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Mini SSTC Inquiry - 220v?

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LAN_403

MArked One

Mon Jul 09 2012, 11:41AM

Registered Member #3688 Joined: Mon Feb 14 2011, 07:39PM
Location: Europe
Posts: 38

Greetings

After searching around, I've found a schematic which I could use to build a SSTC (the mini SSTC). I'm not new to electronics, but this is the first tesla coil I've Attempted. I can understand the schematic easily enough, but I have a few quick questions:

Providing I use IRFP450 Mosfets, I should be able to run the coil off of my native 220v line, right?

How does one ground the secondary coil? By Attaching the end to A pipe fixture or A grounded rod? What about mains ground?

When looking At the interruptor section I noticed that there were two pots, but the sub point was only shown At one wire. In order to change the spark Appearance, must both pots be changed?

Any help with these issues is greatly Appreciated!

Kind Regards,
Ivan Kozlov

Sigurthr

Mon Jul 09 2012, 12:23PM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

I've built Steve Ward's Mini-SSTC a couple times and it works very well, it is an exceptional design. I think you've made a great choice for a SSTC.

Yes, higher blocking voltage mosfets is all you need to substitute, IRFP460s should work fine, as would any n channel mosfet with enough current rating and a Vds of >400V. Personally I think IRFP460s have a rather high RdsON though and would get rather hot. Remember that your GDT insulation must withstand full rectified line voltage too.

One thing though: I recommend you look up Steve Ward's "universal interupter with burst mode" schematic (in the DRSSTC section) and use that instead of the simple single-555 PWM interupter that is included on the mini-sstc schematic, it is much more useful and the increased cost is negligible. If you can build it in to it's own box and then wire it in (short length leads or use optocoupling with fiber optics if possible) as needed so it isn't permanently attached to the coil. Sometimes you may want to run the coil in CW mode (I do that mostly) or use this universal interupter for another coil.

The default interrupter on the mini-sstc has a rather narrow range of operation due to using only 10k pots and the sub point resistor is basically the minimum on (or off depending on where you put it) time value achievable. I found better results by using much larger (500k) multi-turn pots and leaving out the sub point resistor. Even still, the entire range of interuption resides entirely in the audio frequency range and never really gets very low, so your coil will be loud and annoying/piercing in tone. Both pots affect the perceived frequency and vary the output appearance co-dependantly, so you have to adjust both. I also found it to be very sensitive to the finger effect - proximity of your hand changes the output.

Edit: I almost forgot... grounding!

The ground for a Tesla Coil needs to be a LOW IMPEDANCE RF GROUND. This means it has to be thick stranded wire as short as possible between a TRUE EARTH GROUND and your coil. If you have any high license level amateur radio (ham radio) operators in the area ask them how to set up a proper RF ground, or look up the subject online. Mains ground is NEVER to be used for a secondary ground connection for a TC. That is a surefire way to cause massive interference and most likely damage to anything plugged in to your house wiring or even cause fires.

A true earth ground may be a cold water pipe that you know for certain does not lose electrical continuity between where you access it and the outside of your house lines underground (no plastic joints or teflon plumbing tape) or a 5ft copper ground rod pounded in to SOFT MOIST SOIL (not sand or rocky soil).

Dr. ISOTOP

Mon Jul 09 2012, 03:45PM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

Sigurthr wrote ...

Mains ground is NEVER to be used for a secondary ground connection for a TC. That is a surefire way to cause massive interference and most likely damage to anything plugged in to your house wiring or even cause fires.

Not necessarily. SGTC's want their own earth ground; their output contains all kinds of high-frequency garbage from the spark gap liable to interfere with electronics. In addition, they can have tremendous peak currents due to the pulsed nature of their discharges. Small (DR)SSTC's are much more forgiving, and I've run them connected to mains ground with little trouble.

dude_500

Mon Jul 09 2012, 07:33PM

Registered Member #2288 Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179

A lot of schematics people post have doublers on the input, make sure you remove the doubler if the schematic has one and you plan to run on 220v.

