodes) around the HV-stack to "shield" the inner components. Their 300kV-stack is only 1m high, their 450kV-stack only 1.6m.
I'll know my knowledge is far from being an expert in building CW-multipliers, so I shoot for a 300kV-stack with a height of 1.5m, that's only 66% of the field strength (kV per height) than the professionals at Glasman do.  
Their units look like these ones:  
(click the images for full resolution!)
For higher output voltages they use another big toroid in the middle of the stack, dividing the stack in two halves (perhaps a good idea for a better storage of the device in the always crowded cellar).

Circuit plan (only some of the 22 stages drawn ):

An additional cap has to be inserted when joining the lower and upper 150kV cascade. Since I want to change the polarity by turning the stack upside down, I'll house the cap in a way that it can be "clicked-in". On the lower end, a shorting bar has to be inserted:


I plan to start with a 150kV-stack and make a second one which can be added on top of the first one - the CW multiplier doesn't multiply, it adds the voltages... I also want to have the flexibility in polarity, so I'll try to buildt it in a way that I can flip the HV-stack upside down to reverse polarity. Finally having two 150kV-stacks, I can make one positive and the other negative polarity (need two Rout units then!) or stack them for 300kV of either polarity:

Calculation of the height of the construction:

The 5cm at the lower and upper end is due to the stackable approach. The tube connector of the brown 75mm diameter center tube is 12cm, the overlap 4.3cm and 4.8cm (middle section 3cm).

Cross section will be something like this:

The potential rings have to be wider as the routing of the wire between the components (mainly due to wire in white PVC tubes between caps). So far I think I need at least 3.5cm clearence from the components/wires to the potential ring, the inner diameter of the potential ring has to be at least 18cm!

I'll arrange the components (red: capacitor, blue: resistor, black: diode) around a beautiful shiny brown 75mm diameter tube (looks like the brown line isolaters for >>10kV), add some fiberglass plates (yellow) to increase stablity and provide a rigid frame to connect the potential rings (grey) to. The screwed connectors between the fiberglass boards are black PVC tube with screwes at the ends. The orange ring in the schematic above is a bigger tube in which the CW will be stored. It will be removed before going alive on stage of course.

Each 150kV-stack will have a height of 0.60m without toroids when build like depicted above.

At first, I tried to arrange the caps vertically, the caps have a length of 48mm and will be spaced 7mm apart to allow soldering. But that resulted in 55mm per stage. Currently, I make a redesign and try to place the caps more horizontally, so that the height can be decreased. Of course the whole thing gets wider then. But considering the final version with 300kV and 3 toroids, the whole thing else will be ~2.1m tall with the caps standing upright (ooops, I planned 1.5m)! The horizontally arrangement will allow for 43mm height per stage if I place some tubing arount the wires between the caps to prevent arcing (hopefully, not ested up to now...).

Another limiting factor besides diameter (storage!) and height when fully erected (for firing) is the size of the styrofoam rings I plan to use as potential rings. I only can get them in 200x40mm, 220x40mm, 250x50mm and 300x60mm at the moment. Perhaps I'll use selfmade rings of installation tubing (for house wiring) which I have in 25mm outer diameter and which I can get eventually in approx. 40mm outer diameter.

Small model with 4 stages:

I'll build a small modell with only 4 stages at first to get some experience. I don't know how tricky the air insulation is in reality and I don't want to drill any holes in the wrong place all over the big structure. 

Simulation of small model with 4 stages:

Since I'll build a small modell with only 4 stages at first, I tried to simulate it's behaviour. With the student version I only can do a simulation with up to 7 stages, so I can't simulate the whole model anyway.

The simulation was done with 50k and 63k resistores at the diodes, which I planned to use originally. BEcause of their insufficient voltage rating, I aquired some 100k resistors. A first calcuation showed nearly identical results, therefore I leave the old simulation results here: 

Rload is shown as 1Ohm, that's short circuited. Other values for simulation are 50.000kOhm and open (10GOhm).

Since I didn't measure the short circuit current my driver can deliver up to now, I "guestimated" its impedance from the input power and output voltage. The frequency (also not measured under load until now) is set to 15kHz, the voltage 13.5V, that's 1/500 of the measured actual voltage. Therefore all the voltages and currents in the simulation results have to be multiplied by a factor of 500!

