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Brian

Tue May 09 2006, 10:09PM

Registered Member #117 Joined: Fri Feb 10 2006, 05:22AM
Location: WI
Posts: 9

I want a NST filter since I have only one NST. Greg's filter and Terry's filter seem to popular choices. I would like to know the key differences, effeciencies, and which one is better. Thanks.

Greg's Filter:

Terry's Filter:

Terry Fritz

Tue May 09 2006, 11:11PM

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

Oh!! "mine" is better

The component values are not critical at all and are commonly +-100%

The only significant difference is that Greg uses spark gaps for NST over voltage protection and I use MOVs "and" a safety spark gap up front.

Spark gaps are cheap but you "must" set them right or they do no good. MOVs are expensive, but you can't set them wrong

) I use the spark gaps to take normal over voltages and the MOVs are the last ditch protetion incase the safety gaps are set wrong or fail. If the MOVs burn up, they fail shorted. So in mine, "nothing can go wrong" enough that the NST is damaged. But MOVs are pricey so Greg's is the next best choice on a budget.

Be sure to put a fuse on the NST too since there is a core saturation issue that "might" blow an NST now and then.

With my filter and a fuse, the reports I have gotten of NST fialures are "zero"

A few folks complain that the resistors get hot and that is why they are so over rated. Power resistors can reach 300C in "normal" operation which was freaking people out, so I way oversized them. There have been a few people that have fried the MOVs. But the MOVs died saving the NST from the user there

The reason I went with MOVs is because there is a certain percentage (a high one) of folks that "just can't" get the safety gaps set right. The MOVs solved that, but at a price...

If Digikey is sold out of the MOVs, use a few more of the next voltage size down... I hate making things like this and every body orders the stuff and DK goes sold out for three months...

Guess why I have not mentioned the PN of my new Super IGBTs for the Sidac/IGBT gap

Pick your poison, but mine is best

Cheers,

Terry

...

Tue May 09 2006, 11:54PM

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

I should point out that littlefuse will sample mov's from the little smd ones to huge bot down monsters. Use them wisely, but they will usually send you as many as you need... (not going to say anything more)

I haven't done any damage to my nst (7.5kv/30ma) with no filter (not even a saftey gap), but I will admit that I don't really abuse it with my micro coil...

Have fun!

Brian

Wed May 10 2006, 01:06AM

Registered Member #117 Joined: Fri Feb 10 2006, 05:22AM
Location: WI
Posts: 9

Terry Fritz wrote ...

Be sure to put a fuse on the NST too since there is a core saturation issue that "might" blow an NST now and then.

Where in the circuit and what sort of fuse would be needed?

BTW - Thank you Terry for you ever unbiased opinion. I like the idea of idiot proof though.

Terry Fritz

Wed May 10 2006, 05:36AM

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

Hi,

The fuse should be a fast blow type. One of those 1-1/4 x 1/4 inch cartridge fuses is fine (Radio Shack). The value is hard to guess without knowing anything about you NST, gap type, etc. but it should probably be about twice the VA rating of your NST divided by the input voltage. So if your NST is a 12/60:

2 x 12000V x 0.060A / 120V = 12 amps

If the fuse blows too much, go higher.

The fuse is directly on the NST 120V low side. On the Hot wire is best but not a big deal.

The reason for the fuse is that if an NST is driving a capacitor, the output current can go too high and saturate the core and shunts. Then it looses its current limiting and the currents can go far too high. They make a very loud bad sounding buzz when that happens. The fuse simply blows in that case preventing damage.

Cheers,

Terry

JimmyH

Wed May 10 2006, 07:01PM

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

I don't get this whole RC filter thing. It's unnecessary loss.

People used to use LC filters for a while, but realised that the ringing they created was just as bad. Terry tested that out once, if I remember correctly.

So once people realised that, they seemed to have gone RC, since it's safer, even though it wastes a lot of power.

Why not both ?

Instead of a huge power resistor, why not use a smaller resistor with an inductor in parallel with it? If it's set to be critically damped, then there is no voltage spiking to worry about.

At 60hz, the inductance provides much less reactance, so current passes much easier, plus whatever VA flowing in the inductor is purely reactive, so it really cuts down on loss.

Say for example we want to replace our RC filter with a critically damped RLC filter at 360khz (which is 1/RC for Terry's filter). Since the mains frequency is 60hz, and the resonant frequency of the LC part of the filter is 360,000hz, the frequency, and therefore reactance (Xl = 2*pi*F*L) varys by 6000x.

say we wan't to have the same rejection that our RC filter had. That means Rnew has to be 2*Rold, and Xlres = Rnew. Xlmains (hope you understand my notation) is .0003 Rold. The voltage drop when charging the capacitor is now .2% of what it was, and reactive (if the inductor is lossless).

Power = V^2/R = (0.0003^2)Vold/(2*Rold), or ~0*old power dissipation. This doesn't count any high frequency noise which has to be eaten up, or any inductor loss, but the point remains that it is a big power saver.

The inductor would have to be around 870uH and the resistor 2k to be a drop in replacement for Terry's filter. The inductor could just be a 2x3" coil with a couple hundred turns of 28awg for example. The DC resistance would be a mere 6.8 ohms compared to the 1000 ohms that it had to push through before when charging the capacitor.

So what do you think?

ArcLight

Wed May 10 2006, 07:41PM

Registered Member #341 Joined: Thu Mar 23 2006, 07:41PM
Location: Northern Illinois, USA
Posts: 69

It seems that I remember reading someplace that MOVs clamp faster than a sparkgap can strike an arc. But the sparkgap can sink much more power so a combination of the two makes good sense.

Hazmatt_(The Underdog)

Wed May 10 2006, 10:07PM

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

Consider this:

My Tesla system running with a 12KVAC 60mA transformer and 3000 ohm load resistors have a 300VAC drop across them at maximum power pull. This has been measured with my potential divider.

So for each 3000 ohm load, we have 300*.06 = 18 W for peaks. And since they get warm during a 15 minute run, they're probably dissipating 25W each.

Say I'm loosing 100W in the resistor. 760W - 100W / 760W * 100 = 87% power transfer, meh, I'm happy with that. And if I'm really loosing 50W, then I have 93% which I can live with too.

The losses I care about are not having good enough coupling, long leads, bad capacitors, bad leads, bad sec. gnd. connection. etc. etc.

Steve Conner

Wed May 10 2006, 11:53PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

JimmyH: The DC resistance of a NST secondary winding seems to be about 5-10k ohms. There's an awful lot of real thin wire in there. So personally I wouldn't bother adding inductors to decrease the DCR of the filter.

For a pole pig or high frequency solid state power supply, I guess your idea would be good. Mainly because it saves having to buy such big resistors and manage the heat from them. (I guess that argument depends on how much of the heat is RF and how much is DC.)

HV Enthusiast

Thu May 11 2006, 01:23AM

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

JimmyH wrote ...

I don't get this whole RC filter thing. It's unnecessary loss.

People used to use LC filters for a while, but realised that the ringing they created was just as bad. Terry tested that out once, if I remember correctly.

So once people realised that, they seemed to have gone RC, since it's safer, even though it wastes a lot of power.

Why not both ?

Instead of a huge power resistor, why not use a smaller resistor with an inductor in parallel with it? If it's set to be critically damped, then there is no voltage spiking to worry about.

. . . .

The reason is because the resistance of the secondary winding of an NST is considerably greater than the RC filter most would use for an NST filter. Any savings would be quite negligible. Similar to trying to increase the Q of the secondary when its the primary Q that is the limiting factor of Q in the system.

EDIT: Oops. I see Steve beat me to it . . .

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