First test of the SSTC

[LoOdaK]

Hello everyone,
the first test of my first SSTC is coming up.
I'm waiting for a few more components that should arrive soon. I have a couple of questions before the first tests, I've usually seen other coilers test their SSTCs at a lower voltage first. The problem I have is that I don't have a variac. Is there any other way to lower the voltage during the test? What else should I pay attention to during the first tests ?

I hope my tests are going to be successful so I can share my results here

[davekni]

The problem I have is that I don't have a variac. Is there any other way to lower the voltage during the test?

If you have any lower-voltage supply (lab supply, old surplus laptop computer supply, transformer), that is a good first step.  I and many others start with a lab (bench) supply with adjustable voltage and current limit.  But I have also used old laptop supplies.  I'm presuming none of these are available to you.  Next best option is to use line power with an incandescent light bulb in series to limit power.

Presuming this is an interrupted SSTC, start testing with very-low duty cycle, such as 50us on-time at 1Hz repetition rate.  Slowly increase on-time before increasing repetition rate.  Even with a bulb in series, sparks should be possible at 1Hz.  1 second allows time for bulk caps to recharge between on-times even through light bulb.  (Before trying direct line power, use the highest wattage bulb available and/or multiple bulbs in parallel to increase available power while still providing some protection.)

What else should I pay attention to during the first tests ?

Do you have access to an oscilloscope?  That is the best way to see that circuitry is behaving as designed even at low power where no sparks are generated.

[Mads Barnkob]

As Dave mentions, very low BPS and then find a way to lower the input voltage. If you have spare electrolytic capacitors, you could make a capacitive voltage divider and with just 4 capacitors, you can get roughly 320, 240, 160 and 80 VDC from rectified 230 VAC.

I usually use a small fluorescent tube or bulb, near the secondary coil, to quickly see if there is a HF EMF emitted, a clear sign of a coil working at lower voltages than enough for break out.

[davekni]

If you have spare electrolytic capacitors, you could make a capacitive voltage divider and with just 4 capacitors, you can get roughly 320, 240, 160 and 80 VDC from rectified 230 VAC.

This sounds interesting.  Are you proposing a switched-capacitor divider with manual switching from series to parallel connection?  Or some other circuit I'm not imagining at the moment?

[LoOdaK]

Thanks a lot guys.

As for the capacitive divider, I have only polypropylene film capacitors lying around, can capacitive divider be made with them ? I could make about 50 VDC or something with them maybe.

[davekni]

As for the capacitive divider, I have only polypropylene film capacitors lying around, can capacitive divider be made with them ?

Depends on how high value your capacitors are.  If you have some large (ie. motor-run) capacitors an AC divider on line voltage could work.  For example, 30uF at 60Hz is 88 ohms reactance.  So if you had a 10uF and a 20uF capacitor, you could make 1/3rd of line voltage with 88 ohm source impedance.  10uF line hot to output, 20uF output to line neutral.  SSTC from output to neutral.  That should charge your SSTC bulk cap in reasonable time.  1uF and 2uF would be 880 ohms.  Might be usable, but you'd need very low repetition rate to allow time for bulk caps to charge between pulses.

[Mads Barnkob]

If you have spare electrolytic capacitors, you could make a capacitive voltage divider and with just 4 capacitors, you can get roughly 320, 240, 160 and 80 VDC from rectified 230 VAC.

This sounds interesting.  Are you proposing a switched-capacitor divider with manual switching from series to parallel connection?  Or some other circuit I'm not imagining at the moment?

Just series connected and you tap whatever point to ground that you want, its crude, but will give you a lower voltage from mains

[davekni]

Just series connected and you tap whatever point to ground that you want, its crude, but will give you a lower voltage from mains

That works only for AC (and AC-capable caps).  With DC, load will discharge bottom cap (or wherever load is connected) resulting in near-zero output voltage after a short while.

[klugesmith]

Dave's first reply suggested an incandescent light bulb in series with your apparatus.

In addition to that, you can put a resistive load (such as light bulb or heating appliance) in parallel with apparatus.  Now you have a resistive voltage divider, whose unloaded voltage can be as small as you like (even 10% of nominal mains voltage).  Voltage sensitivity to your load current is much smaller than it would be with light bulb alone. 

