[TCML] First (or maybe 2nd) light on my medium-sized SG coil

Wed Apr 16 22:19:27 MDT 2008

Hi Bart,

Yes, you've pretty much confirmed what I pretty much already knew :^)\
80 nFd is a big honking capacitor for a 120 volt fired coil, bar none.
I do have the option to go to 40 nFd though, since my primary cap is
just (2) paralleled .04 ufd, 40 kV units. However, I am already tapped
at about 8 3/4 turns on my primary coil with a total of just 11 available
turns so I'm not 100% sure that I would have enough primary coil
turns available to tune in with a 40 nFd. cap. I do know that the
inductance does tend to increase as the square of the number of
turns, though, so 11 turns may still just get me in under the wire
with a 40 nFd primary cap.

No, my PT is only rated at 1.5 kVa @ 30*C, but as you probably
already know, PTs are generally capable of FAR more output
than their nameplate rating suggests for relatively short runs in Tesla
coil service.  Mine weighs about 95 lbs. I wouldn't mind actually
getting my hands on a 5 kVA rated PT, though ;^)

My topload is a homemade 6"x30" corregated aluminum duct
toroid that's covered in aluminum tape to add to its rigidity.

David Rieben

----- Original Message ----- 
From: "bartb" <bartb at classictesla.com>
To: "Tesla Coil Mailing List" <tesla at pupman.com>
Sent: Wednesday, April 16, 2008 8:46 PM
Subject: Re: [TCML] First (or maybe 2nd) light on my medium-sized SG coil

