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Advice on bad coil performance.

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LAN_403

J. Aaron Holmes

Sun Mar 04 2007, 08:40PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

That is a little mysterious...can you draw an arc on your NST from either output to the case? Are you using a Terry filter to protect your NST? (highly recommended) I'm not too familiar with the ins and outs of bottle caps, so maybe there's something to look for there. Certainly having too *much* capacitance could cause shortness of voltage on the gap, but you said you'd already had it jumping .5" in the prior gap?

Hopefully somebody else jumps in with some thoughts.

I'd definitely recommend grounding the case of your NST to RF ground. You'll give the RF nasties someplace to go and equalize the stresses on your NST secondary. Don't use the mains ground, as a strike from secondary to primary will probably find its way to your RF ground in a very undesirable way (through your house wiring to ground, and from that ground to your RF ground--probably passing by a lot of other expensive electronics on the way!).

Regards,
Aaron, N7OE

Annie

Sun Mar 04 2007, 09:00PM

Registered Member #547 Joined: Sat Feb 24 2007, 01:27PM
Location:
Posts: 9

Hi Aaron; Thanks for your message! I do not use a filter, I think, since the spark gap operates in parallel with the NST and primary LC resonance circuit, that any voltage that is too high, will be shorted by the gap automatically. I think the NST is working correctly, although I did not test the secondary windings separately. I am a little afraid to connect the NST housing (middle of secondary) to the RF ground, because current from the NST to RF ground will flow when sparks go from toroid to RF ground. They will be carried by the streamers from toroid to earth (this is dangerous, because then the current will be very high, and when one is accidentally hit by a toroid streamer, it could be lethal). When the caps arrive, I will test both sides of the NST separately and tell you if they are ok. Tnx for your reply, 73's!

J. Aaron Holmes

Sun Mar 04 2007, 09:40PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

It's true that having the gap across the transformer helps a lot with RF, however you still have the spike caused by the spark gap to contend with. The snubbing action of the cap in the Terry filter will virtually eliminate this. For this reason, Terry filters are generally regarded as indispensable for NST-based coils.

I'm not sure I completely follow the comment about getting hit by streamers. By itself, I don't see how grounding the NST secondary midpoint makes the streamers more of a hazard. To have mains-frequency current flowing in the streamers, I would think you'd really have to have a primary-to-secondary flashover happen at the same time. And if that happened with the secondary midpoint floating, then the chances of the NST insulation standing off 100's of kV between secondary and mains ground (which is, to some degree, connected to your RF ground) to save you ought not to be relied upon!

EDIT: Not sure where you're at, but in the U.S., "neutral" runs straight to the mains ground at the breaker box, so even if you don't connect the green wire to your NST case, mains ground will still effectively be very near your NST's core by way of the neutral lead. Not sure how this works in other places, but I'd still wager you're not going to be saved by the differences.

Regards,
Aaron, N7OE

Annie

Sun Mar 04 2007, 10:47PM

Registered Member #547 Joined: Sat Feb 24 2007, 01:27PM
Location:
Posts: 9

Hi; If the middle of the NST is connected to RF ground, together with the lower end of the secondary coil, there wil be a potential of 4kv 50 mA (8kV center earthed NST) between primary and secondary circuits. This voltage can run through the body, carried by a spark coming from the toroid. That is what I read on other sites, I don't know is this is really going to happen, but that is why I decided to keep the NST free from any earth connection. I doubt if the floating center of the NST decreases performance. Maybe someone has more clues on this ?? P.S. I live in the Netherlands. The mains here consists of Neutral and Live. In addition, an Earth electrode, grounded in the soil under our house, is connected to all earth pins in the wall outlets. Neutral to Earth should be 0V, Live to Earth is 230V, 50Hz AC. Equipment without necessary Earth connection is, of course, connected between Live and Neutral, LOL!

J. Aaron Holmes

Mon Mar 05 2007, 03:37AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Maybe I need somebody else to help me understand the argument here

Just because you have your secondary midpoint grounded to RF ground, the streamers are no more harmful than with the secondary floating. Whether you ground or not, you still need a strike from secondary to primary or something like that. Otherwise there is no circuit for your "4.5kV." Please explain how this 4.5kV gets into the streamers in your scenario.

Without your midpoint grounded, high voltages and RF kicking back into your transformer (esp. in the case of a primary strike) are likely to go bad places. Now, if you really want to avoid primary strikes, then proper spacing of primary and secondary, use of a strike rail, and (in some cases) grounding of the inner turn of your primary to RF ground are often indicated. This latter approach is common on coils powered by large pole transformers but is generally not done on NSTs because the midpoint ground prevents it (you'd be shorting half the transformer).

...but even if you do all these things, streamers can go unexpected places, and so they should always be regarded as dangerous.

Take care,
Aaron, N7OE

Hazmatt_(The Underdog)

Mon Mar 05 2007, 07:58AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

One of the major reasons I use a supply grounded source circuit neutral and separate earth ground strike rail & secondary ground is that I don't want the high RF energies feeding back into the house. Now they still do because of reflected load through the transformer, but in my case that usually just drives the light dimmers crazy. If I were however feeding back my ~750W worth of streamer energy to the neutral, it could be pushing 10-20A worth of RF current on the neutral and high enough voltage transients to arc to the low voltage wiring, causing a huge fire in the conduit.

Separate grounds in this case I think are safe grounds.

I should also mention just as a silly tidbit. One of the new guys working at the electronics store I used to work at and still frequent, has a friend who was testing a coil in his apartment. Turns out he didn't know what he was really doing when he had all of his grounds tied together. Long story short, he blew out his apartment's wiring and 3 other unit's wiring, destroying his electronics and other tenant's electronics. He's being sued for damages now and has to find a new place to live.

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