Safely testing 22 kV capacitors

[Roy L. Fuchs]

I think that I can easily test them at low voltage with a capacitor
testing function of my multimeter.

Low voltage testing is not definitive of their capacity at the
voltages they will be expected to be working at, and will likely be
off by quite a margin.

 

[Roy L. Fuchs]

Maxwell Labs caps are really nice quality pulse discharge
capacitors.


Hmm ... It says "impregnant MIPB" which sounds like something
different from PCBs. You might check whether they are still in
business, and ask them.

And these *might* actually surivive a dead-short discharge --
but it is still kinder to them to not do that.

With those caps, it would be kinder to all elements involved,
including and perhaps even particularly the person causing the short.

 

[Gunner]

It'd look like these:
http://images.google.com/images?q=h...US:official&percentage_served=100&sa=N&tab=wi

Yikes. OK, go to images.google.com and enter
fluke high voltage probe


While I am aware of and respect your design skills and abilities, I
don't think this is the time or application for a homebrew solution.



Sure, but that doesn't give you any evidence of the health of the
electrolyte at high voltage. Wouldn't be "tested" from my perspective
if I were a potential buyer.


I think so, yes. Certainly would get you better bids if you could show
charge being held over time. Hell, give 'em a table of your test data.
Buyers love to know that you've done your homework.


Respectful avoidance is the approach I take. I saw a guy get zapped by
a defib he was fixing once. Didn't take off his ring, and he shorted
from one side of the cap to ground. He didn't get anywhere near the
full current, but it knocked him on his ass, hard, and we got a 12-lead
EKG on him right away (this is when I was working biomed in a hospital).
Nothing alarming on the EKG, but it was pretty exciting for all
concerned. The cardiologist we had take a look at his waveforms was
impressed, and it left a hell of a burn on his ring finger. Obviously
he did at least two things wrong for this to happen, but he's a smart
guy, just made a mistake. So please be careful. A HV probe can
probably be bought on eBay for not too terribly much, and you can
probably resell it for about the same later when you're done.
Apparently, you're better at the "selling" part than I am, I usually buy
something with the intent to sell it when I'm done with it, and then
never get to that selling part.

Dave

Sounds like the fine beginning of a do it yourself rail gun kit

Gunner


"The importance of morality is that people behave themselves even if
nobody's watching. There are not enough cops and laws to replace
personal morality as a means to produce a civilized society. Indeed,
the police and criminal justice system are the last desperate line of
defense for a civilized society. Unfortunately, too many of us see
police, laws and the criminal justice system as society's first line
of defense." --Walter Williams

 

[Roy L. Fuchs]

Ian, would you or anyone have any idea if what I have is HV resistors?


They are blue, about 3 inches long, and are marked VICTOREEN MOX-3 12
MEG.

Those are HV resistors. At least I'd say it is quite likely.
Probably 15kV or higher rating. They are Metal Oxide, likely 3 Watt
units. Without actually hunting up the data sheet from victoreen, it
is no more than an educated guess, however.

 

[Roy L. Fuchs]

Why hasn't anyone mentioned yet that if you DO get a HV probe for your
meter it will serve as a resistor to discharge those caps.

If you were observant enough of the thread, you would find that even
the original poster mentioned the HV probe loading.

 

[Roy L. Fuchs]

Odds are they are just surplus.

More likely, they are change outs. Maxwell caps are only rated for
a specific number of charge/discharge cycles. After that, expensive,
tightly controlled lab environments want new caps so they know exactly
how much energy they are working with on each stroke of the bank.

 

[Roy L. Fuchs]

Ignoramus, I strongly protest your post.

Are you a mexican illegal immigrant, protestor too?

Here is an interesting
conversation that you started, with 32 posts so far, according
to the Google usenet archive, of which you have made 14, or 44%
of the posts. Yet you marked all your posts "do not archive" -
which means they will be removed from Google Groups in 6 days,
thereby severely damaging the stored thread.

