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First of all, let me say hello to everybody in this really helpful forum and let me give a short introduction of myself: HV / HF is not anything that shows up in my profession, so I unfortunately don't have any academic knowledge about this subject. My projects are more or less trial and error. I do know something about the principles of electrical engineering, some old school knowledge and some self-taught things. When explaining something, don't expect an electrical engineer, but a hobbyist who still could be unaware of some fundamental basics. The most important statement might be: I'm old and careful enough not to kill myself dealing with HV. I'm responsible for myself in what I'm doing. This said does not mean that I'm not grateful for any indication of possible dangers - but I can and will not make anyone responsible for what might happen if I'll try out your suggestions. And: I'm not a native English speaker, so my English might be wrong in some points or sound funny. I apologize for that and will try to write it in a different way if something is unclear for that reason.
Okay, now, well I could need some help tuning my tesla coil. The project is private and in an early stage, yesterday I have tested the coil the first time completely. I will describe the components and my test setup in short words to my best knowledge, if you miss some information, just ask.
* HV supply: 4 MOT Stack with one ballast MOT. Mot 1+2 primary antiparallel, grounded. MOT 3+4 primary parallel to 1,2 secondary removed from core, core not grounded. Secondaries all in series. Ballast MOT primary in series, secondary short. After testing with different MOTs, it's in a fixed setup now with a 16A Fuse (230V AC, 50 Hz primary) and a EM-Filter from an old server power supply unit as power input socket as well as a safety spark gap. The safety spark gap is T shaped (HV - GND - HV) and is set to 14 mm (HV - HV). If I set it to 12 mm it will always fire (arc) on full power, set to 13 mm it will fire on switching on / off full power, set to 14mm it will not fire on switching on full power. The safety gap is deionized by an old microwave fan.
* Control panel, fixed setup, tested. Consisting of a 220V / 14 A variac, main switch (contactor), 16 A fuse, and two filters (taken out of old microwaves) in parallel setup on the secondary side of my variac, as well as Voltmeter, Amperemeter, indicator lamps.
First Test: Power Supply with control panel: The ignition starts depending on the distance of the poles of the safety gap and the input power. Once the gap has fired the arc will be blown out soon by the fan and (depending on the input power and the distance of the gap) reignite. The transformer draws about 10 Amps when arcing at full power.
* Sparc gap. I'm planing to construct a SRSG@200BPM but as this will still take some time to complete I am using the following test spark gap: 7 copper tubes, 20 mm length, 18 mm diameter, 0,5 mm gap between each tube. That makes six gaps and 3mm gap in total.
Second Test: Sparc gap in parallel to the HV supply. The supply starts arcing at 1/4 power and draws less then 1 Amp. Increasing the power on the variac, the sparks get a bit more intense and the power consumption raises to 10 Amps at full power. That the same values as in Test 1 with the safety gap only. The sparks make the typical 50 Hz noise. The spark gap is not cooled externally (as it is a test setup) but does not heat quickly either in that setup.
* Capacitor: I'm using 11 3 nF / 40 KV ceramic capacitors in a parallel setup. That makes 33 nF in total, the voltage rating should be more than sufficient.
Third test: HV supply, spark gap, capacitor in parallel. Wow. The spark gap starts firing @ 1/2 power (without capacitor it starts at 1/4) and it does 'single shots' in different intervals that are very loud. No comparison to the prior setup. The power consumption is difficult to read, because of the non continuous firing, but it is about 10 Amps at full power. It does heat very quick.
* Primary Coil: Max 7 windings of 8 mm copper Tube. 30 cm diameter at the inner end, wound with 3cm distance at 45 degrees. That makes a final diameter of about 50 cm. About 10 cm above the primary coil there is one Ring @ 60 cm diameter that is connected to GND. The intention was to prevent sparks from hitting the primary coil and maybe damaging my HV supply by giving an earthwire above. Maybe this was not so a good idea and it can be reversed easily, but I'm not sure about that.
* Secondary Coil: 850 windings of 0.5 mm enameled copper wire, wound with 0.5 mm distance (held by a .5 nylon cord) around a PVC Tube with 11 cm Diameter. That makes a height of 1mm per winding, 85 cm in total.
* Topload: Aluflex tube with 10 cm diameter, the ring has 60 cm diameter in total.
The setup allows me to vary vertical position of the primary coil.
