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Registered Member #4396
Joined: Mon Feb 06 2012, 11:58PM
Location:
Posts: 30
Hello All,
I've tried duplicating Steve Ward's "SSTC-5 (Mini SSTC)" tesla coil with some slight changes in turns.
My secondary is a 1240 turn, #30 wire, 13" tall on a 4" OD PVC pipe. Primary is ~4.5" OD, 5 turns #12. Its made with a MOSEFT half bridge on full wave rectified 110VAC. The control electronics are powered off a 12VAC doorbell transformer, and some LM regulators with input and output capacitor, etc... The toroid is made on a 8" OD piece of plywood with 3" dryer ducting, total OD of 14".
The mains ground is connected by the terminal block in the corner to the bottom of the secondary.
With the top load OFF, I can arc to about 1/2" from a coat hanger jumper cabled to my house plumbing, if i bend the antenna near or far from the primary, the spark increases, and if i turn the PWM up on the controller, the power increases as well. With the top lop ON and a coat hanger breakout point, I can't get it to jump anything at all, the secondary windings just get hot.
I believe that I have made the control and power electronics properly, I've tested it with a scope. I think the feedback is at least sort of working, because I can change the effect a little by moving it to/from the primary. But, very little to no breakout, none with a top load. The secondary only gets warm. What gives? I'm dying to get this working.
There are future plans to make a full IGBT H-bridge, transformer feed back / current limiting and converting to a DRSSTC with a primary MMC. I wanted to crawl before I walked, but now I'm just stumbling!
Any advice? If I don't insulate the antenna, the secondary will jump out and get it, but it wont discharge from the top? I think I'm just way out of tune, but I'm not really sure the best way to go about tuning it. How you you guys look at your coils while operating with your scope? Transformer isolation?
Registered Member #4362
Joined: Sat Jan 21 2012, 03:44AM
Location: Texas
Posts: 98
Hello. Just a few questions for you. In your last pic. I see a big resistor at your main rectifier. Is it there for inrush current protection? Looks like its been way overheated. Might want to test it. If its a bleeder you may want to change the value.
Also have you tried reversing polarity of the primary? Could be out of phase.
Registered Member #4396
Joined: Mon Feb 06 2012, 11:58PM
Location:
Posts: 30
Its a bleeder, but its on a switch... a switch that I forgot to open a couple of times ;) Don't worry, its open circuit now ;) I plan to put a proper bleeder on shortly, maybe another low valued one that gets switched in when power goes away with a relay.
No, I've never really thought about the phase of the primary much until you mentioned it. I have connected it and disconnected it a few times before though, and it seems to behave the same. Who knows, maybe I used the same phasing every time by dumb luck though. So I made a passive negative voltage relative to earth vs a small one? I don't know what to think now...
Any advice on tuning? I thought the whole point of the feedback was that I didn't have to bother much with tuning? I know the feedback is connected properly, maybe it all has to do with phasing... I need to try that next.
Registered Member #4073
Joined: Sun Aug 28 2011, 08:43AM
Location: Australia
Posts: 17
I would try with a longer antenna. I just finished (sortof, still more work to do) the same SSTC. I ran it a few nights ago and it arced on the inside, but before that happened I could see corona on the tip of the antenna. I would try moving the antenna away from the coil, and making it longer.
If that doesnt help, double check the secondary has a solid ground connection, as poor grounds have had similar performance problems for me.
This could be a case of the blind leading the blind, but it's worth a try.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Couple things, SSTC are not tuned, the amount of turns on the primary really only sets the current draw of the system. Easy to calculate. The reactance of the primary at the coil's resonant frequency. Use the reactance and ohm's law to find your current draw at whatever voltage you are feeding it. Generally I wouldn't recommend a reactance that allows anything over 3A to 5A at 170VDC bus voltage.
Second ditch the antenna and go with a CT on the base of the secondary 1:50 to 1:100 would probably work fine in your case. This is much more reliable than the antenna and has about a 0% change of getting hit with secondary voltage. Also as other have already said check the phasing it could just be that you need to reverse it.
Last, for the love of god move your bus capacitors right next to your FETs, that is a high current loop and should be made as low inductance as possible. If this is not done large voltage spikes will develop across those lines and blow the crap out of your FETs. I see new coilers making this mistake time and time again. No one seems to read the old SSTC threads where this is pointed out countless times.
Registered Member #3964
Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332
I also experienced what you have experienced, at first, my SSTcoil runned by antenna, produced only an inch. I then use a black GDT for the feedback with a 1:80 ratio, at first, it did not work because my variac humms whenever I increase the Vin, - I reduced the bps, also made the supply filtered and full wave rectified.
second try, I luckily made a 11 inches spark from my 10" sec. secondary feedback,
When I tried swapping the primary connection, I noticed that It did not change a bit.