The following simulation shows the open circuit voltage of 47.5kV:

and here zoomed to the according input current (25mA at the beginning, dropping down to 9mA after 90ms, output currrent remains zero since this is open circuit):

The next image shows the simulation with Rout=50MOhm. The output voltage is still 26.5kV:

With Rout=50MOhm, input current starts at 25mA and drops down to 11mA after ~180ms:

while the output current through Rout=50MOhm rises to  0.55mA:

Those 0.55mA at 26.5kV in the output resistor is 15W output power at 50MOhm load.

Finally a simulation with a short at the output. The input current is 24mA and the output current 12mA amplitude AC with an offset of approx. -3mA:

Here are the first images of the 4-stage test-CW:

That's the prototype of the potential ring:

After tightening the two turns of heavy wire inside the tube (over a mandrel of 18cm diameter), I wrapped the joint with clear Tesa tape because it has more strength than the electrical tape. Then a layer of electrical tape is applied. After that, a lot of thin small stripes of alumnium tape (DIY store, plumbers section) are applied. finally, all the wrinkles are smoothed out with the help of a spoon (rub it over and over). Now the ring has much more strength/stability and stays in form when bent to a flat (!) and really circular ring (at the beginning it will look like a wobbly egg). I'll place some more images on that topic here soon...  

Here is the brown tube connector, looks like the brown ceramic of commercial HV line insulators:

And that's the 4-stage CW in it's current state:

The curled green wires at the top (and bottom, but on the backside) of the tube just "simulates" a spark gap - it reminds me to put some acorn nuts on a threaded rod when the fiberboard is in place. The blue ones are the resistors which will limit the current through the (black) diodes when the cascade is discharged. The white tubes between the caps contains just the wiring because the caps are not insulated very well. Hopefully this will prevent discharges between the components. This it the gap before installation:

Oh, and yes, I know, I should put some bleeder resistors across the caps. But it's really hard to find some cheap resistors rated for approx. 6GOhm and 30kV (need 44 pieces of them). Perhaps I'll build a nice earthing system like Haefely uses on their SGDA-Marx-generators (see page 11 of A first schematic how this will be wire is shown above.

Width of the fiberboard is 15cm, spark gaps are in a distance of 3cm to the outer edge of the outermost cap, connections between fiberboard 2cm from spark gap (2cm from outer edge of fiberboard):

I bought two "flat" 30x6cm styrofoam toroids. Flat means that I have to combine two halves. Actually, the "halves" are a bit thicker, so total thickness is 8.5cm instead of 6cm. This should be a suitable toroid (actually way oversized) for the top of my 50kV-cascade (4-stage test setup):


I got some cheap (empty ;-)) film containers with 18cm diameter. I'll use them  (well, only the upper parts which are not as thick as the bottom parts) as top plate and bottom plate of the stack as well as the plate of the top toroid.

For the first light, I choose the ignition circuit as the HVAC-source, it deliveres approx. 6.5kV (open circuit, not connected to the CW-multiplier). Testing of the ignition circuit was done with two TV cascades without a suitable cap in the feedline (usually omitted in the commercial TV cascades). Old blue TV-cascade delivered 29.4kV without cap in the feedline, green new TV-cascade delivered 31kV without cap in the feedline. I tried a 0.8nF cap and a 4.7nF cap in the feedline of both cascades, but output decreased significantly. The measurements have to be performed again with a better setup of the peak rectifier.

Since we saw that there was a significant output (lots of hissing), we decided to do the first light of the 4-stage test setup with this ignition circuit as a the source (voltage is perfect for the 30kV rating of the caps in the cascade). We started with negative polarity this evening. Since this was only a test setup, I did not connect the potential rings to the smoothing column. In fact, I just slided them over the acrylic plates without any fastening, therefore the whole thing looks like it was found in the dumpster...:

Below the multiplier stack I placed some cardboard boxes to prevent arcing through the carpet to the screed (wife would be not amused if I would melt some holes in it...). I was not sure if the output resistors would withstand the surges, I just taped them to an insulaing rod placed inside the middle tube of the arrangement.

