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Keep in mind that if you use secondary current feedback for a SSTC you can't run it CW without a pull up resistor/momentary switch on the enable input (provided the gate drivers have an initialization pulse, like the UCC chips do, and your interrupter can sink current / pull down to ground as well as source) as the CT will look like a DC short to ground effectively pulling your feedback input to ground.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Sigurthr wrote ...
Keep in mind that if you use secondary current feedback for a SSTC you can't run it CW without a pull up resistor/momentary switch on the enable input (provided the gate drivers have an initialization pulse, like the UCC chips do, and your interrupter can sink current / pull down to ground as well as source) as the CT will look like a DC short to ground effectively pulling your feedback input to ground.
Ok, stupid question that I should know the answer to but don't--What does "CW" stand for?
But thanks, that's a good point. I will probably be using a pull-down resistor on the Enable input of the 27425, I just forgot to include it in the schematic. For the time being I am using an Arduino for the interrupter, which allows the user to adjust the frequency and duty cycle independently. I will probably replace it with less sensitive circuitry later on.
We don't generally use the term anymore but most singly resonant SSTCs people make or see today are ISSTCs (interrupted SSTCs). To run CW is to run a "standard" SSTC; no interruption - the enable pins are pulled high and held there by either the interrupter (easily done on arduino / MCU) or a seperate switch and resistor or internal pullup resistors on the gate drive chips.
The issue is that when using a secondary base current transformer the input to the schmitt triggers is pulled to ground through the DC resistance of the CT, so there will be NO feedback. When you use an interrupter or operate in pulse mode the transition from low to high on the enable pins cause the gate drive chips to start up oscillation at whatever frequency is natural to them for a few cycles. This is enough to ping the resonator and cause a feedback signal to flow through the CT, which then takes over and sustains oscillation at the resonator's resonant frequency. If you don't use an interrupter or run "hard wired" CW (either from an unswitched pull-up resistor, or the chips internal pull ups) there is no transition from low to high on the enables and the chips don't know to send out the oscillation start pulse. The chips just sit there in the off state because there is no feedback present. Steve Ward's early designs used a 555 timer loosely capacitively coupled to the feedback input in order to start oscillation for this reason, but getting the right level of starter oscillation that doesn't swamp the feedback is tricky and problematic. I've found that a simple current limited oscillation start switch is all that is needed. Simply momentarily pull the enable pins to ground if no interrupter is connected or program the interrupter for a CW mode where output is brought from low to high and held there. The no interrupter + pull down resistor + switch is nice as it also performs the function of an emergency off switch.
Feel free to check out my site about SSTCs, which has all of my schematics, write-ups, and data files freely available for download. There's also videos of my coils and designs working, as well as a video on how to properly phase a GDT.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Use Javatc to calculate the resonance. The secondary coil itself has a distributed capacitance, the primary and topload position also play roles (additional capacitances etc). By using the calculations you wrote, the error can probably be as large as 50%.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Sigurthr wrote ...
CW, or "constant wave" meaning uninterrupted.
We don't generally use the term anymore but most singly resonant SSTCs people make or see today are ISSTCs (interrupted SSTCs). To run CW is to run a "standard" SSTC; no interruption - the enable pins are pulled high and held there by either the interrupter (easily done on arduino / MCU) or a seperate switch and resistor or internal pullup resistors on the gate drive chips.
The issue is that when using a secondary base current transformer the input to the schmitt triggers is pulled to ground through the DC resistance of the CT, so there will be NO feedback. When you use an interrupter or operate in pulse mode the transition from low to high on the enable pins cause the gate drive chips to start up oscillation at whatever frequency is natural to them for a few cycles. This is enough to ping the resonator and cause a feedback signal to flow through the CT, which then takes over and sustains oscillation at the resonator's resonant frequency. If you don't use an interrupter or run "hard wired" CW (either from an unswitched pull-up resistor, or the chips internal pull ups) there is no transition from low to high on the enables and the chips don't know to send out the oscillation start pulse. The chips just sit there in the off state because there is no feedback present. Steve Ward's early designs used a 555 timer loosely capacitively coupled to the feedback input in order to start oscillation for this reason, but getting the right level of starter oscillation that doesn't swamp the feedback is tricky and problematic. I've found that a simple current limited oscillation start switch is all that is needed. Simply momentarily pull the enable pins to ground if no interrupter is connected or program the interrupter for a CW mode where output is brought from low to high and held there. The no interrupter + pull down resistor + switch is nice as it also performs the function of an emergency off switch.
Feel free to check out my site about SSTCs, which has all of my schematics, write-ups, and data files freely available for download. There's also videos of my coils and designs working, as well as a video on how to properly phase a GDT.
