Multiplier Troubleshoot

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Joe Gayo
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Multiplier Troubleshoot

Post by Joe Gayo »

My 200kV custom multiplier has started showing excessive leakage current (defined as the current supplied to the multiplier with no HV cable attached and beyond what the sense divider should draw). The excessive current becomes quite noticeable above 150kV and is about 500uA (previously the current would have not exceeded the sense divider of V/2.5Gohm). I'm thinking the diodes are damaged and they leak with increasing reverse bias. The multiplier has around 300hours of operation time and there have been occasional arcs but the supply has always been ballasted with 100kohm or more. Thoughts?
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Finn Hammer
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Re: Multiplier Troubleshoot

Post by Finn Hammer »

Joe,

Sorry to hear that.
How many stages are there, and which diodes are you using?

Cheers, Finn Hammer
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Joe Gayo
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Re: Multiplier Troubleshoot

Post by Joe Gayo »

10 Full Wave Stages and 2x series 2CL2FM, 30kHz
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Finn Hammer
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Re: Multiplier Troubleshoot

Post by Finn Hammer »

This needs to be adressed by electronic professionals.
Sorry.

Cheers, Finn Hammer
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Richard Hull
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Re: Multiplier Troubleshoot

Post by Richard Hull »

In giant multi-capacitor chains the ESR can degrade over time, especially at high frequency drive rates and high dv/dt conditions. This might be a multi-facetted issue here with a number of causative agents working in concert. Tough to diagnose easily, for sure. With lowering ESR in capacitors, the di/dt goes up and this can create issues not only with heating, (not extreme in your case), but will start to separate vapor deposited metallization on the film, internally. The situation worsens with time and power through-put, of course. The loss tangent shifts in high power pulse situations. The more capacitors in a multiplier series-parallel chain, per stage of this sort, the more complex the diagnosis to pin point the issue becomes.

Breaking new ground in the fusor power supply issue at high voltages over 50kv and high pulse power rates will be interesting and challenging. Few systems are of the type demanded in fusor service are out there.

In the shock excited, (spark gap), tesla coil biz we solved it with National Winding Labs custom making a single .05ufd 60kv capacitor for our 14,400 pole transformer powered systems, ($900 polyprop cap fixed it!).

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Joe Gayo
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Re: Multiplier Troubleshoot

Post by Joe Gayo »

The capacitors are single layer, class 1 (N4700) ceramic. Possibly one or more have cracked. I'm draining the mineral oil tank today for a closer look.
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Richard Hull
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Re: Multiplier Troubleshoot

Post by Richard Hull »

Ceramics while great filters are not good for pulse work. They shed vapor deposited silver on the slug's face like dirty underwear. This reduces plate area and capacitance. Measure your caps capacitance carefully.

In tesla coil service this worked great for a number of hours in smaller systems <<1kw, but in power systems, >>2kw they rarely survived over 1 hour and required more primary turns to be tapped into as their capacitance nose dived. They heated like a big bear, too. Naturally, shock excitement is the worst possible service for a capacitor. Only large, old, oil filled paper capacitors and polyprops survived shock excitement. ceramics were worthless. Mylars (polyester) worked fine, but heated and dissipated a lot of power. Giant G5 mica capacitors were to die for...They could really take it on the chin. Today, such micas are museum pieces.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Bob Reite
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Re: Multiplier Troubleshoot

Post by Bob Reite »

Maybe I should bring to the next HEAS that box of G5 micas I scored from a directional AM station that was being demolished because the land where the towers were was more valuable than the radio station! Then again I might need them to repair a DA that is still on the air at some point.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Richard Hull
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Re: Multiplier Troubleshoot

Post by Richard Hull »

I still retain two giant G5s and a lot of smaller G series cast aluminum bolt-down end micas and the usual large number of Mica-mold tan blocks in the 1-5kv range. The Tesla coil days are gone but not all the classic antique caps.

Our local Scrap yard about 4 years ago had about 150, 6 foot long phenolic tubes filled end to end with ceramic caps of 500pf @18kv. We counted 70 ceramic caps per tube. I think our local HEAS member, "Alex" and one HEAS attendee from Maryland took about 70 tubes each at one visit. I got about 10 tubes. I have sold about 600 of my 700 ceramic caps over those years at ham fests and HEAS October events. I think Alex still has his 4900 caps. Most of the ones I sold went into MARX generators that tend to show up at yearly HEAS gatherings or for sale at hamfests in this part of the east coast.

