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200KV PLUS voltage multiplier

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Sparks

Mon Aug 03 2009, 05:11PM

Registered Member #2263 Joined: Mon Aug 03 2009, 04:57PM
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Posts: 20

Hi everyone, new to this forum and just looking for some advice on building a half wave Cockcroft Walton voltage multiplier.

As you can see I have already constructed the unit and its quite a beast at over a meter tall. The output I would say is a minimum of 200KV.

The multiplier stage is made from 20 hand made diodes, 20 2kv @ 1.5A Fagor diodes soldered on a strip of pref board and potted in paraffin wax using plastic flexi duct.

The caps are 2000pf 40kv ceramic caps and as you can see the support frame is just 40mm dia plastic wast pipe.

The driver was purchased from Information unlimited and is a high voltage plasma globe driver PVM400 20KV

Assuming the output is 20kv then the multiplier produces something like 200kv

As you can see I have not put any resistors on the EHT + output or the multiplier input.
First time I fired up the unit it worked well until I put a grounded copper rod near the output whilst it was operating, it arced and then the unit stopped. Fortunately I only blew the driver transistors which were inexpensive to replace. Its all working again and as I donâ€™t know much about high voltage electronics I need some advice firstly why the transistors blew and secondly how to protect the device from self destruction.

At some point I am also thinking of obtaining/constructing a better driver unit, what do you think the max I can safely power the multiplier stage with?

uzzors2k

Mon Aug 03 2009, 05:52PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Nice setup, though it must have put a real dent in your wallet purchasing that HV supply! When drawing arcs from a CW multiplier you need some sort of limiting resistance to protect the diodes. Remember that when you short the output into an arc the capacitors will discharge through the diodes, resulting in a pulse of current. I've drawn over 1,5kW from my CW Tower, so unless you get some large limiting resistors, you'll need to upgrade to a more powerful transformer driver.

Firefox

Tue Aug 04 2009, 07:05AM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

Uzzors is correct. I recently finished a 6 stage MOT multiplier for 36kV open circuit voltage. I used 3 200W 100k resistors to limit the output. With your limited input power (200W), I would use a few Megaohms at least. Also, your math is a little off. I see you have ten stages there, so with 20kVp, you should be getting 400kV open circuit voltage. Thats 2*Vinpeak*nStages -- two times peak input voltage times the number of stages.

Sparks

Tue Aug 04 2009, 09:58AM

Registered Member #2263 Joined: Mon Aug 03 2009, 04:57PM
Location:
Posts: 20

The high voltage driver was rather expensive for what it is, I think I could have made a much better one for less.

When you fire up this CW its electro static field can be strongly be felt within a 6ft radius, hairs go up and you feel like your walking into a danger zone lol

The output is adjustable. I actually constructed this device to experiment with making Electret materials (the electrostatic equivalent to a permanent magnet) They will be polarised between ground and the tower output so in effect it is like putting a large resistor between tower output and ground as the electret material cures. What size risistor would you suggest I use on the tower output?

Steve Conner

Tue Aug 04 2009, 09:58AM

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

Watch out! You can blow your high voltage diodes if you pull arcs from the output without a limiting resistor.

Also, don't you get a lot of corona from the upper stages? You haven't used any kind of grading rings or other field control.

Sparks

Tue Aug 04 2009, 11:27AM

Registered Member #2263 Joined: Mon Aug 03 2009, 04:57PM
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Posts: 20

Hi Steve,

Yes there is a very strong corona accompanied with ozone production and hissing sound, quite impressive actually which is why I need to figure out the correct resistor to use. I don't want to destroy my hard work,not to mention money!

HV Enthusiast

Tue Aug 04 2009, 11:32AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Sparks wrote ...

Hi Steve,

Yes there is a very strong corona accompanied with ozone production and hissing sound, quite impressive actually which is why I need to figure out the correct resistor to use. I don't want to destroy my hard work,not to mention money!

The resistor won't help with the corona. You'll need to use methods for electrical field control (rings, toroids, spheres, etc...) to help with that.

Sparks

Tue Aug 04 2009, 01:11PM

Registered Member #2263 Joined: Mon Aug 03 2009, 04:57PM
Location:
Posts: 20

Most of the CW towers I have seen don't have collector spheres on the EHT out but rather a wand full of resistors.

The reason I have not opted to put a toroid on the EHT output yet is because I am worried about charge accumulation and generating powerful sparks which could damage the CW whilst I don't have resistors on the output EHT yet.

I looked at plans for a CW using the same driver unit I am using and it suggested putting 4 100K 2 watt resistors in series.

HV Enthusiast

Tue Aug 04 2009, 01:45PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Those resistors are for current limiting, not corona suppression.

Corona isn't necessarily a bad thing, however it can lead to significant losses. And those losses to contribute to the maximum voltage the output can reach.

Sparks

Tue Aug 04 2009, 02:14PM

Registered Member #2263 Joined: Mon Aug 03 2009, 04:57PM
Location:
Posts: 20

Its only the 3 upper stages which really radiate a corona, As you can see from my photo I did place small brass balls at the end of each diode but the tower is obviously too big to put in oil. I do intend to put a nice toroid on the EHT output when I have the current limiting resistors in place.

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