Each stage delivered approx. 12kVDC. With the corona rings in place but not connected to the caps, corona decreased the output of the upper stages a bit. Output of second stage was 24.3kV, of the third stage 34.7kV. Since the high voltage probe is only rated 40kVDC and I had no other suitable equipment at hand at this time, I did not measure the output at the 4th stage (guess is 43kV with all the corona around).  

Last time we buildt our lifters, they won't fly (DC flyback as the source). Now we tried them both with the 4-stage CW. Please click the images to see the movies:
The flights were very unstable, but with the experience of the mini lifter I was prepared. Therefore I added  three 68kOhm resistors to limit output current in case of a short between the wires.

After the successful flight, we wanted to have more fun. A ball of alumnium foil (3.5cm diameter) was placed on top of the 4-stage test cascade and we used a plastic wand with a grounded brass ball (1cm diameter) to draw some impressive fat arcs of 5cm (2") length. Click the image of the screenshots to see the 15s video clip of the arcs:

As the simulations above indicate, the cascade needs approx. 0.5s for recharging, the spark repetition rate was approx. 2Hz when the grounded wand was held in constant distance.

2nd light: I managed to draw 7cm arcs with a bigger topload (the 4 potential rings stacked on top)! Repetition rate is much higher (5-10Hz). The feeding resistors (4pc. 100kOhm in parallel) get warm. I guess max. 3W of heat. That would work out to I=SQR(P/R) = SQR(3/100k) = 5mA. If we assume a drive voltage of 5kV (it will drop under load), that's 25W when arcs are drawn repeatedly (mmh, its hard to count the arcs per second, anyone an idea to do this automatically?). Values to be verified with the help of some better instruments than just my fingers. See below for the real values!

3rd light: I managed to destroy one of the 100kOhm resistors rated for 10kV. It was the top one of the upper stage and happened when I draw arcs with reduced Rout (used only 2 and then only 1 of the 63kOhm resistors). Seems as if I have to readjust the protection spark gaps.... Because of this, I'll use all the 63kOhm resistors I own as the Rout (total of  945kOhm).

Another issue to think over is the feeding resistor. I paralleled four resistors each 100kOhm (rated 10kV) for a total of 25kOhm. If we assume an input power of 100W (far in the future...) at 6kV, current will be no more than 17mA. Lets calculate with a peak value of 10 times that. The voltage drop across the 25kOhm resistor will be U=R*I=25kOhm*170mA=4.2kV. So it still is a good idea to use the resistors with the 10kV rating. Dropping the resistance will increase the possible current, therefore the voltage drop will remain nearly the same. 

First measurements (still without potential rings):
a) multiplier w/o load:
With 230V into the ignition circuit and the 1GOhm high voltage probe (rated 40kVDC) at the output of the CW-multiplier, I got 41kV output. Input current (using the 25kOhm feeding resistor) was 2.7mA with no load (besides the probe), the input voltage was 5.85kV peak. That works out to 16VA input power at no load (just the probe). Efficiency of the 4-stage multiplier is 41kV/(2*4*5.85kV) = 41kV/46.8kV = 88% at this load condition. Lets calculate how severe the load  presented by the high voltage probe really is: 1GOhm at 41kV is 41uA output current of the multiplier, or 0.33mA input current into the multiplier (Iin=2*n*Iout), that's 12% of the total input current. Will be interesting to see how this changes with the 6GOhm probe I'm going to build soon. (Rough estimation: if all the input current will be used for increasing the voltage instead of delivering output current, the output voltage would rise to 2,7mA/(2,7mA-0,33mA)*41kV = 1.14*41kV = 46.7kV, the efficiency would be 99.8%, that means there are no other significant losses in this no-load condition.)   

b) multiplier short circuited
More measurements to come...
c) repeated arcs
More measurements to come...

Again some photographs of the latest progress (compare to the original at the right!):

Now it looks much better compared to the "first light setup" ;-) As in the original, the driver circuit will be located in the base (here an alumnium case).

