Author Topic: Need advice: Cockroft-Walton Voltage Doubler  (Read 4868 times)

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(unknown)

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Need advice: Cockroft-Walton Voltage Doubler
« on: February 27, 2005, 06:20:37 PM »
I am driving a microwave transformer with a > 3000 Continuous Watt Voltage Inverter.

The Secondary of the transformer is part of a Cockroft-Walton Voltage Doubler, to take 2.1kV to 4.2kV DC.  When I powered up the "current charger,"  the Inverter seemed to go in and out of limit and there was no apparent high voltage at the output storage capacitor.  The 15A fast blow in-line fuse to the Primary did not blow.  Nor did the fuses in the Power Inverter blow.


After the device was depowered and discharged (if there ever was any residual charge to begin with), I ohmed out the voltage doubler and was not able to locate any shorts to ground.  When I ohmed out the secondary winding coil, I got an O.L.

reading on my DVM, whereas before it had read 65 ohms or thereabouts.  Apparently,

I had blown my secondary winding.


I noticed on the microwave transformer - for which unfortunately I had no schematics - that part of the secondary winding is tied to the core itself.  I am

wondering if the secondary of the microwave transformer is polarized, and that I had therefore hooked the secondary backwards.  I had actually hooked the connector

typically used for the magnatron to ground reference - earth ground - and connected the other end to HV DC blocking cap.


Can anyone comment on this, or any other mistake I might have made?  Thanks.  

« Last Edit: February 27, 2005, 06:20:37 PM by (unknown) »

Flux

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Re: Need advice: Cockroft-Walton Voltage Doubler
« Reply #1 on: February 27, 2005, 11:46:32 AM »
The normal microwave power circuit is a voltage doubler, using the magnetron as one of the diodes.  I can't see any reason why the transformer shouldn't operate a separate doubler circuit.


I think it is common to ground the low end of the secondary so you should have taken the non grounded end to your multiplier. If you didn't realise and grounded both ends that may have caused the trouble.


I find it surprising that the 15 A input fuse didn't blow before you damaged the secondary. It should have taken minutes to blow that winding. Look for thermal protectors or something.


Flux

« Last Edit: February 27, 2005, 11:46:32 AM by (unknown) »

(unknown)

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Re: Need advice: Cockroft-Walton Voltage Doubler
« Reply #2 on: February 27, 2005, 02:27:46 PM »
So if I understand what you are saying correctly, there is a dedicated end

to the microwave secondary that is grounded internal to the transfomer, and

that it does make a difference which end of the secondary is used for HV.

I had in all likelihood connected both ends of the primary to ground.  I simply

don't understand how there would be a completed ground connection internal to the

transformer, since I am not mounting the core to anything conductive.


I also don't understand why either the inline 15A fuse in the primary or the

fuse in the Power Inverter didn't blow, but clearly they didn't.  


I will make certain that for the next transformer I get, I connect the end intended

for the magnatron to the HV portion of the circuit and I will ground the other end

to chassis ground.


Thanks for the help

« Last Edit: February 27, 2005, 02:27:46 PM by (unknown) »

(unknown)

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Re: Need advice: Cockroft-Walton Voltage Doubler
« Reply #3 on: February 27, 2005, 02:46:11 PM »
One other follow-up point I want to make.  In diagrams of microwaves, such as the one posted at this site http://www.gallawa.com/microtech/doubler.html , the diode cathode is tied to chassis ground.  I however reverse the diode so the anode is tied to chassis  ground and that diodes cathode is attached to the second diodes anode.   This is standard and I assume this configuration should not be a problem when used in conjunction with a microwave transformer.
« Last Edit: February 27, 2005, 02:46:11 PM by (unknown) »

Flux

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Re: Need advice: Cockroft-Walton Voltage Doubler
« Reply #4 on: February 28, 2005, 01:31:33 AM »
Yes the magnetron anode is grounded in a microwave so the diode cathode is grounded to make the polarity right.


If you need positive output you can reverse the diode.


The hot end of the transformer is the one that goes to the capacitor not the one to the magnetron, that is grounded. I suspect this was your mistake.


With your new transformer, find the lead that goes to the hv capacitor, that is the hot one . confirm that the other one is grounded and leave it so.


You should end up with the low end of transformer grounded and also the anode of your lower diode grounded. the low diode cathode should go to top diode anode.


The transformer hot end will go to the original hv capacitor as in the microwave, the other capacitor end goes to the junction of the 2 diodes. This capacitor is vital, don't connect the transformer hot directly to the junction.


Your output comes from the top diode cathode and you will need an even higher voltage capacitor from there to ground to filter the ripple and get peak volts.


Sorry I can't easily post a sketch, hope you can follow this.

Flux

« Last Edit: February 28, 2005, 01:31:33 AM by (unknown) »

jimovonz

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Re: Need advice: Cockroft-Walton Voltage Doubler
« Reply #5 on: February 28, 2005, 02:08:59 AM »
As I just happen to have a microwave open at the moment (funny that...) and most microwaves have a schematic pasted on the inside of the case, I have taken a picture for you. Unfortunately this particular schematic is rather small and my outside light is out so the image is rather pitiful. You can just make out the detail if you squint just right. If it is useful I can get a better picture of a larger diagram tomorrow.



