Cockcroft Walton Voltage Multiplier 2

Cockcroft Walton Voltage Multiplier II

After the great success of the CWM at the Cambridge teslathon. I started to ask around for some more 25Kv capacitors. Dave Reeve, found 10 of them and donated them to the cause (Thanks Dave).

So I added another 4 stages (8 caps, 8 strings of diodes)

This should give me a theoretical 200KV ..

The first problem was that to protect the diodes, I had a string of 1M resistors on the output of the old CWM, this gave me a 20M @ 60KV , underated even then, It now would have to handle 200KV, a rebuild was in order.

After asking around, the general idea was to use a pipe full of water to give me the value I wanted.

After much experementation I managed to get a 20M resistor built with a 30" peice of 5mm ID silicone tubing wrapped around a 13mm former.

The resistor was hard to measure, as the copper/water junctions gave about 0.15V, which totally confused my DVM.

I eventually used a 24v supply and measured the current that the piping took across this, which gave me a  more accurate reading of its resistance.

The picture is of the CMWII with the outer cover and top sphere removed.

The finished safety resistor.

On powering up the CWM stack, a loud crackling was heard

I had acheived breakout on an 8" diameter sphere, It worked!!

By using a small machine screw I could get some excelent discharges to the air.

I then started to play with another eqarthed 6" sphere about 10" away from the CWM's top sphere, I got some really nice arcs between the too.

I then got a single really loud bright spark, followed by nothing..

$%^$K!!!

I found the damage, four of the HV diodes had blown, when the arc happened. these have been replaced, but I need to find out how the arc got past the safety resistor.

I suspect either I'm getting more than 200Kv or the arc tracked along the inside of the outer pipe covering the CWM bypassing the safety resistor.

To prove that this CWM was an inprovement on its predicessor I tried to levitate a 1 ¹/₂" polystyrine ball covered with aluminium foil. Previously I could only levitate a ³/₄" ball, and then only with small strips of aluminium foil to keep the weight down.

The picture is of an attempt I made to suspend the ball from three peices of cotton, this worked for a while , but the ball soon lost its charge and fell back to the sphere.

Click on the picture for a video of the ball attached to the sphere with a single peice of cotton.

After various plays with the CWM, I found that it was rather easy to get a spark big enough to blow the rather flimsey 5mA rated BY8406 diodes. So I started to hunt around for some better ones.

I saw an auction on ebay for some 30Kv diodes rated at 200mA, and thought they would be ideal.

I won the auction and having a few minutes spare, I thought I would replace the diodes.  5 mins later, all 16 diodes replaced with 30KV doides, turned on the power and BANG!!!

The 30KV diodes broke down at around 10KV, my ebay bargin wasn't quite so good and having only 20 diodes I didn't have enough to double them up. 

After a lot of searching around for replacement diodes, I eventually came to the conclustion that cheap off the shelf doides were the way forward, and bought a large pack of 1N4007's these are 1000V 1A diodes and cost me about £0.01 each , so a string of 30 would do for a single stage, 16 stages, 480 diodes would do the job.

To ensure that I could get all of the string soldered neatly, I made a jig from two peices of MDF spaced 4mm apart with a diode sized hole in each. Each diodes leg was cut to 4mm long, and one by one threaded through the jig and soldered together in the gap in the center.

This ment I had fairly even strings of diodes which eased the threading into the 3mm Silicon tubing. .

Each String was connected and wound around the "diode" leg of the CWM .

I powered it up and it worked first time, back to getting breakout on the 8" sphere. I need to replace the rest of the anti corona insulation and I'm done.

The next trial is to reduce the charging resistors as now I can take up to 1A arcs...

Ok, So I had a little play. I haven't tried the experement with the wig with the 16 stage setup and I thought it worth a try.

Click the picture to find out what it looks like with power..

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