glassman hv supply

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colinheath
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glassman hv supply

Post by colinheath »

hi all,
iv been fortunate enough to get a 75kv 4ma glassman power supply for free! the only thing is its positive output not negative.
could i still use this for a fusor?
cheers
colin
Richard Hester
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Re: glassman hv supply

Post by Richard Hester »

Charging the outer shell of a fusor to +75kV is a sure formula for leaving this planet quickly with the slightest mistake. You might be able to do a glass number with all hot components inside, but it sounds like a lot of trouble to me.
Brett
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Re: glassman hv supply

Post by Brett »

If he's only going to be running it for short periods, he could run it off battery power, ground the output, and let the power supply, battery, inverter, and all, float at -75 kv. Ugly, but it would work.
guest

Re: glassman hv supply

Post by guest »

Depending on how complicated the supply is, he may be able to
reverse the polarity by swapping a few diodes and any polarised
caps etc.
colinheath
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Re: glassman hv supply

Post by colinheath »

thanks for the replys. i am fully aware of high voltage safety so my main concern is if pushing ions towards the centre is going to be as efficient as pulling them in with regards to focus
also is 4ma enough current?
many thanks
colin
Richard Hester
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Re: glassman hv supply

Post by Richard Hester »

Generally, the multiplier assemblies in modern high voltage supplies are potted, making it difficult to change polarity, as one must reverse all the diodes in the multiplier stack.
Richard Hester
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Re: glassman hv supply

Post by Richard Hester »

One more safety note - if you let everything float in your fusor setup in order to use a positive supply, there is the possibility that some of the components will remain charged when the supply is turned off. This may be gotten around with care, but is another consideration making it unadvisable to run a fusor with a positive supply. Gas connections will need to be with plastic tubing, unless you want the cylinder and regulator to float to HV. The same goes for the vacuum connections. I refused to bid on a perfectly servicable 70kV Glassman supply for the reason that it was a positive supply and not reconfigurable.
Brett
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Re: glassman hv supply

Post by Brett »

I'm not sure that's entirely true; Isn't it possible to just clip the multiplier stack off, and flip it around? Assuming that the caps are equal in value, of course... If they're not that wouldn't work well.

Alternatively, a multiplier stack really isn't all that expensive to put together. You could use the supply to drive one of your own.
Richard Hester
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Re: glassman hv supply

Post by Richard Hester »

Draw a cockroft-walton type voltage multiplier, with input, ground, and output leads, and try to turn it around backwards to get an opposite potential output - it won't work . You are stuck with digging out the diodes and flipping them around. The capacitor voltage ratings between the input side and ground side of the stack may not be symmetrical either, to reduce costs. To prove this, draw the simplest implemetation, a voltage doubler. The input cap sees Vin, the output cap sees 2Vin.
Brett
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Re: glassman hv supply

Post by Brett »

Yup, my mistake, that wouldn't quite work. Too bad
AllenWallace
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Re: glassman hv supply

Post by AllenWallace »

YES! you can use a positive power supply. You want to make a
single stage marx generator, which will convert a positive DC voltage
into negitive pulses. Also, look at Richard Hester's post "Pulsed Fusor
Schemes" in the Files section of this forum. You can subsitute a spark
gap for the thyratron.

My son's fusor uses this concept and he is getting about 120 neutrons
per pulse, using 25 KV and a 0.1 uf capacitor. The only draw back is
that it takes 5 seconds to charge up the capacitor!
3l
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Re: glassman hv supply

Post by 3l »

The Marx inverter is a really great way to turn lemons into lemonade. I didn't think anyone else was using it. I have used that trick many times at high voltages where diodes are rare and fragile. Try and get any above 20 kv at high amperage >100ma!

YES..I know about the xray rectifier sticks but the amperage is 20 ma tops. They are fragile don't try to pulse them they are slow recovery diodes...I lost two Ebay Specials this way.
I really like spark tech....you can't beat it or even dent it !
It may be low tech but you just can't beat the price or the power delivery.

Keep on Fusing!
Larry Leins
Fusor Tech
DaveC
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Re: glassman hv supply

Post by DaveC »

Colin -
The Glassman supply is a nice one... They actually made some of their units to be either positive or negative. Unfortunately, it was not just a switch change.. it required a second multiplier wired up for the other polarity... You remove the one plarity multplier, plug in the other, change a panel switch and away you go...