> Hi David,
>
> I got a chance tonight to take a closer look at your specs. I assume your 
> PT is in the 5kva range? BTW, what's the top load?
>
> Your total gap distance is kind of low causing the gap to fire early as 
> based on a credit card width of 28 mils (I measured one of my own), so the 
> total gap is about 0.2". The gap should be firing at about 65 to 70% of a 
> 120Vac source. I personally would have set the total gap to be 0.35" in 
> your case (50 mils between each electrode). This would allow the gap to 
> arc at near the 140 Vac available with the variac. But at 140 Vac setting, 
> you will have nearly 50 amp spikes occurring on the line. With 20A 
> service, you don't want to go over 2500 VA. This forces you to run at 1/2 
> power (or less) and in your case, your breakers are forcing this.
>
> As DC mentioned, you could parallel a couple 120V sources (which is not a 
> bad idea). Just because the PT is setup for 120V, doesn't mean it should 
> go over 2500 VA on a single source (and it is certainly capable in TC 
> service). The cap is large. Reducing it only slightly would make things 
> worse (I think your transformer resonance is 65nF). You could go to 40nF 
> and run STR however. These smaller caps can have big sparks also. My cap 
> size is 4.3 times smaller than yours and my sparks are longer! Go figure.
>
> It's one thing to have a 120V source coil. But big caps require power, 
> sparks require power at the bang, and power requires the capability of the 
> circuit it's sourcing power from. This coil can do so much better, but it 
> is limited by the source. You'll have to run at a lower power, consider 
> something like DC suggested, or lower that cap size. I think it's just too 
> much transformer power and cap size for the 120V line.
>
> I run a 200mA 120V transformer for the small coil using an 18.5nF cap 
> size. But as I said, I watch the current, I keep my feelers out for wire 
> heating, and those type of things. I'm required to use some common sense 
> considering the house 120V 20A breakers and I tend to ride the edge. Your 
> PT is even more capable at 350mA current and it's using an 80nF cap size!
>
> Take care,
> Bart
>
>
> David Rieben wrote:
>> Hi all,
>>
>> I have finally gotten my mid-sized SG coil project up and running
>> A quick rough-up of the specs are about 28" of #22 AWG magnet
>> wire wound on a 6" ID (6.5" OD) white PVC pipe for the secondary coil. 
>> The primary is 11 turns of 3/8" copper tubing wound in the classic 
>> Archemdian spiral with between 3/8" and 1/2" spacing be-
>> tween each turn and the outermost turn has about a 12.5" radius
>> (25"diameter). After initial tuning, the primary has been tapped at
>> approx. 8 3/4 turns. The transformer is a 120:1 GE potential trans-
>> former (14,400 volt output). Ballasting was origianlly going to be via a 
>> surplus saturable reactor that was rated at 12 kVA and was origi-
>> nally designed to run a bombarder and thus control 240 volts. Naturally,
>> I assumed that it shouldn't have any trouble holding the reigns on
>> 120 volts input to a PT. However, it seemd that I was having a fer-
>> roresonant issue with the saturable reactor in this setup and I was
>> pretty quickly tripping the 20 amp panel breaker with only about
>> 50 on the dial of the 22 amp Staco variac. The variac would also
>> start to make that heaving and humming sound with the start of the SG 
>> firing, like it was almost dead shorting.
>> So I removed the sat-reactor and replaced it with (2) MOTs with
>> their seconadries shorted and their primaries in parallel to each other 
>> and of course in series with the 120 volt input to the PT. This does seem 
>> to offer better current control although it still seems to start rocking 
>> with Staco variac set at 50 to 60 on the dial (very fre-
>> quent 50 to 60" strikes to my tool cabinet in the garage). The MOTs do 
>> tend to get warm pretty quickly running like this though and I haven't 
>> had a chance to pull it outside to "pull the ears back on her" since I am 
>> confined for space inside of my garage and the arcs start hitting the 
>> ceiling
>> if I try to go beyond this (fire hazard, already been there, done that) < 
>> :^O
>> I don't have any metering on this coil's control panel and the Staco
>> variac IS the control panel at this time. Therefore I'm not really sure
>> just how much current that it is pulling but I can tell that it is up 
>> there a bit. I have at least stopped tripping the 20 amps panel
>> breaker since replacing the sat-reactor with the MOTs, though.
>>
>> O yea, I forgot to mention that the SG is a simple stationary RQ style 
>> copper tube gap with (8) 1" diameter x 6" long copper pipe
>> segments and the resultant 7 gaps each spaced by the thickness
>> of a credit card. The primary cap is (2) paralleled RFI recon mica pulse 
>> capacitors, each rated at 0.04 uFd @ 40 kV for a total pri-
>> mary C of 0.08 uFd. The topload is a homemade 6" x 30" toroid
>> made from two 8 ft. lengths (when fully streched out) of 6" dryer
>> duct. Because of the relatively large primary C, I have found that the 
>> output is smoother when I short about 2 of the 7 gaps in the
>> RQ stationary SG. The gap will start "spitting" with the variac
>> is turned to around 30 on the dial instead of 45 or 50 and it makes
>> for smoother operation. I almost forgot to mention the the stationary
>> SG is forced air cooled by a pretty robust fan (likely several hun-
>> dred CFMs). One thing to note is that I just wired the input to the
>> fan in parallel with the input to the PT so the more power that I push 
>> into the transformer, the faster that the fan will turn. Would
>> you all suggest running the fan on a seperate variac or just running it 
>> seperately all together and letting her rip at full speed at all times
>> the the coil is running?
>> I am pleased so far with its performance, but I was wondering if some PFC 
>> caps would help to reduce the current draw a bit
>> and make sure that I stay within the power draw range of a standard 20 
>> amp circuit on a 120 volt line? After all, that was the
>> main reason that I decided to construct this coil in the first place.
>> I have about (12) each of some 100 uFd, 370 VAC power factor
>> caps that I could cannibalize from another project that I no longer
>> have much interest in :^) I'm sure you would love to crunch the
>> numbers through your Java TC, Bart ;^)
>>
>> I could post some pics if Hotstreamer still lets you post photos?
>> I have been concentrating so much on the large pig driven coils
>> in the last several years that I have just about lost my skills for 
>> building sub-pig sized coils <;^)
>>
>> Thanks,
>> David
>>
>> PS - the primary/secondary coupling is set so that the bottom
>> turnof the secondary coil is just about level with the top of the pri-
>> mary coil.
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>>
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