Divider losses might be more than 100 watts, but the components can handle that safely, and for most of us it's winter so extra heat is welcome.

[Mads Barnkob]

Just series connected and you tap whatever point to ground that you want, its crude, but will give you a lower voltage from mains

That works only for AC (and AC-capable caps).  With DC, load will discharge bottom cap (or wherever load is connected) resulting in near-zero output voltage after a short while.

But it will still work to charge up a larger DC bus capacitance for low BPS testing. I thought we were just talking about how to get a lower voltage for a small test and not a sustainable solution

[LoOdaK]

As for the capacitive divider, I have only polypropylene film capacitors lying around, can capacitive divider be made with them ?

Depends on how high value your capacitors are.  If you have some large (ie. motor-run) capacitors an AC divider on line voltage could work.  For example, 30uF at 60Hz is 88 ohms reactance.  So if you had a 10uF and a 20uF capacitor, you could make 1/3rd of line voltage with 88 ohm source impedance.  10uF line hot to output, 20uF output to line neutral.  SSTC from output to neutral.  That should charge your SSTC bulk cap in reasonable time.  1uF and 2uF would be 880 ohms.  Might be usable, but you'd need very low repetition rate to allow time for bulk caps to charge between pulses.

I went on an expedition to find the capacitors, I only have 680nF and similar lying around so that is not an option for me probably. Thanks for the explanation.

Dave's first reply suggested an incandescent light bulb in series with your apparatus.

In addition to that, you can put a resistive load (such as light bulb or heating appliance) in parallel with apparatus.  Now you have a resistive voltage divider, whose unloaded voltage can be as small as you like (even 10% of nominal mains voltage).  Voltage sensitivity to your load current is much smaller than it would be with light bulb alone. 

Divider losses might be more than 100 watts, but the components can handle that safely, and for most of us it's winter so extra heat is welcome.

Well I have light bulbs lying around so that is probably my way to go. Thanks for the info.

Also I'm looking at some variacs but I'm quite uncertain what amperage my variac needs to handle for a standard SSTC ? I'm never planning to run my SSTC in CW mode.

[davekni]

Also I'm looking at some variacs but I'm quite uncertain what amperage my variac needs to handle for a standard SSTC ? I'm never planning to run my SSTC in CW mode.

Don't know what your budget is, but you might want to consider saving for an oscilloscope before paying for a variac.  Incandescent light bulb voltage divider is less convenient than variac, but works for testing.  Even when using a variac, I often add a bulb in series with variac output for the first tests of a line-powered circuit.  Bulb limits current in case of a fault.

I think somewhere you mentioned that your SSTC design is somewhat close to one of Mads' coils.  Perhaps he has information on power draw vs. duty cycle.  In general, more power will make more impressive arcs.

However, you don't necessarily need a variac capable of full power to your SSTC.  Variac is needed mostly for low power testing.  Once your coil is working well at full voltage and low duty cycle, the variac can be replaced with a direct connection to line power.

One more caveat:  Some people always power their coils through variacs.  That provides one solution to limiting inrush current charging bulk capacitor.  For direct line power connection, you'll need some method to limit inrush current.  Large NTC thermistors designed for inrush current limiting are available.  Or some other resistor (such as incandescent light bulb) can be used in series with line power when first connected, then shunted by a switch after bulk cap is charged and before running the coil.

[LoOdaK]

Also I'm looking at some variacs but I'm quite uncertain what amperage my variac needs to handle for a standard SSTC ? I'm never planning to run my SSTC in CW mode.

Don't know what your budget is, but you might want to consider saving for an oscilloscope before paying for a variac.  Incandescent light bulb voltage divider is less convenient than variac, but works for testing.  Even when using a variac, I often add a bulb in series with variac output for the first tests of a line-powered circuit.  Bulb limits current in case of a fault.

I think somewhere you mentioned that your SSTC design is somewhat close to one of Mads' coils.  Perhaps he has information on power draw vs. duty cycle.  In general, more power will make more impressive arcs.

However, you don't necessarily need a variac capable of full power to your SSTC.  Variac is needed mostly for low power testing.  Once your coil is working well at full voltage and low duty cycle, the variac can be replaced with a direct connection to line power.