And then It came to my mind that why wouldn't I use a primary feed? Ofcourse I had to tune it via javatc as well as adding a primary capacitor. I tried it, but first there were no output.
Then I tried swapping the gdt input, still did not work, Then I tried swapping the Primary, Still did not work, then I swapping the feedback ct output, still no luck, I put back all the swapping, except the ct feed, then it worked,.
on my halfbridge, I am lucky to hit a 21 inches target, but when I am in fullbridge mode, my coil easily struck a 30 inches target..
JUST sharing,, =)...
by the problem I see, It seems that the secret is in this 3 set up,,
GDT, primary winding, feedback CT. =) try every combinations, don't worry it will not explode! =D
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
If you're going to run it as a conventional SSTC, higher coupling improves the performance greatly. Right now you have very low coupling, around 0.15 if I'm calculating it right. I used something like 0.4 in my SSTC, and got great performance (24'' from an 11'' tall secondary running as a normal SSTC). The limiting factor will be flashover from the secondary, so don't overdo it.
Registered Member #4396
Joined: Mon Feb 06 2012, 11:58PM
Location:
Posts: 30
Goodchild wrote ...
Couple things, SSTC are not tuned, the amount of turns on the primary really only sets the current draw of the system. Easy to calculate. The reactance of the primary at the coil's resonant frequency. Use the reactance and ohm's law to find your current draw at whatever voltage you are feeding it. Generally I wouldn't recommend a reactance that allows anything over 3A to 5A at 170VDC bus voltage.
Second ditch the antenna and go with a CT on the base of the secondary 1:50 to 1:100 would probably work fine in your case. This is much more reliable than the antenna and has about a 0% change of getting hit with secondary voltage. Also as other have already said check the phasing it could just be that you need to reverse it.
Last, for the love of god move your bus capacitors right next to your FETs, that is a high current loop and should be made as low inductance as possible. If this is not done large voltage spikes will develop across those lines and blow the crap out of your FETs. I see new coilers making this mistake time and time again. No one seems to read the old SSTC threads where this is pointed out countless times.
Hope this helps Good luck.
I'm a little confused about the reactance calculation, you make it sound very easy, but in practice, I'm not very sure how to get meaningful numbers out without knowing more about the system operating frequency.
Reactance is 2*pi*f*L, it’s easy enough to calculate the inductance of an air core inductor, but how will I know the operating frequency of the system? I suppose I could just guess that its between say 100-200kHz and size for the largest current at ~170VDC.
A question about the CT. You are recommending _secondary_ coil feedback with a 1:50-100 ratio, most of the Steve Ward designs I’v seen with CT FB use _primary_ feedback with a ~1000:1 ratio. Is there an advantage to using a secondary vs primary feedback CT other than not having to cascade transformers or put many, many windings? Is it just simpler to do secondary FB because it’s easier to construct?
About the long leads between the bus caps… yeah, I know better. That was done out of purse laziness… I wanted the construction somewhat modular, so that I could revamp portions of it as I got simple versions of the circuit working. I got some super fancy 600V FETs with really low R-DS-ON, so I thought I’d get away with it because they were so overrated. Maybe I’ll rebuild it soon, I might just throw some large 440V TVS’s around them to limp along while for the time being also. Not that I don’t appreciate the advice… Speaking of soaring rail voltages on high current switching… If I were to calculate this, I want something like V = L * di/dt. How do I calculate the di/dt part and have it be meaningful or close to reality. First, I need the peak I, and then the switching speed of my FET, right? I guess I’m stuck at finding the real working I.
One last comment about the phasing of the primary... If I had this backwards, wouldn’t I wind up with an equally large _negative_ voltage at the top of the primary? Wouldn’t that still want to discharge to earth? It seems like theoretically this would still work? Maybe the charge can’t spread as easily on the top load and concentrate on the break out point if it’s negative? This goes against most of what I thought I understood about it.
Anyway, it’s simple enough to just swap phases on the primary, I’ll try it.
Yes, I’m certainly into changing to a CT for feedback vs an antenna.
Registered Member #4396
Joined: Mon Feb 06 2012, 11:58PM
Location:
Posts: 30
Moohhahhaaa!
It's alive!
The problem was 100% dude to primary phasing. Got 10" or better sparks until i got a hit on the antenna, now its toast. I'm guessing the diodes and inverter IC failed.
Anyone care to give a proper explanation of the reason that primary phasing is obviously so important? I'm guessing it somehow puts the feedback 180deg out of phase and doesn't allow it to ring up....
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SSTC-5 (Mini SSTC) - Tuning Help Needed Please.
