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Registered Member #1721
Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136
Having the need on occasion for a "quick" VM build for a specific high voltage, got me thinking along the lines of how to do a quick VM build, which works well, but does not take on the proportion becoming another project in itself.
I ended up making a 4 stage half wave multiplier, capable of handling 11KV RMS, or about 15KVP input for a close friend who needed it quickly. The 8 capacitors used were 35KV, 3nF polymer / metal foil capacitors. At 20KHz input he could draw up to 5 ma without experiencing a serious drop in output voltage (Input current was limited by his driver). The diodes were 2CL2FM 20KV / 100 ma diodes, with each stage using 2 diodes in series. The final stage was a chain of 2 watt 47Kohm HV resistors for output current limiting, to protect the diodes. The design was totally routine, and not really worth posting. The construction however was different from anything that I had tried before.
To mount all of these components I cut up a sheet of 3/16"polypropylene perforated board. The PP perf. board has 3/16" holes spaced at about (yes you guessed it) 3/16" apart. I cut a strip of the board about 12 x 3.75" As wide as would fit snug inside of common 4" PVC pipe.
To mount the capacitors to the PP board I used 3/16" wide nylon zip ties, 2 for each capacitor, I also attached the diodes with a single nylon zip tie per diode. Then I soldered all of the connections. I put a dab of silicon sealant on the reverse side of the board, so that the silicon sealant extended through the holes in the PP board to reach the components on the other side. I did it this way assuming that the adhesive would have a hard time sticking to the PP board, this way the dabs would act sort of like "silicon rivets", in the event that it failed to stick to the PP board. I anchored the finished board into the inside of a 12" length of 4" PVC pipe.
I did not have time to go to the hardware store to buy o-ring seal screws for the electrical feed through connections like I normally do, so I instead used regular brass screws, sealed with adhesive (Silicon based for oil resistance). I then glued on the bottom cap permanently with PVC glue. For the top I used a rubber cap with a SS band which could be tightened with a screwdriver for easy access. The temporary terminal was a 4" SS gazing ball which had a threaded 1" long shaft already. The last step was the filling with transformer oil. I have a bunch of this oil already otherwise I would have tried some more eco-friendly alternatives.
The total construction time was less than one afternoon, including part of the design work. What I liked about this method is that it is very fast, and easy to do with very basic tools from the hardware store. If your hardware store does not have PP perf board, it can be ordered from Small Parts Inc, for about $4 a square foot pre-cut into 12" squares. As far as the Silicon adhesive goes, use the kind which is acetic acid free, the one which does not small like vinegar.
The next step will be to build a larger 180KV full wave VM, here I intend to use longer maybe 24-36" PP perf board strips in a triangle fashion. If I cut 3 strips of the PP board, and trim the edges to 120 deg, then I can again use nylon zip ties to assemble the 3 strips into a triangle shape, at the edges. One for each capacitor stack, and the third for the diodes. I will try to post a picture of this one before it goes into the oil.
Using PP perf board to build multipliers works very well, and best of all its very cheap, simple, and fast, without having to make any sacrifices. The nylon zip ties make the whole thing a breeze to assemble. I know there are probably other maybe better methods, but this one worked well enough to repeat.
Registered Member #1721
Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136
Hello:
Here is the info of the 2CL2FM HV diodes, several members have asked me about these, so here it is:
These are HV diodes made by Leadsun in China re-sold in the US under the HVAC brand, and some other sub vendors. You can buy them direct from China at 500+ amounts for about $1.50-1.75 each. For smaller amounts you can get them on ebay for about $2.20, to $2.50 each.
The 20KV / 100ma HV diodes listed on ebay are in fact 2CL2FM's if you click on the listing, and look at the diode markings. I became aware of these diodes when I started to see them a lot in several different commercial HV multipliers. I still have one commercial 180KV multiplier which uses these. There are a lot of diodes on the market, but I like these for the following reasons:
20KV single diode rating, proven to work well in VM's High Surge Rating = 10A ifsm Fast Switching = < 100nS High Average Current = 100ma (200ma in oil)
You can often find better "special" deals on HV diodes, but for routine use in multipliers, these seem to offer the best "bang for the buck". The other common 12KV, and 15KV ones, some with even higher current rating, are normaly much more expensive.
The 20KV rating is nice because you do not have to string together long lines of diodes. The 80KV/ 100ma rectifier sticks sold on ebay for $20-25 each are nothing but 4 of these in series in epoxy, for 2-2.5 x the single diode price.
If there are any left, there were some very nice: VMI 10KV / 50-100ma, 70nS HV, diodes on ebay, these have an ifsm of 5A. Very sweet diodes also for VM's!!!
One Note: The Ebay vendor selling them has a miss-print in the spec sheet, it has the ifsm at 2A, instead of the correct 10A figure. As long as they are 2CL2FM diodes its 10A.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Just a thought about insulators in high energy projects capable of creating extremely high rate of temperature rise: My work was operating and repairing power switches, which did fail to clear fault currents and got destroyed. One letter on file from a manufacturer of such gear indicated that the insulation was converted to gasses in the heat, and the damage was far in excess what the simple arc blast could have done.
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