One more caveat:  Some people always power their coils through variacs.  That provides one solution to limiting inrush current charging bulk capacitor.  For direct line power connection, you'll need some method to limit inrush current.  Large NTC thermistors designed for inrush current limiting are available.  Or some other resistor (such as incandescent light bulb) can be used in series with line power when first connected, then shunted by a switch after bulk cap is charged and before running the coil.

Thanks a lot for the tips. I'm actually looking to buy some hantek DSO 2000 series scope soon, I think it should be more than adequate for my projects.

As for the inrush current limiting, I was thinking about designing a small circuit with the 555 timer and relay to limit the inrush current. After some time , the relay would switch from the inrush current limiting resistor to the "no resistance" path. Would that be a good solution also ?

[davekni]

I'm actually looking to buy some hantek DSO 2000 series scope soon, I think it should be more than adequate for my projects.

Expect that will be fine.  I have a bit-older Hantek model, DSO4102C.  Much more portable than my much-older Tektronix TDS754C.  Prefer my Tek scope for most use, but the Hantek one is good enough.  I do find its 2,4,8,20,... sequence for horizontal time/div to be annoying compared to the normal 1,2,5,10... sequence of Tek scopes.  BTW, had to put Hantek scope in a foil-lined box to use around my Marx generator.  Otherwise it got confused by large transient fields.  No such issue with Tek scope.

As for the inrush current limiting, I was thinking about designing a small circuit with the 555 timer and relay to limit the inrush current. After some time , the relay would switch from the inrush current limiting resistor to the "no resistance" path. Would that be a good solution also ?

Yes, that is a good solution, one I've seen before.  It's an automated version of the series resistor/bulb shunted by a manual switch.

Good luck with testing!

[LoOdaK]

Guys thanks a lot for the help.

I did my first test yesterday and it was kind of successful, at least nothing blew up
It is a great milestone for me, I got my "first light" 

I have one problem and I have a few "fixes" in my head but would like to hear from you guys.

I did my tests with a lightbulb in series. The coil was working, and it was producing arcs but the arcs were very disappointing. They were maybe about 1cm in length, I hoped for more.
I think my main problem is the coupling between the coils. I think I will need to increase it a little bit. Currently, the primary coil has 8 turns, I'm thinking about lowering it to 6 turns and seeing what happens.

The secondary is wounded on a 75mm tube (around 1000+ turns) and the primary coil is wounded on a 100mm tube.
One more thing which maybe draws a little bit of energy from the coil is the platform supports. I did my support with M10 rods (I don't know what you call them in English) and nuts which hold the lower platform (where are all the electronics) and upper platform (where are primary and secondary windings). They are resting below a lower platform on the ground so I'm assuming they could be isolated from the ground and maybe improve performance by a little bit.

One thing I could also do is move my antenna a little bit to see how that affects coil performance.

Below is the coil picture which I took yesterday (sorry for the bad quality, I will post better photos soon)



Full picture: https://i.imgur.com/R8BHRrQ.jpg

And yes I know, it is quite big for an sstc

[Mads Barnkob]

Congratulations on first light! It is a major milestone for anyone doing Tesla coils!

If a SSTC is working, but only giving minimal output at a fair input voltage, lets say above 150V DCbus, you should try to reverse the phasing of the primary coil. You might just be driving it out of phase.

[LoOdaK]

Congratulations on first light! It is a major milestone for anyone doing Tesla coils!

If a SSTC is working, but only giving minimal output at a fair input voltage, lets say above 150V DCbus, you should try to reverse the phasing of the primary coil. You might just be driving it out of phase.

Thanks Mads, yes I tried reversing the phasing and when I reverse it, I don't get any sparks at all so phasing is probably right.


Today I played with it the coil a little bit more, I elevated the primary coil and removed one turn from the primary winding (total 7). The performance was better but using 230VAC without a lightbulb and getting sparks about a 2cm max is really bad. I pushed it really hard in music mode with a random song from a phone and sparks grew to 3-4 cm and one of the MOSFETs popped.

I'm thinking about winding a new primary with a smaller diameter to improve coupling, I think that should help a little bit but I'm still not sure If that will improve performance to the level I was hoping for.

Below are a couple of photos of the coil and fets after destruction 









Album: https://imgur.com/a/A5PuwYO

[Rafft]

Cheer up bud!

