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Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
The coax is to provide a low inductance line between the IGBTs and primary coil. This is very crucial because you want all of the inductance to be in the primary. You dont want stray inductance in the circuit for a few reasons: it lowers the overall coupling between the primary and secondary, and it increases your surge impedance unnecessarily so. Ideally the IGBTs would probably become part of the primary coil like Terry did in his first OLTC design. But, since you are dealing with a giant IGBT with a big heatsink, its not an easy way go to. The coax is the next best option, but you need to parallel enough of them to handle the currents.
Registered Member #242
Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210
So is the coax being used for the inner or outer conductor? I'm assuming outter due to the high current involved...so would the braided shielding work then?
Also, Are there any eaisly obtainable alternatives to using the cardboard form for the secondary? I can see that getting broken down the road.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
No, the coax is used as coax (both conductors are used, and seperately). It is used for the send and return of the primary connections. You are basically reducing the loop area of the wiring, thus reducing the inductance.
Yeah, it's "me"...kinda sounds like you recognize me from somewhere?
I had you confused with James Zimmerschied who is also "JRZ". I get confused easy #:-)
The Steve's
Thanks for explaining the Coax and how it is used to reduce wiring inductance!! I never realized what all that was about *:-) I might do that on my new coil where primary inductance needs to be rather low.
Must study the voltage doubler and trippler link! The DC SISG and MOT stuff is all new to me. Mark and some use resonant charging on them too...
Somedays, it is like I don't know anything 0:-)
For a durrable secondary form. I like cut to length polycarbonite from Mcmaster-Carr.
But it costs a whole lot more that concrete tube!! Very nice for little coils though. You can also get sewer and drain pipe from a plumbing supply contractor type place but you usually have to get full lengths that are like 20 feet long... Or just hang out there and follow the guy that just bought 50 of them and see if he has any left over )) You can often call plumbing places and ask if they have an extra bit they would sell you too. Most contractors just throw the extra away. The trick is to be there at the right time.
Registered Member #242
Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210
ouch. thats quite expensive. I'm going to make some calls tomorrow and see if I can find some for cheap...If not, I'll probably just buy a concrete tube, plug the ends and fill it with expandable foam.
My dad has a 18.5" diameter piece of pvc, 48" long, left over from when we built our house...I think its alittle too big?
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
What size are you looking for? Public Missiles Ltd has 48" pieces of phenolic tube for $40 for a 6" piece or $80 for a 12" diameter... If you only need 4" I would get the 'quantum tube', (it is that grey plastic tube that is shown at the top of the airframes section of the webstore) it isn't as strong (but still stronger than you need) but considerably easier to work with.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
When I was making my OLTC 2, I went to one of those plumbing supply places and spoke nicely to them, and they cut off a 3 foot piece of 10" HDPE pipe with 3/8" wall thickness for me. It cost about $35.
The secondary for my DRSSTC was another piece of smaller HDPE piping that I found lying in the road one day.
But it costs a whole lot more that concrete tube!! Very nice for little coils though. You can also get sewer and drain pipe from a plumbing supply contractor type place but you usually have to get full lengths that are like 20 feet long... Or just hang out there and follow the guy that just bought 50 of them and see if he has any left over )) You can often call plumbing places and ask if they have an extra bit they would sell you too. Most contractors just throw the extra away. The trick is to be there at the right time.
Cheers,
Terry
Check EBAY. There is a supplier there that sells acrylic tubes for dirt cheap. Although, i'm not sure what the largest diameter is.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
Now that we have the SISG in our toolboxes, what about using SISGs to create an "off line" Marx generator to power your OLTC? Take three 300V SISGs (one SIDAC each) and make a three-stage Marx that'll spit out 900V. Then attach TC primary. I'm told SIDACs are slow, so perhaps the rule would be that the output of the stack ought not to exceed the voltage of any single IGBT. Else the rise time might outpace the SIDACs after the first stage fired and you'd get deep-fried IGBT. But if you stay below the IGBT voltage, as in my example above, wouldn't that work?
Then you'd have the Sidac Igbt Spark Gap Marx Generator Tesla Coil (SISGMGTC)
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
(edit: Few more thoughts)
Basically, the schematic I had in mind was:
...where each SISG is set for 300V (one SIDAC).
Hmmm... Maybe the voltage doubler ought to be separated out with bigger caps for smoothing to allow for higher break rates, I don't know. Haven't thought enough about it. Add in DC resonant charging and higher-V IGBTs and you might really get something out
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OLTC project
)) You can often call plumbing places and ask if they have an extra bit they would sell you too. Most contractors just throw the extra away. The trick is to be there at the right time.
