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Coronafix

Thu Jun 15 2006, 02:49AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Just a quick question about primary coils. Why is it that in most SGTC setups, people use a flat spiral primary, and in SSTC systems, a helical primary is commonly used?

...

Thu Jun 15 2006, 03:32AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

It depends on how much coupling you need... SGTC's need very loose coupling, so we use a flat or slightly conical one, but the solid state coils need much higher coupling, so we use the helical.

GODSFUSION

Thu Jun 15 2006, 11:45AM

Registered Member #157 Joined: Sun Feb 12 2006, 08:00PM
Location:
Posts: 76

Why do you need or is it required to have less coupling on a sgtc. I.E. Why couldent you wrap the hollow tubing from a sgtc helically like a sstc?

- wayne -

Marko

Thu Jun 15 2006, 01:27PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

SGTC's love low coupling and also love to arc to primary if it gets even little over secondary.

If you really want to use helical primary on a SGTC you should make the first turn of secondary just above the top turn of primary.
Primary should be wound on separate former so you can change it's distance to secondary if necessary (to reduce/improve coupling).

This way helix can pass but because of big primary under it your secondary looks much higher than it should be, and

Steve conner says:

There is nothing worse than puny little sparks coming out of a huge coil.

Flat or conical primaries are much more preffered because they are smaller in physical size, you have no trouble with flashovers and coupling is fair for the type of coil.

GODSFUSION

Thu Jun 15 2006, 03:08PM

Registered Member #157 Joined: Sun Feb 12 2006, 08:00PM
Location:
Posts: 76

No offence but ya havent answered my question. Why does a sgtc like low coupling and a sstc like high coupling?

- wayne -

HV Enthusiast

Thu Jun 15 2006, 03:14PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

I wouldn't say an SSTC uses high coupling. Its better to say that it uses a "higher" coupling than that typically used in a SGTC. DRSSTCs actually use similar coupling to that of a SGTC.

I believe the coupling comes down to how fast energy is transfered from the primary circuit to the secondary circuit. Of course, as you increase coupling, energy transfer does become "faster", however, with a spark gap coil, if this energy transfer is too fast, the voltage build-up on the secondary occurs too quickly which leads to arcing problems including racing arcs.

Coronafix

Thu Jun 15 2006, 10:00PM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Thanks, this is clearing it up a bit, it's about the energy transfer. The spark gap is able to dump a heap more power into the system than mosfets or IGBTs, right? Probably to do with the internal resistance of the component.
But if the coupling in DRSSTCs and SGTC are similar than why doesn't everyone just use the flat primary, seems figuring out the coupling would be easier, you just raise or lower the secondary. With the helical, you have to adjust the diameter to set the coupling. Or is it about aesthetics?
I want to use a helical for a SGTC, this is why I ask.

HV Enthusiast

Thu Jun 15 2006, 10:17PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

A flat pancake coil would work fine for DRSSTCs. Just remember, there is a practical limit to getting a large coupling with a pancake coil, so if you need additional coupling, then a solenoid or inverted cone would work.

Remember, to increase the coupling of a pancake coil, you need to raise it above the secondary. This can lead to arcing problems depending how high you need to raise it.

Remember, just because its not commonly done, doesn't it mean it won't work.

Steve Ward

Fri Jun 16 2006, 11:39PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

SSTCs (without a tuned primary circuit) need the higher coupling to get the best impedance match between the 2 circuits, othwersize SSTCs cant process as much power. They are really quite different than both SGTCs and DRSSTCs. SSTCs often drive the coil after the spark is already there, and keeps pumping in more power to feed the spark. In order to feed the spark lots of power (and at only a few hundred volts), the primary needs to be coupled to as much of the secondary as possible. The limitation is flashover between the coils. An interesting thing about SSTCs is, they never develop more than about 80kV so long as you use a breakout point, so you can make the coupling rather tight compared to other TCs.

With DRSSTCs the coupling can be all over the place. My early coils used couplings around .25 or so. The idea was to transfer the primary energy quickly and continuously so that the primary current wouldnt peak quite as high. But, now im finding that in order to get the longest sparks possible from a DR coil is to lower the coupling, and suffer the higher peak currents.

The spark gap is able to dump a heap more power into the system than mosfets or IGBTs, right? Probably to do with the internal resistance of the component.
But if the coupling in DRSSTCs and SGTC are similar than why doesn't everyone just use the flat primary, seems figuring out the coupling would be easier, you just raise or lower the secondary. With the helical, you have to adjust the diameter to set the coupling. Or is it about aesthetics?

IGBTs are catching up with spark gaps. Only on the biggest of coils (say, 30' sparks) might spark gaps win. Fets are no good in this high peak power arena.

You seem to have some misconception about coupling. A flat primary makes it no "easier" to figure out the coupling. Also, with a helical coil, you dont need to change the diameter of the primary, you can simply raise or lower it as well. Many of my coils were made when i thought i needed the higher coupling, so they are either conical or helical. One benefit is that you can fit a big primary in a small footprint when the turns are stacked vertically. Just raise up the secondary and you will get coupling similar to your common flat primary.

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