I can't believe that you wasted so much of your time doing a
statistical analysis on a technical thread which you haven't even
posted any contributory response to. You are the antipathy of what it
is to be "smart".

With this kind of
callous action on your part, one wonders why others should even
take part in the already-broken, soon-to-be-bogus conversation.
What the h____ are you doing here?

You're an idiot. Enough of ALL of the responses provided are
archived, and would actually make reading the question for a given
response easier than scrolling back up through the thread chain.
What the **** are you doing mouthing off? You COULD actually try
posting on topic, contributory material.

 

[Roy L. Fuchs]

Are you sure that Fermilab is the actual seller? Based on what I know of
DOE labs, it's not very likely that they would be selling direct on Ebay or
even to the public at large.

<snip>

However, the seller would not likely be able to get away with using
the moniker either. I think it is legit, and I think they have
SEVERAL "liquidation expediters" in their employ. The prices of these
caps new means that whatever they can recover from the old change outs
is that little bit more toward the whole budget.

 

[Ignoramus27098]

Nah- 9kVAC is 12.7kV peak, call it 15kV peak. The rectifier has to stand
off full voltage on the backswing, so you need 30kV diodes, minimum. 40 or
50kV would be even nicer.

Tim, I lost you a little bit here, sorry. Are you saying that I need
single 30kV diodes and that putting 1 kV diodes in series is
unsuitable? Or are you saying that twenty 1,000 V diodes is not
enough?

Yeah, but conversely, I recall reading an article which stated that modern
diodes are avalanche rated, meaning that if the voltage across one diode
increases to say, rated PIV, current starts going up (it looks like a really
high voltage zener diode), pulling it back into balance.

Yes, tat would be very nice.

i

 

[Ignoramus27098]

You bought them just to resell them?

Yes, although I will probably keep one for myself.

Ok, we will use ONE cap as an example.

A: You do not need to charge the cap up very far, voltage wise, to
test/verify the value of the cap.

B: If you wish to test it for leakage, you need a voltage that is
closer to the max rated value than your 9kV supply. Do you have an
old PC CRT monitor or a TV laying around? The anode voltage of those
devices are typically pretty high, and you can look the model up to
see exactly what each particular one operates at.

C: You need a high quality high voltage probe to do what you are
asking.


I am not sure about your choice of comparative mechanisms, but you
shouldn't use these old caps at 22kV. Even when they were new.

One does NOT operate a cap AT its rated voltage (at least, it isn't
considered "proper"). They are ALWAYS placed into designs where the
applied voltage will never exceed about 70% of the maximum rated
voltage. This is true for ALL capacitors actually. Some should even
be designed in at only 50% of the expected voltage, meaning that one
should use a cap at half its rated voltage in such a case.

For testing, however, with caps that old, I would go no higher than
90% rated voltage.


This is a strange remark. I am not sure that we should be giving you
further assistance after seeing this.


The test is very simple, but the procedures are what are strict, and
somewhat difficult, and the parts requisite to construct a safe test
circumstance may be outside your budget.

If you think that your declaration that "they have been tested"
would change their value somehow, you are likely wrong. HV caps are
NOT all that easy to test, and your declaration is likely to be taken
with a grain of salt by anyone knowledgeable in HV settings.

The test:

Procedural rules.

1: YOU WILL NOT EVER work on or make changes to a live setup.
You only get to break this rule once.

2: Making "big fun" arc discharges is a bad thing for the caps, and
you supplies, and basically just not a good practice.

3: Ideally, you should have a bird cage or other large metal cage
around these test setups, and you should energize the circuits from a
distance of several feet.


Parts requisites:

A: A high quality HV probe. 40kV is typical for the level you are
at.

B: Twenty to twenty-five feet of 30kV TV anode wire, or better
quality "soft" 40kV HV wire, solid ignition wire, or hard teflon HV
hook up wire at 30kV or 40kV or even 60kV insulation strength.

