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power supply...NST vs. FBT

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HM_Murdock

Sun Aug 29 2010, 04:25PM

Registered Member #3075 Joined: Fri Aug 06 2010, 02:44PM
Location: Athens, GA
Posts: 148

When considering a small, portable SGTC, what are the advantages and disadvantages of using the following power supplies:

1- Single transistor FBT circuit, with a modified PC CPU @ 12V

2- 9kV 30mA NST

I am leaning towards the NST, as it seems to be a simpler way to go (and I want to keep it simple for the first one). Less points of failure and all...

modularduck

Sun Aug 29 2010, 04:33PM

Registered Member #3094 Joined: Tue Aug 10 2010, 03:12AM
Location: portland, or
Posts: 30

if weight is a concern i would say the FBT would weigh less, but would indeed be more complex. but that said i would still go with the NST because of its simplicity.

HM_Murdock

Sun Aug 29 2010, 04:54PM

Registered Member #3075 Joined: Fri Aug 06 2010, 02:44PM
Location: Athens, GA
Posts: 148

Thanks modularduck...

Weight is not really an issue (to a degree, obviously). I would gladly trade the weight of the NST for the complexity (relatively speaking) of the FBT driver

I am also thinking the output from an NST would be more predictable, and make it easier to design a more efficient primary tank circuit?

Adam Munich

Sun Aug 29 2010, 05:13PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Yes. And don't follow deepfried neon's calculator. It tells you to design a resonant cap bank, and those often fry NST's. You want to build a cap that is larger than resonant for you're primary.

HM_Murdock

Mon Aug 30 2010, 02:06AM

Registered Member #3075 Joined: Fri Aug 06 2010, 02:44PM
Location: Athens, GA
Posts: 148

Most of what I have read says the primary cap bank should be designed in resonance with the power source so it will charge and discharge at the same cycle as the NST?

But, this is not the first I have read where it is recommended going larger than resonance on the cap bank...what is the current conventional wisdom on this? Good reading on this subject somewhere?

My studies so far point me to .0088 uF for the 9kV 30mA NST (including deep fried neon's page). As I understand it from a design standpoint, the main thing the primary capacitance does to the primary coil is to dictate where in the coil you need to tap into it to achieve resonance with the frequency of the secondary (which is going to be 810.66 kHz in my design so far)?

I think I am starting to get there with understanding the design concepts...in particular getting the secondary and primary in resonance and coupled just right. The areas of the primary cap bank and the protection mechanisms are still fuzzy to me when it comes to understanding the how's, what's, and why's.

Arcstarter

Mon Aug 30 2010, 02:10AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Grenadier wrote ...

Yes. And don't follow deepfried neon's calculator. It tells you to design a resonant cap bank, and those often fry NST's. You want to build a cap that is larger than resonant for you're primary.

Well, you want the cap to resonate with the primary. You just do not want the capacitor to resonate with the NST's secondary, because of resonant rise causing over voltage, and over current. The deepfriedneon calculator will show you the resonant cap, which you want about 1.7 times that size for LTR.

Adam Munich

Mon Aug 30 2010, 02:13AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Arcstarter wrote ...

You just do not want the capacitor to resonate with the NST's secondary, because of resonant rise causing over voltage, and over current.

That's what I meant.

HM_Murdock

Mon Aug 30 2010, 12:15PM

Registered Member #3075 Joined: Fri Aug 06 2010, 02:44PM
Location: Athens, GA
Posts: 148

Ok, so I need to adjust the .0088 up by 1.7, then re-adjust from there.

I know I need to factor in for other components (such as RF protection for the NST, seperate earth ground, strike ring, etc).

Does the 9kV NST take me into the realm where it is not a good idea to use the mains ground (I assume so)? If so, what are my options for seperate ground?

Also, what protection measures should be taken to protect the NST?

Adam Munich

Mon Aug 30 2010, 01:21PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

I'd go with the 15kv, more power and an easier to make spark gap.

As for grounding, a wire to a water pipe is fine.

Duality

Mon Aug 30 2010, 02:00PM

Registered Member #1951 Joined: Sun Feb 01 2009, 01:59PM
Location:
Posts: 105

Grenadier wrote ...

I'd go with the 15kv, more power and an easier to make spark gap.

As for grounding, a wire to a water pipe is fine.

that's when the water system is properly grounded of course :)

i fried stuff just because i thought my water pipe lines were properly grounded. they ain't :(

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