A SEGMENTED LOW CURRENT CW MULTIPLIER

Things at the limits.

A SEGMENTED LOW CURRENT CW MULTIPLIER

Postby William A Washburn » Mon May 30, 2011 10:11 am

I decided to change the way I build high voltage low current Cockcroft-Walton multipliers. Rather than build an N-stage multiplier to fit a given need I decided to build a lot of small 2-stage multiplier boards. When the need arises for a given voltage I can bolt several of the 2-stage multipliers together and drive them with a JKL BXA-12529 CCFL supply powered by a 12 volt 1 amp wall-wart supply. Because the 2-stage boards are small I made four (4) copies of the 2-stage multiplier on each of two boards. Then I used a diamond scribe and scribed separation lines on both sides of the boards as accurately as I possibly could. Then I headed for a sheet metal shop. They will usually charge me a few bucks to separate the boards on their bench shear and unlike using a saw there is no board loss eaten up by the saw. You will want to clean up the cut edges of the small boards as some glass fibers will be projecting ready to embed themselves into your fingers. Medium wet or dry sandpaper works well for this. Below is the pattern I etched on the boards (top and bottom). Since I have no workshop yet and all my chemicals have dried up I used “Express PCB” to do the actual board work:

SegmentedCWGen2.jpg
MAIN BOARDS


The board on the right is oriented as if you had flipped the board on the left to the right about its vertical axis. I made one major mistake and you can see it near the left edge of the board on the right. The square pads are to be bolted together from the output of one board to the input of the next board so the boards must overlap a bit. As you can see there is one component hole that lies underneath this overlap area. This was a hole for an HV diode so I had to drill a hole to the right of the mistakenly positioned hole and its companion hole below to move the diode over. If you decide to build these boards make this correction in the pattern (or use my corrected pattern) so that no components will run into the overlap. Each of the boards in the photo is 3.8 in by 2.5 in. Each of the small boards is 1.9 in by 1.25 in.
I populated three boards with .01uF 6KV caps and 12KV x 10mA diodes and then tested them starting out with a single board then two in series and finally three. They are connected using .06 in x .26 in fine machine thread screws and nuts with smooth washers top and bottom to keep from ruining the copper cladding of the small square connection pads. Driven with a JKL BXA-12529 CCFL supply powered with the above mentioned wall wart they come out a little lower in voltage than the CW formula states but this is probably due to forward diode loss and other normal loss factors. Below is a table of voltages measured after each stage of multiplication:

2,800
5,500
7,400
10,300
12,700
15,000

I have tried no more than three in series at this point since these small boards were not meant to generate 30KV but maybe 1.5KV to 20KV. Any needs for 30, 40, 50KV would be better realized with a specially built multiplier since it would probably need to put out more than 10 mA. Additionally, I have not tried to drive these boards with a supply capable of more than a few mA. This would be useless since the diodes are rated at a forward current of only 10 mA.
Below is a photo of the three boards as they were after my testing.

IMG_3088_2.jpg
THREE SMALL BOARDS


Below it is the copper pattern of each of these three boards as it would appear if you were looking thru the boards to the bottom copper. This pattern is the corrected one with the rightmost diode pads moved over to clear the board overlap when two or more boards are connected. Input from the JKL BXA-12529 CCFL supply is applied to the left end of the board or linked stack of boards. Ground is applied to the lower left square pad and the positive 1KV is applied to the top left square pad. Positive output is taken from the lower right square pad and the ground pad at the voltages listed earlier in this document.

small.jpg
SMALL BOARD PATTERN
small.jpg (5.01 KiB) Viewed 5662 times


I have corrected the error in the complete board pattern in case anybody wants to replicate this adventure:

You will have to download the ExpressPCB application from their website to read the PCB file. Then you can load the attached file into ExpressPCB and print it on the transparency film you use to expose your boards. Just remember that the pattern is oriented as if you were looking thru the board to the bottom layer. Therefore, make sure you turn the transparency over so that the printed side contacts the board.