C: A ten-pack or two of standard medium sized alligator clips with
the rubber sheaths.

D: A good Digital Multimeter with at least 4.5" digits.

E: Four 5" diameter pyrex dishes at about 1.5 to 2" tall.

F: About one quart of good, clean transformer oil.

G: Twenty-five ten Meg Ohm ten watt resistors at 5kV or 10kV voltage
rating (for Capacitance Value test load and Quick Discharge Wand).

H: A good anode supply at 20kV set point (use probe to set) (for HV
leak test at max voltage charge).

I: A simple, lower voltage supply (300V-500V) (for Capacitance Value
test)

J: A 3 or 4 foot length of dry wooden broomstick handle (for Quick
Discharge Wand)

K: A 2 foot length of solid #12 or #10 bus or house wire (unsheathed).

L: Some string (2 feet?) for tieing the resistor "wad" together.

M: The biggest cheap teflon cutting board you can find. (18x24, 20x30
whatever the biggest you can get is).

N: A couple or few four ounce sticks of modeling clay.

O: A stopwatch or timer capable watch.

The pre-prep:

A: Cut 4 24" lengths of the HV wire, and strip and tin both ends
carefully at a quarter inch length. If it TV anode wire, it will
likely already be fully tinned.

B: Attach (install the boots first) two alligator clips to the ends
of each wire. These are your HV test jumpers.

C: Take a short piece of the #12 wire, and make a pencil diameter
(inside) loop with a two inch tail on it. Sharpen a point on the tail
(not needle sharp, just taper it down on the end, it can even be
rounded on the point after that. A good x-acto knife will cut copper
a bit (enough for this). Bend the loop over 90 degrees. This
attaches under your HV probe's end point (unscrew it, then
re-tighten).

D: Take two of the Ten Meg Ohm resistors, and snip the lead on one
end of each at 3/8". Lay the two short cut ends together, and solder
them in "lap joint" fashion. Use a shear to cut the leads )on your
wire stripper, not cinch type snips (side cutters or diagonal
cutters). Shear cut the remaining ends at approximately 1", and use
some of the string to tie each resistor of the pair at the tip of the
broomstick handle. Two strings per resistor at their end caps (just
inside). Take an eight inch segment of the #12 inch wire, wrap two to
four turns of it around the end of the broom stick handle, and leave a
two inch length pointing off the end of it, or cut it to that length.
Lay the 1 inch resistor lead from the top resistor (closest to stick
end) along the coil of large copper wire, and solder it. Wrap the
resistors and coil with electrical tape to keep them from shifting.
Cover all four string ties, and the center node, and the end coil with
a few to several wraps at each location, leaving to remaining resistor
lead accessible. Attach one end of an eight foot length of the HV
wire to the exposed resistor lead, solder the connection, and tape
down about five inches of the HV wire to the top of the stick. You
can even zig zag it to remove pull stresses from the lead attachment.
Tape it down with several wraps, and include the remaining solder node
you just made. This should result in a stick with the last ten or so
inches taped up, and a point sticking off the end. Add an alligator
clip to the remaining end of the free flying seven foot segment of th
HV wire. This is your Safety/Quick Discharge Wand.

E: Place the cutting board on your test bench. Keep the pyrex dishes
handy as well as the transformer oil, and clay.

The set-ups:

Capacitance Value Test:

Cutting board components will be the cap under test, the probe, the
meter, one HV jumper, one pyrex dish, the medium voltage power supply
and the timer. Optional components are one or more of the load
resistors, requiring two additional HV jumpers, and one additional
pyrex dish.

The long, slow, high statistical reliability test simply uses the
probe to load the cap, and the timer to test for the discharge rate.
The probe tip, and HV jumper end go into the dish, and the leads get
held down to the cutting board by a clump of clay. Oil covers exposed
leads in the tray, and the probe rear end can be held elevated by a
empty paper towel roll tube cut to length to fit the angle that points
the probe tip down into the pyrex dish. The probe should always be in
place when energizing the test circuit/setup. Never use the probe on
live gear in "free hand" mode.

