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JimmyH

Thu May 11 2006, 06:28PM

Registered Member #358 Joined: Sat Apr 01 2006, 06:13AM
Location: UCSB
Posts: 28

Hmm.. I guess I was thrown off by there being 2 100W resistors on the schematic, instead of much smaller ones, and reading stories about the resistors getting really hot (IIRC, that was Gary Lau, who had saturated his shunts, and was pulling too much current anyway, so he could have gone less R more C).

I guess it's just a question of whether you'd rather wind a couple inductors, or buy a couple big power resistors.

If you don't mind winding more turns, you can get away with a lot higher noise rejection before power loss becomes a problem, so you can get some extra safety, and possibly skip out on the MOVs instead. =\

Terry Fritz

Thu May 11 2006, 07:21PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

I studied RLC filters back in 98:

Also RC filters thet the present "terry filter" is based on:

Today, with computer modeling, it might certainly be possible to "tune" the mess to get better performance out of the RLC filter. But the present RC design seems to work an just about any system as is. Back then, there were no nice models and the knowledge was limited. Today, there may be many more options as Jimmy mentions!!

One does need to take into account the enormous inductance of the NST secondary and its resistance too.

I think 90% of the good it does is in the spark gap and MOVs absolutely prevent overvoltage though.

It's claim to fame is basically that "it works really well" even though it is costly, burns power, and might not be the brightest design... There is plenty of room for inovation though espicially if it brings the cost down.

Cheers,

Terry

HV Enthusiast

Thu May 11 2006, 09:01PM

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

If you are truly concerned about efficiency, just eliminate the filter altogether. Remember, engineering is all about trade-offs, and it may be worthwhile to increase efficiency at the expense of reliability, should efficiency truly be an issue. Personally, i have never used filtering on my NST coils and never had any NST failiure or other problems.

Also, one needs to look at the entire system. The amount of losses introduced into the system by a filter may be swamped out by the losses in the primary circuit (i.e. spark gap) so any improvement here really won't matter much in the end.

And as Terry nicely said, the claim to fame is basically that it "works really well" at the expense of cost and power losses. The goal of the filter is to gain protection and reliability. If efficiency is a concern, just eliminate it completely, of course at the expense of reliability.

Hazmatt_(The Underdog)

Wed Aug 23 2006, 01:54AM

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

I just wanted to post a picture of my NST line filter.

The capacitors were chosen as 10pF because throughout the simulation, the larger value capacitors such as 200pF and larger did not arrest large enough transients. The 10pF capacitors return the line surges back to normal operating nameplate current.

The ground pad connects on both sides of the fiberglass board for a firm connection. I could possibly add a spark gap on top of the capacitors, but the simulation results are showing normal operation of the transformer.

The board is coated with a silicone spray to protect the copper from 'aging' and will hopefully last many years into the future.

HV Enthusiast

Wed Aug 23 2006, 02:34AM

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

Actually, my filter is better. This is the filter I use and i have never had an NST fail during Tesla coil use. I've used NSTs ranging from small 4kV/20mA units, up to a paralleled 15kV/60mA systems.

Dan's Filter

williamn

Wed Aug 23 2006, 02:43AM

Registered Member #55 Joined: Thu Feb 09 2006, 04:56AM
Location:
Posts: 149

Even though EVR's NST filter (using a radical mid point ground protection system) may be a bit complicated for the average coiler, I too have used it with great success. I have never had a NST fail while using it.

Dr. Drone

Wed Aug 23 2006, 03:21AM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
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Posts: 1673

Hazmatt_(The Underdog)

Wed Aug 23 2006, 08:49AM

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

That's great that some of you guys have never lost a NST, and I say more power to you.

Some of us have lost NST's due to dielectric breakdown. I have lost 2 12/60's at different times. The latest being the victim of performance testing. Basically I wanted to find the LTR multiplier myself and verify the findings first hand, and doing this more scientifically with simulation and data.

SO, if you're going to push every watt out of an NST, I would suggest using a filter. There is no appreciable loss in power, and mine will eliminate the large reflected current pulses.

Terry Fritz

Thu Aug 24 2006, 07:41PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

That's great that some of you guys have never lost a NST, and I say more power to you.

I should note than most of these people are probably using the larger "LTR" cap values that naturally resist over voltage. The LTR caps have really made a big difference in saveing NSTs too!! Back 7 years ago, they were all running resonant and some people made piles of dead NSTs...

With LTR caps, filters, fuses... If the NST dies then, it was a "natural" death... The LTR caps give bigger sparks so no reason not to use them. The fuse is cheap and easy...

One could ask if the filter is needed for LTR coils? Sync LTR coils actually drop in voltage if the gap does not fire... "Assuming" everything is wired right, then the filter only blocks RF from the outer windings. I have tested that and most of the HF voltage drop is on the outer windings for sure... With people running coils so hard these days, that might be a real concern...

But it sort of depends one your budget and situation. If you just bought a brand new 15/60 NST for $250, the choice is easy. But if your whipping together a little coil and can get another NST easily enough, then the choice might be easy too...

Cheers,

Terry

Hazmatt_(The Underdog)

Thu Aug 24 2006, 09:02PM

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

I'm having to take a lot of precautions because I am trying to get the exact right LTR cap value for my difficult to replace 9/120.

In my case it is no throw-together coil. I want it designed to be a show coil that lasts for 50 years, including possible abuse. So this means simulation, calculations, and real work.

I'm trying to pull maximum currnet and voltage possible from this NST without exceeding either nameplate value.

I am getting there, but it is a long process of validating exactly what value for LTR. And in this case I am not going to just look at other people's numbers, I need to verify my numbers for power, efficiency, and make sure all the hard questions are answered with the closest figures.

Matt

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