Below is a ZIP of the PCB file and a PDF file containing a PDF995 print-to-file bottom copper image.

SegCWDocs.zip
PC and PDF Files
(11.91 KiB) Downloaded 395 times
Last edited by William A Washburn on Tue May 31, 2011 6:13 pm, edited 1 time in total.
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William A Washburn
 
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Re: A SEGMENTED LOW CURRENT CW MULTIPLIER

Postby Doug Coulter » Tue May 31, 2011 10:00 am

Nice job William! Good to see you doing stuff -- and hoping it's the start of a lot more good stuff :) I've still been too lazy to make the board I had planned around these, which would have footprint for the inverter and a 3 terminal regulator as well.

One thing to explore with these for portable detectors is to go ahead and use more stages than you'd think needed at first, then run the input of the CCFL down low in voltage -- they'll run down to about 1 volt input. For what is essentially a no load situation (eg, any gas tube, but not phototubes) the quiescent current drawn by the CCFL itself goes way down with lowered input voltage, so you can have lower voltage and smaller batteries in a practical device. You lose some power output (the 12 ma limit of that inverter is still there) but for geiger counters or neutron proportional tubes, it just doesn't matter at all.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: A SEGMENTED LOW CURRENT CW MULTIPLIER

Postby William A Washburn » Tue May 31, 2011 1:04 pm

Thanks Doug,

I was going to include measurements with the inverter fed thru a variable LM317 circuit but
couldn't get it to work. Finally found out that the 15 volt laptop supply I was using was bad
and kept shutting itself down. That's why I used the 12 volt fixed supply.

I also was going to attach the ExpressPCB file and a PDF file containing the board bottom pattern.
I could not see that attaching these files was possible. Is this possible to do? If so how do I do it?
If not I'd like to send you these files by email so if someone needs them they won't have to reveal
their email addresses to God and the world!

BTW...Received my 6x6 in cylinder of HDPE from McMaster and purchased a brand new sharp 3/4 in hole
drill for wood. I think I may take both to a machine shop since my drill press is very small and have
them make the centerline hole for the CHM-14. I plan to go in 5 1/2 inches.

I have also purchased the components to build the premium op-amp amplifier and digitizer you
described in the neutron detector thread. This includes a counter which I sill set up to reset
at 1, 10 60, etc. seconds so I will be able to read approximate rates. I will begin laying out
the pattern on one of the good old 0.1 in prototype boards.

Dislocated my collar bone a few weeks ago and that's slowed me down a bit but I'm on my way back.

Bill
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William A Washburn
 
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Re: A SEGMENTED LOW CURRENT CW MULTIPLIER

Postby Doug Coulter » Tue May 31, 2011 1:16 pm

I wound up just driving that Stanley spade bit through with a (fairly powerful but not excessive) hand drill - I didn't need my super horsey 1/2" Makita for this one -- a drill press won't have the quill stroke to be able to pull it out often enough to clear the chips before they solidify and freeze the bit. The main problem is clamping that thing down to the floor some way so it doesn't spin the whole thing while you work. No harm in making the hole deeper and putting the whole tube down inside it...But since I got a long rod of this stuff, it's still a pain to say the least to chop it to length, as it seizes my metal bandsaw. I still am using a chainsaw, with frequent picking of the chips out of the clutch housing -- it's a real exercise.

While you are editing any post, look at the bottom of the screen where you'll see two tabs -- options, and upload attachment. If you select the latter, you can browse for a file to upload and put a little comment on it. Once you've clicked the add the file button and the board has taken up the file, you can put it inline in the text wherever the cursor is there by clicking the inline button.

There are some filetypes the board software doesn't like (.bmp for example) because they can be used for various malware more easily. If you try one of those and get the error message in red at the top of the edit-post page, just rename it and warn us, or put your stuff in a .zip file.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Doug Coulter
 
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Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA


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