This leads to what you need to do. First open the case, and check whether you have an air insulated multiplier. If so, you are in luck. To reverse polarity, you have to change the direction of all the HV rectifiers in the multiplier.IF it is a positive supply now, you will have a rectifier string with cathodes at the bottom. They need to be inverted. That's "all" there is to reversing the multiplier polarity. You will also need to switch meter leads around on the panel voltmeter and ammeter.. so they read upscale,

If the diodes are not trimmed to closely, it should be straight forward to reverse them.. probably an half hr job.

Hope this helps.. if you have more questions.. I will try to help.

Dave Cooper
jlheidecker
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Re: glassman hv supply

Post by jlheidecker »

This post by Dave is very helpful. I am in a similar boat as Colin. I
have a Glassman positive 20kV, 15mA supply that needs to be
reversed. It seems as simple as 1, 2, 3..

1. Change direction of HV rectifiers in the multiplier.
2. Invert cathodes at bottom of rectifier string.
3. Switch meter leads to make a practical readout.

My question to Dave:
Just to be crystal clear, when you say "it should be straight forward to
reverse the diodes," are you referring to the diodes that make up the
HV rectifiers?

And finally, do you have any personal (direct or indirect) observation
of this being completed successfully or unsuccessfully?

Thanks a million,
Jason Heidecker
Richard Hester
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Re: glassman hv supply

Post by Richard Hester »

Your supply probably has a potted HV multiplier assembly, so it wouldn't be straightforward to change anything in the multiplier.
DaveC
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Re: glassman hv supply

Post by DaveC »

Not all Glassman supplies have potted multipliers.. the older ones are air insulated and work very well. Typically they are 80 kV in the 1 - 10 mA range. It the multiplier is potted, Richard is absolutely correct.. You are stuck. But if not.. you need to simply change the polarity (direction of all the diodes in the multiplier. This will invert the output polarity. The voltage divider will now read the opposite polarity too, so you will need to either simply switch leads on the meters. Remember, the multiplier receives high frequency 20 kHz or a bit more AC. So the output polarity is governed solely by which way the diode string goes. The one caveat here , is the interlock and trip protection circuitry. This could be polarity sensitive.and you will need to plot out the circuit to see how or if it needs to be changed...

If you can find the caps and diodes, and some G-10 board, you can probably make a duplicate multiplier (again assuming it is air insulated.) Glassman does some interesting things to provide stress control, but in general, multipliers are quite well behaved because the voltage builds in stages, which helps control electric stresses.

I build multipliers all the time... typical small 120 kV couple mA devices, about 4 1/2" X 4" X 1" .. 20 stage (20 caps, 20 diodes = 20 stages by multiplier reckoning). Fits nicely into a one gallon paint can for an oil container, but could use SF6 gas at a couple atms or even nitrogen, but pressure must be higher and full voltage may not be possible.

Real challenge is to get the HV out, without flashovers I use HV receptacles (Alden or Fed Std,) or build my own, or direct cable. Okonite EPR 100 to 130 kV Xray cable is a good choice... at about 7- 10 bucks a ft new. Also, High energy spark plug wire can be okay... depending on what type of conductor it has. Copper is obviously the best. Hope this helps a little

Dave Cooper
seadat_mis
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Re: glassman hv supply

Post by seadat_mis »

you can just change the polarity in the output, because its a DC output, the only thing you have to be carefull about, is the insulation of high voltage...
jlheidecker
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Re: glassman hv supply

Post by jlheidecker »

I took a look inside, and it seems quite simple. I attached two photos
in case you were curious. Other than changing the diodes and caps
(if these too are polarity sensitive), I believe the only other problem
might be the interlock and trip protection circuitry (as you said in your
post). I will call Glassman tech support to see if these are polarity
sensitive. If they are, I suppose I'm screwed.

Does the above changes look complete to you? I understand you
were originally giving advise to Colin about his much larger supply.
Can you foresee issues arising out of the the fact I have a different
model? If this doesn't work, what are my chances this switch will ruin
my supply? Or do you believe I can just switch everything back and
it'll work again?
Attachments
PDRM0003.jpg
PDRM0002.jpg
DaveC
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Re: glassman hv supply

Post by DaveC »

Yup that's one of their air insulated multipliers. I would recommend making a circuit schematic first and taking the time to be sure you know how the voltages build. Once you see the zigzag diode string (in your circuit diagram) it will be obvious what to change.

