Hi,
Go the lazy route and just use some online calcutors- but beware, at least
half of these are wrong, so go for the best two of three answers from
different sites.
Here's an simple looking one:
https://66pacific.com/calculators/coil-inductance-calculator.aspx
At the moment I can't locate the last chokes I made, but assuming they
were 2" PVC with 6" of windings and 28gauge that gives diameter of ~
2.375" and assuming 0.015" wire width and 400 turns we get 3.19
millihenrys per choke. Is there anything special about this configuration?
No, but it's cheap and was easy to make. I show a whopping impedance of
4.7kohms at 250kHz for such a coil. A wider, shorter coil and form will
give a larger inductance than a long slender coil. If I make a new set,
they'd be on 4" PVC as I have more of that laying around and it's still an
easy to work with size. No amount of inductance by itself will protect a
NST from burnout. You still need to protection gaps and to NOT run at
nameplate voltages. Another post mentioned resonant effects where voltages
can double, by accident in your primary. Nonense like this was another
reason I only ran off rectified DC. I even got strange resonant effect at
60Hz with certian variacs and transfomers. The variac would suddenly
sound like it was going into saturation, which really made no sense, but
that's how it worked.
There's a (slim) chance you're even running off a harmonic of the real
secondary coil resonant frequency. Tesla coils will run at the wrong
frequencies. The Q isn't stellar with the air gap and all, so this does
work.
The shunt resistors I used were just a junk box 0.1ohm metal oxide
resistors, rated maybe 2 watts. I connected that to coax and just attached
oscilloscope probe to that. You really don't need anything special. Oddly,
a tesla coil is more destructive to itself than surrounding electronics.
You might double up resistors for redundancy and add a neon light across
them as token protection at both ends of the coax or whatever you use.
They'll fire at ~70volts or so, and this won't destroy a decent scope. You
just want the scope on the same ground as the tesla coil itself. Just
remember the alligator lead on a scope is connect to the chassis and
ground at the plug. All channel grounds are connect to each other. If you
connect something like a large 12volt lead acid battery + to one clip on
channel 1 and the - to the ground clip on channel 2, you will smoke the
leads as they are a dead short. This is the "limitation" that calls for
dangerous stuff like cutting the ground lead off the power cord, or using
fancy probes.
If you can't get a frequency generator, one option is to charge your caps
with DC and fire them once (that is once at a time under your control)
with the spark gap and catch the waveform with the scope. You can prod at
your spark gap with a sissy stick with wire on it to trigger it. The
characteristics of a spark gap are always somewhat mysterious as they do
not have a fixed resistance or impedance so real measurements always beat
calculations.
Sadly the days of magnetic neon sign transformers are basically over, and
that GFCI stuff seems like complete nonsense.
On Sun, 21 Nov 2021, Joshua Thomas wrote:
> Hello unnamed person :)
>
> Thank you for your feedback. I'm unfamiliar with winding my own chokes.
> That is, I'm unclear on how to design my own chokes for a particular
> specification, and I lack an LCR meter to determine I have enough
> inductance. If I use my existing 500ohm/100W resistors to form a RL lowpass
> filter, I will need two chokes of approximately 0.25mH each.
>
>> The real key to not burning out your NST is proper tuning.
>
> Yes, I'm not quite sure what other tuning I need. Changing the tap position
> has not made an appreciable difference. I have tried a smaller topload; a
> 1" torroid vs a 4" torroid, and did get increased spark length, but not
> significantly. (Obviously I had to change the tap point).
>
>> I always meter off the ground connection of secondary with a shunt
> resistor.
>
> What type of resistor do you use? I would really like to not destroy an
> oscilloscope! Do you require differential probes for this?
>
> Thanks again.
>
> Joshua Thomas
>
>
> On Sun, Nov 21, 2021 at 7:59 AM <pupman.com@xxxxxxxxxxxxxxxxxxx> wrote:
>
>> Hi,
>>
>> I just use large air cored chokes and a spark gap. It's simple, and
>> impossible to burn out. There's no need for some rube goldberg setup. HV
>> capacitors are expensive and like fuses can only be abused once. The last
>> set I made was probably on 2" PVC with 6" of #28 windings or something
>> similar to that. Protection spark gaps should be pointy and just on the
>> verge of firing. If they arc at full line voltage and no load, even
>> better. You don't get full nameplate voltage on a NST at full load
>> anyways, so don't expect that with a coil running. It's a not that
>> different from how you should never run a microwave oven empty. They must
>> have a load to prevent arcing.
>>
>> The solid state ignition transformer sounds fun. I ran my coils off DC,
>> rectified from NSTs or even unshunted transformers.
>>
>> The real key to not burning out your NST is proper tuning. I could tell
>> from the video you posted a week or so ago that the coil was still not
>> tuned correctly, or other adjustments are still needed. The sound of the
>> spark gap and how the arcs looked was the key. You should be able to get
>> streamers that grow in length and do not not look like DC arcs from a
>> power pack or electrostatic generator. Even for a coil the size of yours
>> the tuning will change due to the proximity of objects nearby, even your
>> arm or a ground lead. For maximum fun, tune the coil to you and the ground
>> wand you draw arcs off.
>>
>> If you can look at the waveform for the coil with an oscilloscope, that
>> helps too. I always meter off the ground connection of secondary with a
>> shunt resistor. It's the safest place to obtain a low voltage with respect
>> to ground for taking measurements or attaching instrumentation.
>>
>> Congrats of the first light!
>>
>>
>> On Sat, 20 Nov 2021, Joshua Thomas wrote:
>>
>>> Hello all,
>>>
>>> I've been struggling to build a lowpass ("Terry") filter to protect my
>> NST
>>> from RF feedback. The first one suffered arc-overs because
>>> the capacitor leads were too close together, and the second one had the
>>> capacitors fail - apparently only 9kV per rail wasn't enough. It's a 12kV
>>> NST so I assumed 18kV > 12kVx(sqrt 2) and therefore would be sufficient.
>>> Apparently not.
>>>
>>> What suggestions are available for those who have made a NST filter that
>>> was successful? Particular capacitor types/brands would be welcome, as
>> well
>>> as how the physical layout was accomplished. I use two 500ohm/100W
>>> wire-wound resistors for the R half of the RC filter.
>>>
>>> On a different topic I got a used OBIT for $35, but it appears to be a
>>> solid-state one running pulsed DC. The model is Allanson 2275-628G.
>> 17.5kV
>>> "peak", 45mA, at 20khz. I mainly bought this for curiosity to see how it
>>> might perform on a coil. I'm a little suspicious of the voltage and
>>> amperage, as the total VA rating is only 87.5VA - which is a fraction of
>>> the 560VA that would be expected from 12.5kV RMS (17.5kV peak) at a full
>>> 45mA!
>>>
>>> What experiences has anyone had with one of these?
>>>
>>> Cheers,
>>> Joshua Thomas
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> --
>>> Joshua Thomas
>>>
>>> My new email address is: joshuafthomas@xxxxxxxxx
>>> Please update your information if you have not already done so.
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>>>
>> _______________________________________________
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>>
>
>
> --
> Joshua Thomas
>
> My new email address is: joshuafthomas@xxxxxxxxx
> Please update your information if you have not already done so.
>