Been in the same boat as you do (13yrs ago). Ive made my coils following from the web pages w/o ever using a -proper- scope, or any scope for that matter. I just relied in popping mosfets as my teachers.

If you live in.an area with 230v, get a 110v 500w step down transformer. (Or a variac if you have). Banging parts are less likely.

Eventually youll get it right as you progress

Here are some of my videos... SSTC from 110v to 220v

Dont loose hope. And get a scope 

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[davekni]

I think my main problem is the coupling between the coils. I think I will need to increase it a little bit. Currently, the primary coil has 8 turns, I'm thinking about lowering it to 6 turns and seeing what happens.

Yes, looks like coupling might be low for an SSTC.  Entering your coil parameters into JaveTC will give you coupling, inductances, resonant frequency, etc.  Highly recommended.  Posting JavaTC results here will help too, as it could improve value of replies.  Raising primary does increase coupling.  Going as high as you did for the next post risks arcing from secondary to primary.
A couple other things that help to increase coupling:  Spread out primary turns, and pair primary lead wires close together (ie. fed through same hole in frame and twisted or taped together along their length.

I did my support with M10 rods (I don't know what you call them in English) and nuts which hold the lower platform (where are all the electronics) and upper platform (where are primary and secondary windings). They are resting below a lower platform on the ground so I'm assuming they could be isolated from the ground and maybe improve performance by a little bit.

I don't think those rods are close enough to cause any problem.  BTW, a counterpoise (ie. section of aluminum foil under entire coil frame and connected to ground) can help with performance.

The performance was better but using 230VAC without a lightbulb and getting sparks about a 2cm max is really bad. I pushed it really hard in music mode with a random song from a phone and sparks grew to 3-4 cm and one of the MOSFETs popped.

Probably better to focus on optimizing performance at lower duty cycle before pushing the coil hard.
Where did you purchase IRFP460 FETs?  There are plenty of counterfeits of that part on the market, mostly from China.  Counterfeit parts often have smaller die with lower current and avalanche energy capabilities.
Also, is this a purchased ECB or one you designed?  A half-bridge layout with high parasitic inductance adds stress to FETs, making failure more likely.  (Some purchased ECB's aren't that great either.  Wide variation in design quality exists.)
Whenever you do get a scope, that will make optimization easier, especially if there is some circuit issue limiting performance.

I'm thinking about winding a new primary with a smaller diameter to improve coupling, I think that should help a little bit but I'm still not sure If that will improve performance to the level I was hoping for.

JavaTC will help.  Enter your new ideas before constructing them.  Will also show if secondary impedance is way too high or low.  Conventional wisdom says ~50k impedance at resonant frequency is good.  Coils from 25k to 100k can work reasonably.

Good luck!  Nice to see progress.  Almost every project I tackle takes much longer than I initially think it should.

[LoOdaK]

Thanks a lot Rafft, those sparks are pretty good. I'm definitely saving for a scope.



Dave, as for the fets, they could be fake. I got them from a marketplace in my country. Below is the photo of the fet taken apart, I was curious about how it looks like on the inside



I compared it to the fets from this forum conversation: https://www.eevblog.com/forum/beginners/fake-irfp460-mosfets-from-aliexpress/

and the photo from it:


According to the post, real fet is on the left and fake on the right. I notice a smaller die size on the fake part. When I compare it to the my fet I'm somewhat confused, it doesn't look like a fake one but it doesn't look exactly like the real one also. Unfortunately, I don't have the equipment to test it and compare it to the datasheet values. So possibility of the fet being fake is high


As for the JavaTC results, I forgot to upload it. Here is the picture from the JavaTC:


The coupling coefficient is 0.307. What is the general rule of thumb about sstc coupling ? Is 0.4 a bit too much ?

JavaTC will help.  Enter your new ideas before constructing them.  Will also show if secondary impedance is way too high or low.  Conventional wisdom says ~50k impedance at resonant frequency is good.  Coils from 25k to 100k can work reasonably.

From the JavaTC I can see it is around 27k. Is that bad ? If it is, is there a way to fix it or I need to construct a new secondary ? Also, why is 50k a good thing, shouldn't lower values be better ?

Thanks a lot for the help and explanations Dave, this means a lot to me. I love learning new things and this forum is an awesome place to do so