Okay, I was very close! I had the right definition for "CW" in mind, I just wasn't sure what it stood for. I just assumed "C" stood for "continuous" and figured it meant "without interruption". I do not plan to run this coil in continuous mode at all, but I will still have a pull-down resistor. I only want the coil to run when I tell it to via the enable pins on the UCC27425.
Definitely going to check out the link, thanks very much!
Dr. Dark Current wrote ...
Use Javatc to calculate the resonance. The secondary coil itself has a distributed capacitance, the primary and topload position also play roles (additional capacitances etc). By using the calculations you wrote, the error can probably be as large as 50%.
I completely forgot about that. When I first started thinking about the coil I knew that the secondary coil would have some capacitance, but when I got to the design part I forgot to consider it. I ran a quick test with JavaTC but my values weren't very accurate. I will run it again and will get back to you. Thanks for pointing this out!
I ran JavaTC, only filling in information for the secondary and topload, and this is the output:
So it looks like I was off by around 34KHz. I am thinking I will most likely add a second toroid on top of the current one. I'm about to add that into JavaTC to see what I get.
By the way, the image JavaTC creates based on the information seems to be in the wrong place for me. Does anyone else have this issue?
Sigurthr wrote ... Feel free to check out my site about SSTCs, which has all of my schematics, write-ups, and data files freely available for download. There's also videos of my coils and designs working, as well as a video on how to properly phase a GDT.
Hello Sigurthr,
I did not realize you ("Sigurthr") were Matt Giordano. I have watched several of your Youtube videos for inspiration!
After adding the second toroid I get ~232kHz, which sounds much closer to what I was looking for.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
DerStrom8: You have managed to make 7 double posts and 1 quadruple post.
You need to read the site rules before you post again!
Use the edit function and if you need additional attachment in a edit, use the attachment forum for that.
Consider this a fair warning as I have to merge so many posts.
A topic related note, SSTCs does not always like huge toploads, with the increased load to drive it can more easily flash over at the primary that usually have a very tight coupling to the secondary in a SSTC.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Mads Barnkob wrote ...
DerStrom8: You have managed to make 7 double posts and 1 quadruple post.
You need to read the site rules before you post again!
Use the edit function and if you need additional attachment in a edit, use the attachment forum for that.
Consider this a fair warning as I have to merge so many posts.
A topic related note, SSTCs does not always like huge toploads, with the increased load to drive it can more easily flash over at the primary that usually have a very tight coupling to the secondary in a SSTC.
My apologies, I was not aware that I was breaking the rules. Do you consider "double posts" to be consecutive, or duplicate? I am a moderator on another forum and a member on another one after that, and I guess I'm used to their rules, since they seemed fairly standard. I assumed they were the same here, though perhaps I shouldn't have. I'll take a look at the rules for this specific site.
Thanks for the note regarding the large topload. I had not thought of that. Perhaps I should rethink the overall size of the secondary. I was hoping to re-use my SGTC secondary, but maybe that's not the best idea, especially for such a low-power TC.
Thanks, and once again I apologize for the multiple posts in a row. I'll remember that they're discouraged from now on. Thanks for merging them for me.
Yep, that's me, heh, thanks for the views and appreciation! One day I'll get around to changing my given name, but it's very expensive and quite a big hassle. I originally had an actual youtube channel name that wasn't my legal name but when google merged with youtube they forced my legal name upon my channel.
I've had luck with pushing up to a 12"x3" toroid on a CW standard SSTC, but beyond that I ran into issues as Mads has mentioned. You should look into adding turns to lower the fres instead of a second topload. You won't have nearly as much trouble with eddy currents heating your secondary when running ISSTC though, so you'll be able to get away with pretty tight coupling if you insulate well.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Sigurthr wrote ... I've had luck with pushing up to a 12"x3" toroid on a CW standard SSTC, but beyond that I ran into issues as Mads has mentioned. You should look into adding turns to lower the fres instead of a second topload. You won't have nearly as much trouble with eddy currents heating your secondary when running ISSTC though, so you'll be able to get away with pretty tight coupling if you insulate well.
Okay, then my thought for a second topload is probably a very bad idea. The lower one is 4"x12.5", so that will give you an idea of the scale. I considered picking up some cheap 3" diameter duct from my local Kmart, so I might use that to make a toroid instead of re-using the one I have. Furthermore, I have reconsidered reusing my other secondary. I'm thinking I might just get a 6" diameter PVC pipe and wind a new coil about 16" high. Calculating for a toroid 3" x 10", I get a frequency of ~191kHz. I can't complain about that! It might look a little goofy, but it should work (assuming I didn't miss something).
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First SSTC design - Need some critique
