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  #1  
Old 06-14-2017, 01:15 PM
Kregus Kregus is offline
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Question Hairpin Circuit: Strange Spark Gap Behavior

Hi,

After reading "The Inventions, Researches, and Writings of Nikola Tesla" and watching videos from Karl Palsness, I decided to build my own Tesla Hairpin/Lecher Line circuit.

>> Schematic & parts list <<
Schematics.com | Tesla Hairpin Circuit

The spark gap of this circuit works well in the following conditions:

1. When no capacitors are connected
2. When one capacitor is connected
3. When the copper short bar at the top is removed

However, in it's intended state, so with two capacitors and the copper short bar connected, the spark gap does not fire at all, no matter how large or how small I make the gap. See the video below for a demonstration.



When I use two capacitors and connect the lamp to the copper bars, the lamp flickers once when I flip the ON switch, but then does nothing.

My question is: what could be the reason for my spark gap not firing when two capacitors and the short bar are connected?

Thanks in advance for the help!
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File Type: jpg tesla-hairpin.jpg (47.7 KB, 17 views)
File Type: jpg tesla-hairpin-closeup.jpg (86.6 KB, 16 views)
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  #2  
Old 06-14-2017, 04:43 PM
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dR-Green dR-Green is offline
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Quote:
Originally Posted by Kregus View Post
My question is: what could be the reason for my spark gap not firing when two capacitors and the short bar are connected?
In the first test in the video, are the bars connected, or is it ONLY the spark gap?

The power supply frequency may be too high, and/or the capacitors may be too big.

If the power supply is AC, there's a short circuit across it through the capacitors, so if the capacitors are not small enough to filter the frequency then it won't work as you've got a short circuit = no spark. In that case you need capacitors that are small enough to filter/block the power supply frequency so it sees an open circuit, then it should produce a spark.

[edit] From the picture it looks like the power supply output is 14kHz, so you essentially need capacitors that are small enough to act as a high pass filter with a cutoff of at least 14kHz to allow the open circuit condition.

What happens if the capacitors are connected in series in test 2?

Also, you shouldn't be doing experiments on the floor when cats are walking around
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Last edited by dR-Green; 06-14-2017 at 05:13 PM.
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  #3  
Old 06-14-2017, 07:27 PM
Kregus Kregus is offline
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Hi dR-Green, thanks so much for your quick and detailed reply!

Quote:
Originally Posted by dR-Green View Post
In the first test in the video, are the bars connected, or is it ONLY the spark gap?
Literally only the spark gap, I disconnected the leads to the caps in this test, and the caps lead to the bars, so they are automatically also disconnected.

Quote:
Originally Posted by dR-Green View Post
The power supply frequency may be too high, and/or the capacitors may be too big.

If the power supply is AC, there's a short circuit across it through the capacitors, so if the capacitors are not small enough to filter the frequency then it won't work as you've got a short circuit = no spark. In that case you need capacitors that are small enough to filter/block the power supply frequency so it sees an open circuit, then it should produce a spark.

[edit] From the picture it looks like the power supply output is 14kHz, so you essentially need capacitors that are small enough to act as a high pass filter with a cutoff of at least 14kHz to allow the open circuit condition.
I attached an image of the Seletti NST I am using to this reply. It is rated at 220V AC, 50Hz, 0.42A input and 10kV, 30mA, 34kHz output. The capacitors are 40kV, 2000pF ceramic doorknob capacitors. Is there a way to calculate the required capacitor size based on power supply output frequency?

The behavior of the circuit does seem to indicate a short circuit through the capacitors, but the 10kV input in combination with these exact 40kV/2000pF caps is what Karl Palsness used in his circuit. Except he used a furnace ignition transformer instead of an NST.

Quote:
Originally Posted by dR-Green View Post
What happens if the capacitors are connected in series in test 2?
I'll give this a try.

Quote:
Originally Posted by dR-Green View Post
Also, you shouldn't be doing experiments on the floor when cats are walking around
Haha I am painfully aware of that It was 4am at night and I had just finished building the circuit, so I was super eager to fire it up. I'll be more careful next time, promise


While searching for a solution, I learned that many Tesla Coil builders have problems with newer NST's (mine is brand new), because they contain a Ground Fault Interrupter (GFI) which shuts off a circuit when it senses something odd is going on.

"An NST with a GFI circuit CAN NOT be used to power a tesla coil."

Could this be causing the spark not to fire?
Attached Images
File Type: jpg IMG_20170609_211638.jpg (208.9 KB, 10 views)
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Last edited by Kregus; 06-14-2017 at 07:36 PM.
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  #4  
Old 06-14-2017, 08:41 PM
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dR-Green dR-Green is offline
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Quote:
Originally Posted by Kregus View Post
I attached an image of the Seletti NST I am using to this reply. It is rated at 220V AC, 50Hz, 0.42A input and 10kV, 30mA, 34kHz output. The capacitors are 40kV, 2000pF ceramic doorknob capacitors. Is there a way to calculate the required capacitor size based on power supply output frequency?

The behavior of the circuit does seem to indicate a short circuit through the capacitors, but the 10kV input in combination with these exact 40kV/2000pF caps is what Karl Palsness used in his circuit. Except he used a furnace ignition transformer instead of an NST.
C = 1/(2pi*F*Z)

Where
C = capacitance in Farads
F = frequency in cycles per second
And Z in Ohms is probably the impedance of the bars, but since you don't know that either I would guess that 100-500pF should do it.