« Last Edit: February 28, 2005, 02:08:59 AM by (unknown) »

(unknown)

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Re: Need advice: Cockroft-Walton Voltage Doubler
« Reply #6 on: February 28, 2005, 07:45:21 AM »
Clearly,  your schematic shows the secondary connected to the transformer core,

and it is this end that is grounded.  So whichever end of the secondary represents a very low impedance to the core is the end I should ground.  Thanks flux for the

help!


I also see that I should have added a parallel 1 Meg resistor across the DC blocking

cap.  I thought that that was only necessary for bypass type capacitors, because

ultimately any high voltage charge would dissipate through the secondary coil.

Guess I'm mistaken.


JC


 

« Last Edit: February 28, 2005, 07:45:21 AM by (unknown) »

Flux

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Re: Need advice: Cockroft-Walton Voltage Doubler
« Reply #7 on: February 28, 2005, 08:44:06 AM »
The 1 meg resistor is purely a safety discharge resistor and is usually integral with the capacitor, it is not a part of the multiplier function.


The original microwave capacitor is ok to keep here, but I don't think the rating would be sufficient to use one to filter the output voltage, you need something about 10kv working but the capacity needn't be very large.


P.s. You have to thank Jimovonz for posting the diagram not me.


Flux

« Last Edit: February 28, 2005, 08:44:06 AM by (unknown) »

(unknown)

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Re: Need advice: Cockroft-Walton Voltage Doubler
« Reply #8 on: February 28, 2005, 01:18:20 PM »
Thanks Jimovonz
« Last Edit: February 28, 2005, 01:18:20 PM by (unknown) »

(unknown)

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Re: Need advice: Cockroft-Walton Voltage Doubler
« Reply #9 on: February 28, 2005, 01:33:08 PM »
  I am using a 24uF 5.2kV storage cap at the output of the doubler,

not to be confused with the filter cap.  By saying a need a rated

10kV filter cap at the output of the doubler, are you saying that

the storage cap voltage rating is inadequate to carry the 4.2kV?


Thanks


JC

« Last Edit: February 28, 2005, 01:33:08 PM by (unknown) »

Flux

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Re: Need advice: Cockroft-Walton Voltage Doubler
« Reply #10 on: February 28, 2005, 01:50:18 PM »
Your 5.2 kv capacitor should be ok, I was being a bit conservative as I suspected you may use something from a tv or monitor that works under pulse conditions.


I am not sure what you are building but that capacitor is rather a frightening beast.

I suspect you may need some resistance in series with the output of the multiplier to limit the charging current or some pretty high current diodes. Be careful that's a potential bomb.


Flux

« Last Edit: February 28, 2005, 01:50:18 PM by (unknown) »

richhagen

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Re: Need advice: Cockroft-Walton Voltage Doubler
« Reply #11 on: February 28, 2005, 05:34:00 PM »
Needless to say, be very careful with 4200 volts in a capacitor.  If it was a scrap microwave, and stored outside, might you have had moisture or something in the transformer that would cause it to short out right away?  


You can send 211 Joules most anywhere very quickly with your capacitor at that voltage.  Just a few Joules can be fatal to a person. Please be very careful.  What are you building?  An instant hot dog cooker? - ur um I mean exploder? Not powerful enough for a quarter shrinker, and probably not a HERF. I give up, what is it?  Rich Hagen

« Last Edit: February 28, 2005, 05:34:00 PM by (unknown) »
A Joule saved is a Joule made!

(unknown)

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Re: Need advice: Cockroft-Walton Voltage Doubler
« Reply #12 on: February 28, 2005, 08:45:13 PM »
Thanks for the concern.  I am aware of the fact that these voltages and

currents are potentially very fatal!  Fortunately, the power inverter I am using

is remote controlled - i.e., I can keep my distance when I turn the circuit on,

and the circuit I am operating can exhaust the charge on the storage cap after several cycles.  The "current charger" is housed in a container and is not accessible during operation.  Unlike PCBs with low voltages which I am

not intimidated to troubleshoot and have done so many times as an engineer, development of this circuit has been very slow because of the dangers involved.


I am trying to reproduce an effect observed by Tesla back in the late 1900s.

This is very interesting science, though I cannot really discuss it on this board as it is possibly considered taboo.  I would strongly recommend a couple books

and video.  My greatest recommendation is the Book by Gerry Vassilatos called

"Lost Science."  Outstanding book. A+++. Can be found at Borderland Sciences -

borderlands.com -- and the chapter relevant to what I am building is on Tesla,

but there are many chapters in this book that will make your jaw drop.


An excellent video from this same website is "Tesla's Longitudinal Electricity"

by Eric Dollard.  Also will leave your scratching your head.


And the book which is most relevant to my work is Secrets of Cold Electricity

by Peter Lindemann.

« Last Edit: February 28, 2005, 08:45:13 PM by (unknown) »

(unknown)

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Re: Need advice: Cockroft-Walton Voltage Doubler
« Reply #13 on: February 28, 2005, 08:48:11 PM »
I have done a slight redesign of this test bench -- these are old photographs --

but the current test bench looks very similar to what you see here.





src="http://www.otherpower.com/images/scimages/3297/tesla3.jpeg" width=80%>

« Last Edit: February 28, 2005, 08:48:11 PM by (unknown) »

jimovonz

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Re: Need advice: Cockroft-Walton Voltage Doubler
« Reply #14 on: February 28, 2005, 10:28:52 PM »
:)
« Last Edit: February 28, 2005, 10:28:52 PM by (unknown) »