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SSTC stops working after so many volts

Discussion in 'High Voltage' started by corbin, Sep 3, 2015.

  1. corbin

    corbin New Member

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    Hello, i just finished building my sstc, and things are going kinda great, everything works correctly, but to a certain point. after I exceed around 180v the arcs stop and i get nothing. if i go back down in voltage it starts right back up again, but i have to go to zero first. when it stops arcing theres a fairly quiet clicking noise or discharging noise coming from the H-Bridge. Ive tried figuring it out as best i can but i just need someones knowledge. my setup is Kraizer's SSTC II which is basically just steve wards SSTC 5. im feeding 120 fully rectified into a voltage doubler, other then that everythings a exact copy
     
  2. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    Hi corbin,

    Could you post the schematic please? I've seen so many versions of SSTCs, and can't remember which is which.

    My guess would be that some of your drive electronics are latching up. It would be good to see a photo of your setup too. And if you can, perhaps a video of it doing what you describe?

    Regards,
    Matt
     
  3. corbin

    corbin New Member

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  4. dave

    Dave New Member

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  5. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    What are you using for a bridge rectifier (D3)?
     
  6. corbin

    corbin New Member

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  7. corbin

    corbin New Member

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    IMG_3830.JPG IMG_3829.JPG IMG_3831.JPG
     

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  8. corbin

    corbin New Member

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  9. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    This form of SSTC operates based around a feedback signal it receives using the antenna. The antenna directly determines when the TC switches. If this antenna loses a signal, it can't tell the drivers what to do, and the coil stops. Something is causing that antenna to lose its signal. Where is the antenna located?
     
  10. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    Ahh, I did not realize you were interrupting it at such a low frequency. Usually these things work better when interrupted at a higher frequency. It's easier for the antenna to keep track of the signal and keep it oscillating properly. Have you tried cranking up the frequency?
     
  11. corbin

    corbin New Member

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    Yes ive tried that, really any combination gives out the same results, if you think its an antenna issue, should a direct feedback? like 1:60 turn on a ferrite toroid? from the secondary negative to where the anteana would be, cause ive seen that, does that run risks?
     
  12. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    I expect it has something to do with the clipping diodes. When the feedback voltage (voltage on the antenna) reaches a certain level, the diodes conduct, negatively affecting the feedback signals. I have spoken with some friends of mine who are experienced with Tesla coils and we've reached a few conclusions:

    1) Use a current transformer around the base wire of the secondary coil. A current transformer is a far more reliable type of feedback than an antenna. Use clamping diodes on that.
    2) Build your circuit on a PCB. Building it on Protoboard leads to all sorts of signal integrity/quality issues.

    I very highly recommend you take these two suggestions into account and make some changes. I think the issue you are seeing is due to a screwed up feedback signal. Antennas are by far the worst option. Current transformers are not difficult to make, provided you use the right type of core.

    Out of curiosity, what core did you use for your GDT?
     
  13. corbin

    corbin New Member

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    okay, those sound like great ideas. I really appreciate the help! you where so quick to it too. thank you so much, and your friends. I didn't know bad feedback would cause that, since it seems to be working so well in the beginning. as for the toroid its out of something i took apart at one time... which could very possibly means its very bad quality and probably not good for a gdt. something thats helping ive noticed is letting it run for a while, and slowely increasing the voltage. I manage to get farther that way, and time isnt a problem, my mosfets barely heat up
     
  14. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    Hi corbin,

    I'm always happy to help! Tesla coils have been a hobby of mine for nearly a decade, and an interest for even longer. I have been blogging the build of my dual resonant solid state Tesla coil here on ETO, though it's been on the back burner for a while now due to certain life events. I have been learning as I go. Tesla coils are a fascinating subject!

    GDT toroid material is critical, as is the length of the secondaries. Do you have the ability to scope the output from the GDT? The following link is very thorough and helpful when designing a good GDT:

    http://www.richieburnett.co.uk/temp/gdt/gdt2.html

    It explains what can go wrong with GDTs and how to fix it. As I mentioned, GDT core material is critical. It must have a very high AL value (provided in the datasheet), preferably of 10,000 or higher. They are much better at higher frequencies and are more reliable at switching the transistors. It is also very important to keep secondary lengths identical, otherwise the extra inductance will cause delays in the switching signals, which can cause your H-bridge to switch unevenly. This could cause damage to your transistors. It is also very important to wrap the primary and secondary conductors very tightly around the toroid to reduce leakage inductance (which can also cause switching delays). A great way to build a GDT is to use CAT5 cable (leave the conductors in the sheath) and wrap it tightly around the core. This ensures that the conductors are identical in length.

    In case you're interested, here is the blog for my DRSSTC: http://www.electro-tech-online.com/blogs/building-a-dual-resonant-solid-state-tesla-coil.248264/

    Keep in mind that I was learning as I went, so lots of things change throughout the design/build process.

    I expect your bridge is staying cool because you are running it at such a low duty cycle (a very good idea). It gives the transistors time to cool down between bursts.