Final Test: HV supply, spark gap in parallel, capacitor and primary coil in series, connected to the spark gap. -> The spark gap is not as loud as in the third test, just a bit louder as in the second test. It fires constantly and does make a noise that is a bit alike the 50 Hz hum (not very sure about that, but it's no constant arc, it's firing, quenching, firing). It starts firing at 1/4 power consuming less than 1 Amp, on full power it draws about 10 Amps. There are no visible streamers at the topload. If I connect a conducting rod to the earthwire-ring and bring it in about 20 cm distance to the topload I can see a small corona discharge at the tob (about 5 mm high, glowing). at about 8 cm distance, sparks jump from the topload to the ground wire. When those sparks jump, I can notice very small sparks between the windings of my variac. (Does that mean my filters are ineffective or is it maybe a Ground problem?) I used a neon tube as an indicator in about 50 cm distance to the secondary coil. It glows bright, but in the lower part brighter than in the upper part. After a few seconds of operation there is a clear smell of ozone.
I tried to tune the coil, varying the power-pickup points of the primary coil from 1 to 7 in 1/2 winding steps. The result is that there is no great difference. I think the best results are at about 3 windings, but the brightness of the neon-tube indicator and the spark-to earth length does not vary so much that I am able to measure. There are no streamers and, well the coil is probably not tuned. I started with the primary 20 cm above the base of the secondary and lowered it in steps to zero with no visible effect for me.
Some problems that I have thought of are:
* Ground: I used the PE of my house as ground in my test setup. In the final setup I will use a 'real ground', I'm not sure if the PE (1,5 mm^2 copper wire) is suitable for that purpose at all.
* Contact problems - like it is a test setup, the wiring is not soldered yet, but instead of brought in contact physically and held in place by some tape. a low voltage measurement with a cheep multimeter shows a few Ohms, though.
* Primary coil might have to few windings (out of tune). My calculations showed 5 windings as the resonance frequency, so I thought 7 would be enough, maybe they are not.
* Spark gap quenching - I described how I constructed my spark gap, I expect better results from the final version - SRSG.
* Bad design / Bad calculations. Maybe I did a fundamental error and the coil can't be tuned at all. That would mean: Redo the coiling part. Is there any 'ideal design' for that size?
So: I did find tons of tesla cloil project pages in the web and did read several of them. I searched this forum and found it very helpful so far. I would not have get to the point where I am if I wouldn't have done that. But I'm not sure about the next steps now. Maybe you could give me some hints, tips and tricks, links, anything that might bring me further and get me some streamers. Might I be out of tune and will have to construct another primary with more windings? I do not have a frequency generator or an oscilloscope. Maybe I should look for those to borrow somewhere to measure the resonant frequencies of the coils? Any hint will be helpful. Thank you in advance!
First light. From the point 'hook up and look what happens' it has taken some hours systematical variation of the primary coil length (with a fixed routine from switching off, disconnect power, discharge cap, manipulate, pre-power-on-check, test). The neon tube has shown to be a bad indicator as burning bright even if the coils are not tuned. The brightness can be adjusted with the input power, but seems not to be much correlated to the tune. A grounded stick at a defined distance to the torus gave sparks, so I used that as an indicator for the tune. Increasing the spark distance to the ground stick in steps and varying the contact point of the secondary coil in 1/2, 1/4, 1/8 and finally 1/16 steps, the 'best tune point' was found. All at the same low power input level. A finer tuning has not been possible with this method, as the difference of some cm coil length is not visible in a different spark effect. I have been able to estimate a 1/8 'tune segment' though and the clamp was placed in the middle of that segment. Still, there have not been visible streamers at this point, even at full power. Next thing I did was widening the spark gap a little. Widening it too much had the effect of frequent firing of the HV supplies safety-gap and non-continuous operation. Widening it to the maximum point with continuous operation on 1/2 power resulted in more Amps on my supply, in combination with more heating of the gap itself. The hot gap fires much earlier, what makes it necessary to let if cool down fully before testing. The power consumption are the 10 Amps on full power as in recent tests. But even with the wider gap there have been no streamers so far. The trick was done by some pointed screws that were put upside down on the top of the torus. Now there are constant deep purple streamers comin out of the screws with lots of corona at about 5cm length, what is less than I expected from that size of coil. When switching on and off, single white / blueish flashes reach up to more than 30 cm though. The best 'streamer operation results' are at a bit more than 1/2 power level, where the gap fires smoothly. Turning up the power makes the gap-noise louder, the neon tube burn brighter but the streamers are shorter or gone.
We can let this count as a successful test, next thing to do is to make a rotating spark gap and hope to get better results that way. But still: If you have any hints in what to do / what to check to make the streamers longer and nicer, please feel free to post them here. What length of streamers could be expected in my configuration? Some quality fine-tuning links would be very appreciated, keywords are welcome...