The HV jumper lead that attaches to the HV side of the cap can
remain bare, just like the cap node already is (as pictured). The
Grounded side of the cap gets a lead to the MV supply ground, as well
as the ground lead from the HV Probe (very important). The MV supply
positive lead will also go to the cap's HV node. The banana jack
probe leads go into the meter, typically set on the 10 volt range.
Again, make sure the ground lead of the HV probe is grounded to the
low side of the cap. The probe is typically 1V per 1kV. They are
generally 1 Gig Ohm loads, or that is what you should get.
You can calculate the discharge rate for a 1G Ohm load at a given
voltage on a given cap value. You can then charge the cap with the MV
supply, read the voltage on the meter, the remove th MV source lead,
and time the discharge period to verify the cap value.

The alternative would be to fashion a set of the load resistors in
series, tie the bundle together, immerse it in a second pyrex dish
with transformer oil in it. Attach the HV jumper from the cap to one
end, and from the other end to the grounded side of the test setup.
Attach the remaining HV jumper to the HV cap output node, and to the
HV Probe dish. This presents your load resistor as well as the HV
probe's loading value to the cap. Charge it up, and time the new,
quicker discharge rate, calculate cap value. Test(s) complete.
Always ensure that meter reads zero volts before touching any setups.
A good practice is to take the discharge wand, and attach the lead
clip to ground, and touch the pointed barb to the immersed node in the
pyrex dish for a few seconds.


High Voltage Max Charged Leak Test:

Need: One dish, probe, meter, HV supply (danger)

Test: Without cap, attach ground lead of probe to ground side of HV
supply. Attach high side of supply to probe tip in oil bath. Turn on
meter. Turn on HV Supply, and set output to 20kV. Turn off supply.
Ensure that meter reads zero volts before touching anything, or use
the discharge wand to ensure that there are no charges hanging around.
Place the cap on the cutting board, and attach the ground lead from
the HV supply to it. Attach the HV supply output lead to the cap HV
node. Attach the HV jumper from the probe dish to the cap's HV node.
Make sure that the HV probe ground lead is attached to the HV Supply
ground lead. Turn on meter. Turn on HV supply (use single hand/hand
in pocket techniques). Read meter. If cap is bad it may arc
internally, but will discharge into the HV probe load at a much higher
rate than expected. Use wand to discharge after testing, and always
reattach shorting jumper that ALL of these caps should have on them
(remove them of course during testing)This test isn't 100% definitive
or ideal.

Testing to only 9kV will not allow you to declare that "they have
been tested" with any validity if that is your goal, since as you say,
you are selling them.

See... it isn't as easy as it appears.

Yes, you are right, it is indeed not easy. Thank you for your post, I
saved it for future reference.

i

 

[Mike Henry]

However, the seller would not likely be able to get away with using
the moniker either. I think it is legit, and I think they have
SEVERAL "liquidation expediters" in their employ. The prices of these
caps new means that whatever they can recover from the old change outs
is that little bit more toward the whole budget.

It's more likely that lab "junk" is sold off for salvage and that the
salvagers or someone they've sold to are auctioning off the stuff. It seems
really unlikely to me that anyone at Fermi Lab, or DOE for that matter,
would bother to check Ebay seller names for usurpers of their "name". I
haven't checked, but would guess that Fermi Lab's budget is at least a few
hundred million dollars per year and technical staff probably bill out at
around $125 and up per hour. They'd have to get pretty good prices for
those caps at that rate to break even - did they?

Mike

 

[Ignoramus27098]

However, the seller would not likely be able to get away with using
the moniker either. I think it is legit, and I think they have
SEVERAL "liquidation expediters" in their employ. The prices of these
caps new means that whatever they can recover from the old change outs
is that little bit more toward the whole budget.