    Regards,
    Matt
     
  15. corbin

    corbin New Member

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    Thank you very much! unfortunately I don't have the tools to scope a GDT. and i found out the issue! but it only leads to more issues haha, kinda how it goes. So I was simply running my coil when all of a sudden a huge cloud of smoke appeared.. oh dear. one of the driver chips, specifically the UCC37322 caught violently on fire haha, practically melting the surrounding components. I cleaned out the bad chips, and replaced them with some i had from my last PLL. popped them in hoping my sockets still had good connections(considering one was melted pretty bad) and and fired up again. It FIXED the issue of going over 180v. turns out the chips where bad. i just got done running it a gain for a bit experimenting with different secondaries and what not, when all of a sudden it started to only run in CW mode, the interupter stopped working as if it whereat there, a little confused as to how it was still running i ran it a little while again to see what was going on and the magic smoke was back, fried another pair of chips. Im gonna go ahead and assume this is due to a poor quality GDT? does that cause the chips to heat up? I still have yet to lose a mosfet so that good, atleast the bridge side of things is going well. im now all out of driver chips, so ill be ordering more of those. so if its the GDT thats bad is there anyway of testing it without a scope? Also ive had absolutely zero problems with mosfets heating up. theyve never even got very warm. And back to the feedback transformer, i attempted that but got nothing from it, no output. i again used some random toroid, is the feedback transformer required a high quality ferrite core as well?

    edit: some info about my current GDT i actually used cat 5 cable, i took it out of the cable and used 3 groupings. since each grouping contains 2 wires twisted together, i used 2 for the mosfets and 1 which was just paralleled for the drivers. The toroid these where wrapped around i belive to be are from an old TV i took apart, which was probably just off the mains filtering winding. which i assume to be nothing of the quality i need
     
    Last edited: Sep 9, 2015
  16. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    Hi Corbin,

    I suppose a poor quality GDT may cause stress on your driver chips. I would say more likely, though, is that your secondary windings are different lengths, which affects the switching times of your FETs. If they don't switch in sync, it could cause strain on your drivers. This is why I recommended keeping the CaT 5 cable in its sheath. It ensures the secondaries are all very close in length.

    Each of the driver chips is rated for 9A peak, so unless something is shorted (check your connections) I don't think they're popping from over-current. Have you been monitoring your 12V source? Where is it coming from?

    Also make sure that C5 is connected correctly. If it is not, it could (theoretically) cause the outputs to latch up which would cause excess current draw.

    There should be a 0.1uF capacitor between pin 1 (VDD) and pin 4 (AGND). There should also be a 1uF (or possibly even higher, perhaps 10uF) in parallel with it. This will allow the device to source a bit more current when it needs to, and I would definitely recommend it in an application such as a SSTC. It will reduce the strain on the drivers. Note that these capacitors should be located as close to the devices as physically possible.

    How many turns is your primary on the GDT?

    Matt
     
  17. corbin

    corbin New Member

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    My GDT is a 15/15/15/15/15. and the chips doesn't seem to be shorting out, the only time ive noticed them to really get hot is in CW mode, where just no interrupter is connected, but I thought that was just because the high switching speeds. could this again be an issue of bad antenna feedback maybe? could that ever cause the chips to fail? also something else to think about, CAT 5 makes great cable right? did you know the bundles of wire inside are each twisted at different amounts? making some of the bundles actually longer then others. i would think that would be a problem, or is the difference not large enough?
     
  18. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    CW will certainly cause the driver chips to heat up because of the high switching speeds, yes. In general I do not recommend running SSTCs in CW mode unless the drivers and FETs are designed to switch at the resonant frequency of the coil. I doubt poor antenna feedback would cause issues like you describe.

    CAT 5 cable is made up of twisted pairs, yes, but most of them are wound fairly evenly, making them a great choice for GDTs. Winding by hand is much more risky as your windings may be shorter, further apart, etc. There is a much higher chance of uneven windings which, as I mentioned, will cause delays in the switching times. This would generally cause issues with the FETs though, not the drivers.
     
  19. corbin

    corbin New Member

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    i just removed my GDT and the secondaries going to the fets are indeed different lengths, its not a high amount but im sure it was effecting it. i just cut them equally and twisted the pairs together to keep things neat and reduce noise(which i should have done from the start). ill order more driver chips and some good ferrite cores. unfortunately digikey is on back order for the 221 so it'll probably be a while.
     
  20. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    Hi Corbin. If you have any CAT 5 cable left that's still in the sheath, I would recommend winding a new toroid. I would expect ~8 turns to be enough, but feel free to pack as many turns as can fit. I don't think you can wind too many on there. I suggest using the white wires for the primary (twist and solder them together) and use the colored ones for the secondary. This will give you a 1:1:1:1:1 ratio transformer that is easy to connect, and has very similar lengths and shapes. If you wind the wires by hand, if some turns are closer to each other than others even that could affect the GDT quality. Keeping them all together in the sheath helps keep things nice and even.

    For my DRSSTC I used a few of these cores from Digikey: http://www.digikey.com/product-search/en?keywords=495-3868-ND

    They are perfect for Tesla coil use, and I highly recommend them.

    Matt
     

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