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Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
Here is a complete protocol how to make a cheap diode capable to handle voltages up to 50KV. In pulse mode it should stand 30A peak, because it is based on the 70pc of 1N4007. These little diodes you can buy very cheaply and I tested them in pretty hard HV voltage discharges.
Pictures are more than words so here we go…
I like the HOW TO HV-wiki idea a lot so please Bjorn add this thread there in case you feel it could be valuable for the project…
Chris has a good comment and it is fair to say that the diodes were tested only with a special power supply based on a ignition coil (50KV) which gives relatively small amps at 6KHz. Rectifying 50Hz 50KV higher amps power supply (like neon transformer) will probably destroy the diodes by local voltage peaks because lacking balancing resistors and caps. This design is a compromise which I believe can be still very useful for HV fun…
The required tools and supplies.
Solder the diodes together. Try to make each joint as a "pearl" to avoid corona forming.
70pc soldered together. A lot of work but once you trained you can make it in 30minutes.Thoretically it should be able to handle 70KV. I tested 50KV AC…
Use a small piece of a silicon tube which has little bit smaller diameter then the final PVC tube which will be filled by the oil. The silicon as material is very important in this case.
Fill the tube with hot-glue and let it cool down.
Remove the silicon tube from the filling (silicon will never adhere to the hot glue, that’s why it should go off quite easily)
Cut a piece of PVC aquarist tube 8-10cm longer than the string of diodes.
Place the string of diodes carefully inside the PVC tube. Use a lot of the cooking oil to make this process easier.
Put the string completely inside the PVC tube.
Use your mouth sucking power to fill the PVC tube with the cooking oil (in case of using mineral oil be extremely careful – it can be toxic)
Break the PVC tube and use a kitchen gum to block back leakage of the oil.
Use the kitchen gum to squeeze the PVC tube around the filling made by the hot-glue before. This will make hermetic enclosure of the oil. Make the same on both ends.
Use more of the hot-glue. This will fix the gum on place and also improve the squeezing by heating up the filling.
Final product is string of diodes inside an oil bath. It can be carefully rolled (not more then three turns) to make it more compact.
ct…
Testing the diodes connected as two stage CW multiplier powered by the 50KV ignition coil. More about this project here
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
Weston wrote ...
what oil are you using?
It is a sunflower-seed cooking oil. I tested also silicon oil, but that one is extremely expensive and I see no difference in the function of the diode
Registered Member #8
Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214
I see you have nothing to balance these diodes. Without a resistor (it can be Mohms or so) across each one, some of them may see far more voltage than expected. Even with resistors, I wouldn't use them much past half their rated voltage (50kV peak per 100 diodes, etc). I have had a balanced string like that actually fail at barely half its theoretical rated voltage, with frightening results as it shorted out the 20kJ capacitor bank I was charging with it.
I wouldn't trust your string of 70 past 30kV even with balancing resistors.
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
Chris wrote ...
I see you have nothing to balance these diodes. Without a resistor (it can be Mohms or so) across each one, some of them may see far more voltage than expected. Even with resistors, I wouldn't use them much past half their rated voltage (50kV peak per 100 diodes, etc). I have had a balanced string like that actually fail at barely half its theoretical rated voltage, with frightening results as it shorted out the 20kJ capacitor bank I was charging with it.
I wouldn't trust your string of 70 past 30kV even with balancing resistors.
My first strings of diodes I had made before for rectifying MOTs were also made with parallel balancing resistors and 1nF cap each. It was a lot of shitty work.
I made the first 1N4007 string like is presented in the protocol only for fun – just to know what I can afford and it workes perfectly, I understand your doubts I was also surprised.
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
Chris wrote ...
Probably good luck you got a good batch or something, as clearly my diode string actually failed well under spec. I would be weary indeed.
It can be also little bit about the power supply you have used for your experiment. With higher amps the local voltage peaks can be much bigger problem. I'm using ignition coil at 6Khz. Also, but this is a pure speculation I'm always trying to make the joints between the diodes as big and as rounded as possible. Maybe the joints made like that can have significant capacitance and reduce the voltage peaks.
Registered Member #8
Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214
I was using a 7.5kV 60mA NST, and had four strings arranged as a full wave voltage doubler, thereby outputting about 21kV peak. If I remember right each string had 35 1N4007 or similar under Exxon Univolt dielectric oil and balanced by 1Mohm resistors. One of the strings failed once the caps were charged to 18-20kV, just shy of the target voltage (I can't remember exactly when it failed, but I was watching the voltmeter). The current flow at a voltage so close to maximum was very small as it was starting to level out. I suppose it is possible that with your ignition coil at such higher frequency, the capacitance across the diodes becomes significant enough to balance them well.
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
Chris wrote ...
I was using a 7.5kV NST, and had four strings arranged as a full wave voltage doubler, thereby outputting about 21kV peak. If I remember right each string had 35 1N4007 or similar under Exxon Univolt dielectric oil and balanced by 1Mohm resistors. I suppose it is possible that with your ignition coil at such higher frequency, the capacitance across the diodes becomes significant enough to balance them well.
As I said, the design you are describing is something very different than what I'm using these diodes for. I'm sure my string would die also in that kind of setting. You should have use 1nf/1KV ceramic capacitor across each of your diodes. Only resistors is not enough. Than they would most likely survive. Anyway, I added your comment as warning into the beginning of the thread description…
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I see from the 1N4007 data sheet that Vr[rms] is 700V max, so a string of 70 should hold their own until 49kV if the devices have similar properties.
I've read recently that modern rectifiers don't need the equalizing R and C used in older designs, as manufacture has improved so much that there is little variation between devices. I don't know how true this is, but Mates' experiments seems to bear it out.
I always rate things very conservatively indeed, so there is lots of head room for spikes and surges, and as the 1N4007's are so cheap at the moment - I bought 500 for £5 [CZK 167] on ebay - I had planned on using a string of 150 for 50kV - but will perhaps reduced this a bit after reading Mates' very helpful report.
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Home made HV diode 50KV, 30A peak - complete protocol