It is, doubtless, definitely Fermilab. I will go to him on tuesday
into Fermilab itself. I find it difficult to imagine a private
individual sellings his own things in this manner.

i

 

[Ignoramus27098]

Those are HV resistors. At least I'd say it is quite likely.
Probably 15kV or higher rating. They are Metal Oxide, likely 3 Watt
units. Without actually hunting up the data sheet from victoreen, it
is no more than an educated guess, however.

Roy, thank you. Sounds like they will be helpful.

i

 

[Ignoramus27098]

[alt.marketing.online.ebay -- I am talking about an item that I won
on ebay recently, see

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7603382621
]

It's more likely that lab "junk" is sold off for salvage and that the
salvagers or someone they've sold to are auctioning off the stuff. It seems
really unlikely to me that anyone at Fermi Lab, or DOE for that matter,
would bother to check Ebay seller names for usurpers of their "name". I
haven't checked, but would guess that Fermi Lab's budget is at least a few
hundred million dollars per year and technical staff probably bill out at
around $125 and up per hour. They'd have to get pretty good prices for
those caps at that rate to break even - did they?

Mike, I am going to Fermilab to pick up these capacitors on Tuesday
and they want payment in form of checks made to Fermilab. They take my
check on the spot and do not require it to clear. I would be greatly
surprised if that was not a legitimate Fermilab operation.

I think that what we are observing is typical government/public funded
surplus bullshit. I see the same stuff all over in military surplus.
Everyone is going through the motions ("we are auctioning surplus to
get highest competitive value"), but no one really cares to even
understand what they are selling or what it costs to sell that
stuff. Some things sell well for them (like high dollar electronic
test equipment sold one piece at a time), some do not.

Now if this item was adequately described with love and care, then, I
would suspect that it was not Fermilab but some private person who
actually cares to get money for his stuff.

i

 

[Tim Williams]

Tim, I lost you a little bit here, sorry. Are you saying that I need
single 30kV diodes and that putting 1 kV diodes in series is
unsuitable? Or are you saying that twenty 1,000 V diodes is not
enough?

30-50kV PIV "diode", collectively. You can make that "diode" out of as many
1N4001 diode*s* as you need.

Tim

 

[Dave Hinz]

Ignoramus, I strongly protest your post. Here is an interesting
conversation that you started, with 32 posts so far, according
to the Google usenet archive, of which you have made 14, or 44%
of the posts. Yet you marked all your posts "do not archive" -

News flash: the person doing the writing gets to decide how, or if, his
work is distributed and stored. Demanding that he do it differently is
no different than the people who ask a question and demand an email
response. (equally stupid and arrogant, is the point I'm making).

 

[Ignoramus27098]

30-50kV PIV "diode", collectively. You can make that "diode" out of as many
1N4001 diode*s* as you need.

Thanks, now I understand a little better. I will soon make a stack of,
say, 30 1N4007 diodes. That would let me test the capacitors with 13
kV.

i

 

[Ignoramus27098]

News flash: the person doing the writing gets to decide how, or if, his
work is distributed and stored. Demanding that he do it differently is
no different than the people who ask a question and demand an email
response. (equally stupid and arrogant, is the point I'm making).

Dave, I sort of agree with both of you. I think that I have a right to
hide my posts, but, on the other hand, I decided not to exercise it
anymore. At least in this newsgroup.

i

 

[Jim Stewart]

Discharging caps that way can ruin the cap. Just turn off the supply or
apply a resistor load. The internal plates violently move - ripping
themselves up.

Given that they were made by Maxwell and sold to
Fermilab, they are probably spec'ed for pulse
discharge. That said, I'd not want to be on the
same city block when iggy crowbars them.

Putting a hard metalec short across them will no
doubt blow half the terminal away, create a deafening
blast and generate a nice EMP pulse. There's no
way I'd do it.

 

[Ecnerwal]

Do you thikn that it has PCBs?

Likely not, based on that label - it's simply something that, if you
have ever had to deal with, you NEVER make assumptions about from that
point on, because they were so widely used when they were in vogue.