I will be curious about the issue with Joes increasing leakage current if he figures out the causative agent.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Multiplier Troubleshoot

Post by John Futter »

Nearly all multipliers I have serviced that use ceramic caps ie murata or clones thereof have had cap failures that if left will cause diode failure.
I now test all caps individualy with a hipot tester at near their max voltage rating and check for leakage a N4700 should have much less than a microamp of leakage at its rated voltage.
My suspicion is that supplies that have many output arcs
ie di/dt / dV/dt occurences tend to get leakier with abuse
this is a major reason thar you need to use a ballast resistor and not be tempted to draw pretty arcs in air
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Joe Gayo
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Re: Multiplier Troubleshoot

Post by Joe Gayo »

Quick Update:

Fully drained and disassembled the multiplier assembly. Each component has been individually tagged and bagged so I know its location in the multiplier assembly.

All diodes have been tested for forward voltage at a given current (30mA), and both forward and reverse recovery time and recovery currents have been measured. All are within spec and grouped closely. This doesn't eliminate the possibility of some high voltage phenomenon, but I would have expected differences if some had issues.

All capacitors have been measured for capacitance at the switching frequency and hi-pot tested to 15kVDC. All capacitances were within 5% of the marked value. The leakage current for all caps measured the same, less than 0.01 uA (limit of measurement).

I'm now turning my attention to the output divider board and its capacitors. Another possibility is that the dielectric oil is degraded in some way.
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Dennis P Brown
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Re: Multiplier Troubleshoot

Post by Dennis P Brown »

I agree that the oil, under that voltage strain, could be losing some of its dielectric properties. Also, and this is a MAJOR issue with equipment that people I know have and it requires that they periodically 'dry' their oil to remove trace water that is drawn from the atmosphere - otherwise they too have similar issues. They work in the same voltage range as your supply, but in the thousands of amps range for a few nanoseconds ... current wise, maybe a bit higher then your system ;)
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Re: Multiplier Troubleshoot

Post by John Futter »

site is still a bit flakey --just dropped me during this reply that now has to be redone
joe
do the "crackle test"
put some oil in a pyrex test tube heat it until smoking if any crackling is heard the oil is useless
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Joe Gayo
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Re: Multiplier Troubleshoot

Post by Joe Gayo »

The divider board caps/resistors are fine.

I heated several samples of the oil to 120 degC and small bubbles formed and an occasional "crackle" was heard. I continued to heat until no bubbles formed.

I'm going to bake the oil at 60 degC under vacuum to remove the moisture.
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Re: Multiplier Troubleshoot

Post by John Futter »

Joe
all high voltage transmission line transformers use the crackle test and the oil is discarded if found to crackle
dielectric breakdown of the oil also causes crackles ie voltage stress breaks the hydrocarbon chain with available oxygen causing OH's and HHO's
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Re: Multiplier Troubleshoot

Post by JoeBallantyne »

What was the final resolution on the oil? Did baking the oil under vacuum, restore its properties, or did you have to replace the oil with new?
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Joe Gayo
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Re: Multiplier Troubleshoot

Post by Joe Gayo »

Upon further analysis the water content wasn't as significant as previous though once more samples were tested. I also performed standardized breakdown testing on the oil and the leakage was negligible.

I since have uncovered another more serious issue that I'm exploring.
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Re: Multiplier Troubleshoot

Post by Dennis P Brown »

Sorry to hear - seems to be a common effect of late for many of us.
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Richard Hull
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Re: Multiplier Troubleshoot

Post by Richard Hull »

The higher the voltage toyed with or engineered with, the more the geometric progression of issues must be tackled. "Going for broke" at the high voltage end will see a lot of stuff broke, strained, and unsuspected current paths grow like unwanted weeds in a garden. When solved and fully functional, valuable lessons in HV engineering will be under one's belt.

Assuming one is not prepared to spend $10,000 for a manufactured HV supply, any suitable, stable supply over 100kv must be a personal, "from scratch" build. Quite an undertaking.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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