Sigurthr

Tue Jul 10 2012, 02:39AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

I still say err on the side of caution. I first ran my SSTC too close to a wall with a grounded outlet's house wire running through it (behind sheetrock) and the coil's arc struck the wall and burned through the sheetrock and in to the earth wire. On that line I had a coffee maker and a toaster oven - both had their electronics fried.

Dr. Brownout

Tue Jul 10 2012, 02:50AM

Registered Member #2405 Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140

Sigurthr wrote ...

I still say err on the side of caution. I first ran my SSTC too close to a wall with a grounded outlet's house wire running through it (behind sheetrock) and the coil's arc struck the wall and burned through the sheetrock and in to the earth wire. On that line I had a coffee maker and a toaster oven - both had their electronics fried.

Sigurthr is right it really is not the best way to ground a tesla coil secondary. Will it work if you do use mains grounding, yes it will work, HOWEVER the amount of trouble it can cause is not worth the few minutes it takes to make a suitable ground. When I am running the coil in the house (basement shop) I attach the secondary to the water pipe coming in from outside, I just attach the ground on the city side of the meter not the house side. I have never had a problem doing it this way.

Killa-X

Tue Jul 10 2012, 02:59AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

I use voltage Quadrupler on mine with a variac, I run my mini sstc at 400-450v on 500v mosfets, never once had to change out fete and I've had the coil for over a year of use...it's a lovely easy schematic that works for just about any size coil...I use it on a 4.5 x 24 inch, kicking 24-36 inch sparks depending what mode

Daniel Kramnik

Tue Jul 10 2012, 07:19AM

Registered Member #3885 Joined: Sun May 15 2011, 12:47AM
Location: Newton, Massachusetts, United States
Posts: 94

Killa-X wrote ...

I use voltage Quadrupler on mine with a variac, I run my mini sstc at 400-450v on 500v mosfets, never once had to change out fete and I've had the coil for over a year of use...it's a lovely easy schematic that works for just about any size coil...I use it on a 4.5 x 24 inch, kicking 24-36 inch sparks depending what mode

Amazing! Is this with a full bridge? Minibricks, or TO247?

Do you use laminated busses, TVS? I'm surprised you've been able to get away with running the bus voltage so close to the transistors' ratings. I suppose it's less of a problem with SSTCs than DRSSTCs; I was only ever able to get 9" - 10" out of my mini SSTC, although it was definitely limited by primary-secondary flashovers. What sort of coupling/insulation do you use?

Sigurthr

Tue Jul 10 2012, 11:53AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

I'm using IRFP260s on rectified 120V mains (170VDC), with ZERO mosfet failures. You can run them pretty close to ratings as long as you have good free-wheeling diodes and keep them cool.

Wow! 2 to 3 foot streamers is amazing!
I'm only getting four inch streamers (in CW mode) but my coil is VERY high frequency... ~1MHz, which limits streamer growth greatly. Plus I have a lot of turns (10.5) on the primary because it is drawing nearly 5A as is. If I were to remove any primary turns I wouldn't be able to run it on my variac and I'd probably need a dedicated line/breaker for it. According to the build notes from Steve, removing turns does help with the mosfet heating (probably lower inductive kick voltage), but I'll just deal with running it at lower power or sticking to short run times at full power.

The only addition I made to Steve's circuit that I forgot to mention earlier is zener gate protection for the mosfets, not necessary but worth it imo.

Dr. Brownout

Tue Jul 10 2012, 01:24PM

Registered Member #2405 Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140

I use TO247 style in my 9 inch coil 120 in with a doubler, getting around 38" to 40" being my longest however they are not IRFP260's. i am using a full bridge of the FGH40n60 in there if I remember correctly. I built it about 4 or 5 years ago and its still works great.

In my small table top I use the IRFP260 in a half bridge with 120V input. getting about 6" spark from it I audio modulated that one but it does get hot at the full 120V in thats pushing those 260s' to their limit I think.

I built that one just as a plasma speaker and at 30volts in to around 90 volts in it sounds quite good. The heat sinks are really small in fact my 12" plasma globe has a larger heat sink than the SSTC does it uses a IRFP260 as well in a single IGBT flyback configuration.

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