The furnace transformer probably outputs 60Hz so the 2000pF easily isolates it.

Quote:
Originally Posted by Kregus View Post
While searching for a solution, I learned that many Tesla Coil builders have problems with newer NST's (mine is brand new), because they contain a Ground Fault Interrupter (GFI) which shuts off a circuit when it senses something odd is going on.
That might be a secondary effect, but if there's a ground fault which would trigger it to cut out then a short circuit due to the AC passing straight through the capacitors is probably it.
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Last edited by dR-Green; 06-14-2017 at 08:45 PM.
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  #5  
Old 06-14-2017, 10:12 PM
j dove j dove is offline
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Hairpin circuit

Quote:
Originally Posted by Kregus View Post
Hi dR-Green, thanks so much for your quick and detailed reply!



Literally only the spark gap, I disconnected the leads to the caps in this test, and the caps lead to the bars, so they are automatically also disconnected.



I attached an image of the Seletti NST I am using to this reply. It is rated at 220V AC, 50Hz, 0.42A input and 10kV, 30mA, 34kHz output. The capacitors are 40kV, 2000pF ceramic doorknob capacitors. Is there a way to calculate the required capacitor size based on power supply output frequency?

The behavior of the circuit does seem to indicate a short circuit through the capacitors, but the 10kV input in combination with these exact 40kV/2000pF caps is what Karl Palsness used in his circuit. Except he used a furnace ignition transformer instead of an NST.



I'll give this a try.



Haha I am painfully aware of that It was 4am at night and I had just finished building the circuit, so I was super eager to fire it up. I'll be more careful next time, promise


While searching for a solution, I learned that many Tesla Coil builders have problems with newer NST's (mine is brand new), because they contain a Ground Fault Interrupter (GFI) which shuts off a circuit when it senses something odd is going on.

"An NST with a GFI circuit CAN NOT be used to power a tesla coil."

Could this be causing the spark not to fire?
Hi ,

Was just reading your thread and thought that's would give my thoughts on your problem.
Though DR green has given you some very good advice here.
You can't use a newer NST as yes they do have a GFI and will not work for Tesla coil or the like. I made the same mistake as you, so is only a learning experience.
I then switched to a 10,000 volt furnace transformer and it worked quite well. I have several sets of home made high voltage capacitors that work well for it. There values are 1200 pf , 491 pf and 300 pf . All are made of aluminum and polycarbonate .

You can learn much from this design and even repeat some of Eric Dollards experiments with it. I have been able to replicate his metallic attraction with a light bulb. All different metals were used, Cu , Al, Pb, Fe, Ar, Au, Zn , all were attracted to the bulb and remained so even for a time after the power was off. Was not static electricity as I made sure to ground support system. Very strange effect, but you don't need a tesla coil to do it. Also some work with gas discharge tubes as per tesla writing that you quote in the beginning.

I would recommend to build your own capacitors as is a great learning experience and is not as easy as it sounds. I made nine of them before I got one that didn't short out, catch fire, melt or just plain fall apart. But now I know how to make very good high voltage capacitors.
Mine are several years old now are are still as good as new. And there are other experiments on can use them for in the future.

Best of luck with your experiments,


Jeff
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Last edited by j dove; 06-14-2017 at 10:17 PM.
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  #6  
Old 06-15-2017, 08:32 AM
Kregus Kregus is offline
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Hi Jeff and dR-Green, thank you both so much for all your valuable info! I will experiment with lower pF caps and will try to get a different type of transformer, without GFI.

I was really hoping to be able to use this off-the-shelf NST from Seletti instead of having to browse eBay for an old transformer, because this would make it a lot easier for people to replicate the device.

EDIT
I was just reading more about Capacitive Reactance, or the opposition to current flow in AC circuits, and found:

"At DC a capacitor has infinite reactance (open-circuit), at very high frequencies a capacitor has zero reactance (short-circuit)"

Since the NST I use outputs 34kHz, the short-circuit hypothesis seems extremely likely. The article also said:

"If either the Frequency or Capacitance where to be increased the overall capacitive reactance would decrease"

So this indeed means, as dR-Green mentioned, that the higher the output frequency of your power source, the smaller the Farad value of your capacitors has to be to block the AC current. However, since DC is completely blocked by capacitors, regardless of their Farad value, it might make sense to use a HV DC power source instead?
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Last edited by Kregus; 06-15-2017 at 10:18 AM.
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  #7  
Old 06-15-2017, 01:42 PM
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blackchisel97 blackchisel97 is offline
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Hi Kregus,

I have done several experiments using similar hv transformers. GFI isn't a problem but it's output frequency makes it impossible for the capacitors to charge and they will appear as a short. OBITs output 50 or 60Hz, depending on the country and capacitors can "catch up". You can overcome this problem by putting hv diode in series (between the transformer and capacitor). Diode direction doesn't matter. Also, adding small hv capacitor across the spark gap (boost cap) may improve firing. It's value can be from couple hundred pF up to 1nF. I usually use 470pF rated accordingly to the power supply being used.
OBITs are generally designed to work as igniters in oil furnaces and I would exercise caution when running it at full voltage for extended periods of time. I used OBIT in my build but added variac to regulate the output.

V
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