As to the high voltage divider which is in there, too....and the output current sense resistor, if the meters are analog, all you need is to reverse the connection on the back of each meter. The Positive (+) meter terminals will connect to ground. If the readouts are digital, you may have to work you way through the signal conditioning circuitry to be sure how polarity is handled.

The only other thing is the overcurrent sense. At worst it might need an inverter to change polarity. These aren't much to build, but add-ons can be a problem getting them installed nicely.

But.... Glassman makes reversible supplies. We used to have one. You purchase a positive multiplier and a negative multiplier. Somewhere on the case, there is a polarity switch. It fixes the meter and sensing problems. If yours doesn't have the switch, you may be able to figure out how to install one. To that end I would suggest trying to get schematics from Glassman to help you.

Best regards,

Dave Cooper
jlheidecker
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Re: glassman hv supply

Post by jlheidecker »

I successfully converted my Glassman power supply from positive to
negative polarity. It was only slightly more complicated than
switching the direction of the diodes. Glassman sent me instructions,
drawings, and replacement diodes/caps for free. Of course they only
bent over backwards like this after I purchased a $7000 60 kV supply
from them! So the HV rectifiers were flipped, one cap was moved,
and two jumpers were replaced by three others. There were a few
calibration procedures, but nothing actually needed adjustments. The
entire process took one afternoon. It works great!
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Adam Szendrey
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Re: glassman hv supply

Post by Adam Szendrey »

Hi!

Umm ..maybe i don't understand something here....but if i charge the outer grid to +75 kV , wouldn't ground be -75 kV compared to it? (if i say that the +75 kV outer grid is the reference). Sooo...if i ground the inner grid it would act like a cathode wouldn't it? Maybe i missed something...

Adam
Richard Hester
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Re: glassman hv supply

Post by Richard Hester »

If you had a fusor that could take 75kV, then it would not be glass, as stray electron beams would very quickly melt a hole in the glass and cause it to implode. Every fusor builder that has tried to build a high power fusor with a glass chamber has experienced this. Inappropriately placed view ports can also have holes melted into them. X-rays would also come sleeting through a glass chamber They will at least be partially blocked by a metal chamber. The X-ray hazard from a hot amateur fusor is no joke. The soft x-ray output can fry a cornea or give you skin cancer. Anything operating over 40kV or so should have supplemental lead shielding. A real 75kV fusor would have a metal (stainless or aluminum, preferably stainless) vacuum chamber. For safety purposes, the outer vacuum chamber should be grounded. If you ground the inner grid and float the outer shell at +HV, you are begging to get killed.
jlheidecker
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Re: glassman hv supply

Post by jlheidecker »

So where would you draw the line for glass fusors? I currently have
a bell jar chamber, but would like to run at 60 kV. Would you say a
metal chamber is necessary at this voltage?
3l
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Re: glassman hv supply

Post by 3l »

Hi Richard:

I have used glass up to 50 kv.
The metal can inside the glass belljar will work nicely.
But as you say xrays are very strong. (10 rhem)
A dental xray is around .001 rhems.
Shortly I will be testing at 100kv pulse level.
The higher power xrays will need a bunker for sure.
Pulse can't be safely watched without going blind.
It is just like the initial flash of any special weapon.
So no view ports...the backwash would just ruin them any way.
My first wakefield demo will be under glass.
I have found if you point the opening down towards the baseplate
and gap up to two inches the electrons will stop at the baseplate.
No way am I hanging out ...this is all remote work.

Keep on Fusing!
Larry Leins
Fusor Tech
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Carl Willis
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Re: glassman hv supply

Post by Carl Willis »

Hi Jason,

You cannot do 60 kV in a bell jar fusor. It's just not practical for a number of reasons. Right now, I think you should:

1) Put some strips of masking tape around your bell jar, or otherwise make sure it's covered so that if an errant electron beam, or too much heat, etc. busts it under vacuum, nobody will go to the hospital full of shrapnel. Just a good idea with this sort of project.

2) Try to eliminate all plastic from the vacuum. I think you have a yogurt container (?) in there. You won't be able to reach your limits of even the bell jar if poissor electrons can see big pieces of plastic.

Damaging the bell jar will be very easy to do once you surpass maybe 30 kV. There's no hard limit to how far you can push it, but careful observation (look for bubbling, incandescence; feel the sides when it's off for areas that are much hotter than others) should tell you when you have gone too far. Note that replacing the bell jar might be even more expensive than making an appropriate metal chamber. Yet another incentive to just set aside the 60 kV unit for later.

-Carl
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