I'm still quite new as a coiler but I'll say this; in a vertical secondary (monopolar TC) setup a good RF ground is essential. RF grounds are different than standard mains wiring grounds in that the grounding path/conductor needs to not only present low resistance at DC but also present low reactance to AC for an overall low impedance path to earth.
Lots of how-to info is out there for a good RF ground just look under amateur radio grounding for it, or ask any operator. A ham worth their salt will know how to spot and install a good RF ground.
As for actually how to tune a SGTC, I can only tell you how I tuned mine.
1) I set up my spark gap. DISCONNECT THE CAPS AND PRIMARY FIRST. It should be set so that it fires consistantly at the open circuit voltage of the transformer. This means turn your variac to 100% (not overwinding area) and then set the spark gap. It should not fail to fire at this setting. If it fails to spark as soon as power is turned on, the gap is too wide. Turn the power on and off repetitively to check that it ignites reliably. You want it as wide as you can get it without it failing to ignite. Using 40kV caps and only <10kV input and a likely unmatched capacitor bank you could probably expand the gap for increased output LATER ON once you get things tuned up. For now you want the spark gap to fire reliably more than in oncontrolled large bursts.
2) Reconnect the primary circuit (caps and primary inductor) with the power off.
3) I put all the info I had into TeslaMap and JavaTC simulators and was given a reasonable starting point for the number of turns needed on the primary. I then started there and systematically increased or decreased the turns in 1/8th turn increments until I found where the longest streamers were present. I found that when the simulators resulted in a smaller number of turns, even a very small change (1/10th a turn) made a huge difference. Make sure to include the primary lead lengths, as this really affects a smaller coil set up. I actually got more than a 30% increase in output by eliminating excess wiring and retuning the primary. 4) Beware of high coupling factors (coils too close together or similar geometry between the coils) which can cause racing sparks (sparks along the length of the secondary).
Hello Sigurthr, thanks for your reply. Your keyword RF ground made me find a lot of interesting sites, also I checked my calculations with JavaTC. I have not been able to reproduce the numbers exactly, but they are close enough. The coil should be designed at least in tunable range. Still, there are some open questions that I am not sure about.
On grounding: On the next run I will try to use ground rods and as short as possible, thick wiring as the ground the secondary and the strikerail will be connected to. The variac, used for driving the HV Supply will use mains ground (including casing). Now, I'm not sure about the HV supply. The casing of three MOTs (including Ballast) are grounded, the other two are floating. The safety gap is using the same ground. Using RF ground for the HV Supply without connecting RF ground to mains ground would mean cutting the yellow/green wire between variac and HV supply. Is this the way it's done?
On filters: I currently use two filters with nearly identical values from old microwaves in parallel. They are located at the secondary side of the variac. The HV supply is equipped with an EMF Filter taken from an old server power supply. Does this configuration make sense (the HV supply should be able to run without variac, too)? If not, what would be needed to keep the line safe?
On the strikerail (seen in the background of the foto): It's a grounded ring above the primary to prevent the tower being hit by a streamer. Does the ring have to be open or is it ok to form a loop for this purpose?
I would recommend isolating everything from mains ground and just using the RF ground instead. Use an extension cord for powering the Variac and snip the earth/ground wire at the wall socket side or use a 3 to 2 prong adapter. Then just ground the case of the variac manually. This keeps any high frequency kickbacks that do get past filtration completely out of the house wiring. So, your filtration ground, variac ground, strike rail, strike target, and MOT ground should all be bonded together close to the TC, and then run to a 5 to 8 foot ground rod by a heavy gauge copper cable or (even better) a copper braid.
Some mains ground systems are of such high impedance to high frequency current that it does not actually anything at all. Despite being physically connected to earth, the path is so high in impedance that electrically it is basically not connected to earth for those frequencies.
As for the grounding of the individual MOTs... I do not know, they're pretty robust though so I wouldn't worry too much. I would think secondary insulation flashover from any HV kickback would be more of an issue. Is there any reason why the other two MOTs casings are floating (perhaps they are out of phase with the others?). I'm terrible when it comes to worded explanations of circuits, show me a schematic and I'll understand! Sorry.
If you're worried about RF getting back to the mains wiring through the hot/neutral conductors look for a simple mains RFI low pass filter rated for the VA of your mot stack's primary side draw. Use the RF ground for the ground connection on the RFI filter as well. Filters bascally provide a low impedance path to ground for what they filter out, and a high impedance path for what they allow to pass. If your mains ground is high impedance to RF then no matter how good the RFI filter is, if it is still connected to mains ground, it will do NOTHING at all.
