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120kv 10-20mA SMPS

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Adam Munich

Sat Apr 23 2011, 02:03PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

That's a nice multiplier, but hot glue + oil = dissolved hot glue.

Did you spark the multiplier without any limiting resistor? If so you probably killed the diodes...

They put the diodes in series with each layer because it cuts down on insulation requirements and reduces inter-winding capacitance. DC is also easier to insulate than AC.

My tip for you, make multiple pancake secondaries, then link them up in series using 20kV 20mA diodes.

Linas

Sat Apr 23 2011, 02:10PM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

Grenadier wrote ...

Did you spark the multiplier without any limiting resistor? If so you probably killed the diodes...
My tip for you, make multiple pancake secondaries, then link them up in series using 20kV 20mA diodes.

nope, all diodes work as it should. i test them with 10KV as bridge for rectification, all are ok, also made some test with DC power supply to find out if they work as valves, and they do. so where is no reason why they don't work.
maybe speed/current limitations (dv/dt)

Adam Munich

Sat Apr 23 2011, 02:13PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Hmm.. what type of oil did you use?

Linas

Sat Apr 23 2011, 02:19PM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

Grenadier wrote ...

Hmm.. what type of oil did you use?

from hv power distribution transformer.
don't worry, oil is good, i made some test too, because i thought that was reason why i have no output...

Conundrum

Sat Apr 23 2011, 04:42PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Yup, I fried a multiplier like that

Apparently series connected wirewound resistors can work, as long as properly shielded in case of pressure buildup.

-A

Antonio

Sat Apr 23 2011, 08:50PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

120 kV is very difficult to contain in a small space. The main "suspect" in your multiplier assembly is the oil. It may be leaking a lot and acting as excessive load to the multiplier. Strings of diodes in series are not guaranteed to work. If a diode leaks more than the others it increases the voltage over the others. The effect is that the maximum voltage that the string can support is significantly smaller than the maximum that a single diode can hold multiplied by the number of diodes. And if a string breaks down, the charge stored in the capacitors may be enough to damage all the diodes in the string. A common failure mode is a large increase in the leakage current. I suggest a measurement of the leakage current of the assembly with high voltage applied at the output and the input grounded. I like to do tests like this using a simple electroscope connected to the output, charged by an electrostatic machine. If the electroscope can be charged and doesn't discharge immediately the insulation is ok.

Linas

Sat Apr 23 2011, 09:06PM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

i was using MOT and mili-Ammeter
i just connected one end of MOT to oil, and other through mA to oil. even then the distance was very low, i had 0mA current.
Just holding electrodes i can profuse more current.
so oil is good.
also i made experiments without oil, and had same results

i know that problem is diodes, but for now, i have no money for HVHF diodes...

Dr. Dark Current

Sat Apr 23 2011, 09:08PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The leakage current of a diode increases quite a lot (the diode starts avalanching) before the actual damage by overvoltage occurs, I think this effect should contribute to make the diodes balance their voltages properly.

James

Sat Apr 23 2011, 09:26PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

Multipliers work just fine for this.

You really ought to give that approach another try. You are likely to spend more money and time trying to get a transformer alone to produce the necessary voltage. Two copies of the Kodak dental head circuit wired opposite polarity should meet your needs.

Antonio

Sun Apr 24 2011, 01:51AM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Today I was making measurements in a device that I built that has diode strings, and noticed that a string of five 1N4007s was holding only 1500 V instead of 5000 V. The diodes look good when tested separately. Equalization may work, but at a quite high leakage current. In my case I could not tolerate more than a few nA.

Correction: I made measurements to see what was happening and found that the diodes were ok, holding the expected 5000 V with little leakage. The problem was in other part of the assembly, where a dust filament was causing corona. High insulation is not something easy to achieve, even at relatively low high voltage.

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