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Registered Member #3324
Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276
Hey guys, im working on a driver for a large x-ray transformer.
Been messing about with it as a half-bridge to the point that i was getting 1 1/2 ft long white arcs from it. then i found an 80uF 1000VDC capacitor and put it in series, was getting arcs over 3ft....
Unfortunately i killed the capacitor. Thing is I need another one with higher peak currents.
Does anyone know where i can get such a capacitor?
Im running it at around 25kHz to 35kHz. I have tried normal Tesla coil capacitors but they don't seem to have enough capacitance to work properly. Does anyone know where i can find any large uF High voltage film capacitors with a high RMS Current. Im guessing figures like:
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
That's a very impressive X-ray transformer, I can't believe it's capable of such high powers without melting! The only X-ray transformer I've played with was an old Gendex 50/60Hz unit capable of about 75kV peak. The HV windings would get quite warm after about 30 seconds at 10mA or so.
The only film capacitors I can think of are the big screw terminal filter caps like these: However they definitely aren't cheap and seem to be designed for high current DC filtering. Another alternative could be a whole bunch of the direct-mount IGBT snubber caps in parallel, but again they are fairly expensive.
Registered Member #3324
Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276
GrantX wrote ...
That's a very impressive X-ray transformer, I can't believe it's capable of such high powers without melting! The only X-ray transformer I've played with was an old Gendex 50/60Hz unit capable of about 75kV peak. The HV windings would get quite warm after about 30 seconds at 10mA or so.
The only film capacitors I can think of are the big screw terminal filter caps like these: However they definitely aren't cheap and seem to be designed for high current DC filtering. Another alternative could be a whole bunch of the direct-mount IGBT snubber caps in parallel, but again they are fairly expensive.
Thanks for the tip, now i know what to look out for. The transformer is rather insane, i actually got even more output recently... again this is only a half-bridge. I need to do some changes to the casing however before making it to a full-bridge. We had an incident yesterday.....
The wire terminals were getting very hot and the pressure relief valve for the oil was broken. I believe pressure got enough to push the hot metal connector though the plastic a little bit, one of the terminals then shorted to the case i think, causing an arc, then a huge amount of transformer oil-steam shot out around the connector. Was rather worried i had actually killed the transformer, but i did a continuity check and it seems ok, i may give it another try tonight.
-Thomas
EDIT: using a clamp meter, i was actually measuring 350mA on the output....
Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Very impressive Thomas!
I'm still bewildered at where you guys find these monster transformers, the best I've ever found is some 6kV standoffs still attached to an abandoned pole in a field.
As to your question I hate to be "that guy" but have you checked ebay? For example this might be suitable It is not cheap but I don't know where else you could source one.
Gotta love that series load resonant voltage rise action, if you tune it just right you can achieve zero current switching and greatly reduce MOSFET heating. You would need a Phase loop locked driver though such as this one to keep it in tune
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Nice arcs. Does the X-ray transformer have a built-in divider to measure the output voltage? They often do.
I had a look on eBay UK and these are the best I could find. All 8 in parallel would give you 16µF at 1600V, at over 200A RMS. The auction accepts offers so it could be worth a try giving them a lower offer than the listed price.
If you have a shipping proxy in the US, three of these in series would be just about perfect.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well done! Very impressive results Please be careful though as "Transformer oil steam" is quite flammable. You don't want a mini version of this in your living room.
I think Anders' Ebay UK link is probably your best bet. 8 of those in parallel will be a pretty heavy duty cap bank.
Registered Member #3324
Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276
Steve Conner wrote ...
Well done! Very impressive results Please be careful though as "Transformer oil steam" is quite flammable. You don't want a mini version of this in your living room.
I think Anders' Ebay UK link is probably your best bet. 8 of those in parallel will be a pretty heavy duty cap bank.
Thanks for the tip, i placed an offer on them. If they don't do a nice enough price (im pretty skimp) then i might go to a capacitor manufacturer and ask if they have any similar of x'x in stock.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I think I can help you out here, Tom.
I have a case of 4μF 2.5 kV RMS 4kV pk knocking around somewhere - surplus to my Rayleigh line experiments.
They are film types, cylindrical about 2.5" high and 4" diameter with axial M8 terminations at either end. Their main use seems to be in high power snubbers, but they are also used in high power medium frequency RF circuits. (I never met anyone else who used them in a pulse forming line.)
They are still in current manufacture and so have data sheets. I'll try and find them in the next few days, and then I'll be able to find the data sheets.
I don't want any money for them, but the postage is likely to be about £20 for five of them to get your 20μF.
Registered Member #3324
Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276
Proud Mary wrote ...
I think I can help you out here, Tom.
I have a case of 4μF 2.5 kV RMS 4kV pk knocking around somewhere - surplus to my Rayleigh line experiments.
They are film types, cylindrical about 2.5" high and 4" diameter with axial M8 terminations at either end. Their main use seems to be in high power snubbers, but they are also used in high power medium frequency RF circuits. (I never met anyone else who used them in a pulse forming line.)
They are still in current manufacture and so have data sheets. I'll try and find them in the next few days, and then I'll be able to find the data sheets.
I don't want any money for them, but the postage is likely to be about £20 for five of them to get your 20μF.
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x-Ray transformer series capacitor.
Very impressive Thomas! 
Please be careful though as "Transformer oil steam" is quite flammable. You don't want a mini version of this in your living room. 