Hello, thank you for your advice on grounding. Here's the ascii schematic of the HV supply circuit, consisting of microwave oven transformers, each about 2 kV, 900 W. The cores of the inner MOTS are grounded, the secondaries of the outer MOTs have been removed from core which is not grounded.
Ahhh, thank you very much! I understand clearly now. It looks perfectly fine the way it is, no need to change the grounding on the MOT stack. Just remember that the cores on the outer MOTs may be live with some floating voltage and DO NOT TOUCH THEM WITH POWER ON. MOTS CAN KILL.
Hello, yesterday there has been a lovely spring weather here, hot all over the day with a nice thunderstorm coming up at the evening. Best weather conditions to test tesla coils :) Here is a 2:30 min compilation video of the run:
What I did is basically the same as the first time, with the difference that an RF ground was used this time. The RF ground consists of three rods with 60 cm each, driven into the smooth and humid soil. The connection is made by 16mm² copper braid wire.The rods have been place at one side of the coil in a triangle, not in a circle around the coil. Another rod has been used for the variac (because I did not have a long cable to connect the grounds directly). The grounds have been connected via the 10m extension cord (1,5mm²) used to connect the coil to the variac. Mains ground had not been used at all. One result: No sparking in my variac that time.
The 'tuning segment' that I found on the primary has been 1/8 like in the last run, more precise adjustments did not have any visible effects. The tuning segment has been 1/4 off to the first run, though.
Setting the spark gap (with cap and coil connected) first narrow, then wider bit by bit, the following effect could be observed: Turning the variac up, makes the spark gap fire form a defined point. At low power levels the spark gap does not fire constantly, turning up the variac higher gives the typical 50 Hz buzz then. At the higher power levels, more voltage does mean more current draw. The best streamers show at low power levels, when the spark gap is just firing. At a little bit lower power level, the safety gap is firing from time to time, too. At higher power levels there are almost no streamers. The 'chicken stick with ground wire' test shows sparks, though.
I conclude, that the quenching of my spark gap is poor on higher power levels. Can you confirm that conclusion?
The total streamer length achieved is about 50 cm. The sparks to ground are a bit shorter, but about 50 cm, too. Streamers are thin and purple, sparks to ground are thick, white and noisy.
Like I did run the coil for a longer period this time, the ballast mod did get hot pretty fast and I did not want to take the time to let it cool down, so I shorted the primaries, thus kicking the 'extra ballast' out of the system temporarily. The result has been as expected: a much higher current flow at the same variac setting. With the variac set to 3/4 the system draws more than 15 amps now. Interestingly the 16 A fuse has not been triggered at this level but I didn't try out more power 'cause I did not want to break my power supply. A cable drum with about 20 m still rolled-up did get handwarm after some minutes of operation.
One more observation: Using the coil without a defined breakout point (screw) at the torus, the most and longest streamers have been always in the direction where my ground rods were placed. Next time I'll try to place multiple rods around the coil, to see if there is a causality or if this has been just coincidence. I also thought about using a square of grounded wire mesh fence below the coil to act as counterpoise when firing the coil in the shack. Any suggestions on that?
Well, the coil is working - but not very good. This thread could be named: "Debug my coil", too, 'cause it's about more than tuning. I am not very pleased with the streamer length, yet and I'm grateful for any advice.
One critical part would be the spark gap, still with the behavior described above. Can I expect much better results with a rotating gap?
Another point might be the relatively small (33nF) cap. My power supply should be able to charge caps up to nearly 200 nF, but my calculations show that the system might be out of tune with 60 nF or more. This caps are not cheep anyway.
So, any hints on what else to do to get longer streamers? What spark length could be expected with my setup? Can I get close to 1m or even a bit more?
Thank you for helping me so far and thanks in advance for further help.
Not to forget: Safety. Any time when testing, I stick to a fixed routine. When the coil is on power, noone is near. Before approaching, I switch off the main power and unplug the power system. First thing I do then is discharging the capacitor with a grounded chicken stick. Before powering on, I do check the wiring and look around that nobody is coming close to the coil. That's why my neighbors and I are still alive. Additionaly, the control-box is build in a way that I have to press a button to fire. Releasing this button cuts of power (using a contactor). This may tempt to leave the fixed routine, because the coil is powerless, even when the variac is plugged in and switched on. Also, the cap does not use bleeding resistors but discharges very fast through the transformer ( I never got any sparks discharging). Nevertheless, the system is always unplugged and the cap discharged manually when touching anything (e.g. primary, for tuning purposes). You'll never know...
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SGTC tuning howto