Here is the wiring scheme for the case:

I now have finished the case and the output resistor stack. Everything can be screwed together:
(red cable from top toroid is for grounding when not in use)

More detailled images will follow, stay tuned....

First test with different polarity showed that positively charged top toroid led to only half the discharge length than negativly charged top toroid (earthed electrode was 1cm diameter brass ball in both experiments). More to come...

Since the diodes are rated 18kV and the caps 30kV, I can drive the current setup with max. 18kV per stage (should be below 15kV to have some safety margin). I plan to add a second diode everywhere in series to be able to use up to 25kV per stage (that's 66% more!). So I still have 5kV safety margin on the caps...

My analog HF current meter showed me approx. 6mA input current into the cascades HV stack (ignition circuit as the driver), still have to perform a complete measurement of all parameters at the same condition...
Another measurement at the GTL-Teslathon2006 resulted in 42.4kV output voltage (open circuit).

The image above shows from left to right: the flying styro flake, the spark table (showing "GTL" when energized, see image and videoclip below), earthing stick (yellow/red), connector rod (black), 40kVDC peak voltmeter (VOM with red HV-probe and brown cap tube with white rectifier rod on top), analog HF current meter (black box in foreground), 3.3nF@40kV pulse cap (brown tube with ball on top) suitable for direct shorting (to increase the SNAP and brightness of the discharge from the CW-multiplier), CW-multplier, isolated stand.

Future experiments:


Isolated Table:
This isolated table is made from two stacked indoor insulators from a 20kV-installation (here the original dust is still on...):
I use it to perform some hair raising experiments. The last time I was charged only by my whimpy DC-source at the GTL-Teslathon2005. I bet the CW-multiplier will raise them a bit more.
At the next meeting with my fellow coiler Toni, I placed a current limiting resistor between the table and the CW-multiplier. I consists of 30pc. 100kOhm resistors rated 10kV each and limits the current to 20mA at 60kV. It's not in its beautiful final housing but quick and safe ;-) 

We had a lot of fun slowly charging and then rapidly discharging the girls with a ground rod (look at her hair, you can see it twitch every time a spark discharges her):
click it to see the movie!
Other Experiments:
I showed some nice experiments at the Teslathon 2006. This experiment is a so called spark table (or spark board). Alumnium foil is glued to a sheet of glass and then partially removed so that there is only a meander shaped foil at the end. Now the meander is cut where a spark should occure. This way you can decide where a spark will appear:

The next image shows the setup for driving the spark table. You can see the connecting rod exiting the image to the left going to the upper right corner of the spark table (see image above). The peak voltmeter in the background is not used in this setup. The brown tube with the small alumnium ball on top is a 3.3nF@40kV pulse cap suitable for direct shorting (to increase the SNAP and brightness of the discharge from the CW-multiplier) made of a string of 10 Wima FKP-1 caps rated 33nF@6kV (yet without balancong resistors!). This cap is directly connected to the output of the CW-multiplier. On the right you see the CW-multiplier itself. As you can see, the black connector rod is not directly connected to the output of he CW-multiplier but goes to another grey tube on top of the output toroid instead. Inside this tube is a spark gap which is set to approx. 1cm. This is a very inmportant feature because it allows the CW-multiplier to generate significant output voltage (read energy or brightness of the flash!) before discharging through the spark table:

Here is a short video clip of the spark table  
(without additional spark gap and without pulse cap )
in action at the GTL-Teslathon2006
(right click it to download and watch it multiple times
 with your favourite video player).
This second short video clip
shows increased performance of the spark table
the pulse cap and additional spark gap.

Um die Spannung zu erhöhen möchte ich mal probieren, einen Zeilentrafo an einem Hochfrequenzheilgerät zu betreiben wie auf Jogis Röhrenbude nachzulesen (
"ich habe die Primärwicklung eines Zeilentrafos (die Wicklung mit dem dicksten Draht) an diesem "Heilgerät" angeschlossen. Zunächst hatten die Sekundär-Drahtenden einen Abstand von ca. 2 cm, danach drei, zum Schluß 5 cm..."
Ebenfalls auf der genannten Seite befindet sich die Idee, einen ektronischen Halogen-Trafo zu verwenden, der einen Zeilentransformator ansteuert.