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Registered Member #102
Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169
Erm, Iam no expert but I do know that driving above resonant frequency will cause the secondary to go into 90 degree phase difference with the primary. Which will entirely disagree with the feedback method used by most tesla coils. And phase leading by the secondary can cause failure in the mosfet/igbt stage amplification via over voltage. this is at least what i have read in the past, it is not my personal experience to my knowledge.
Which will entirely disagree with the feedback method used by most tesla coils. And phase leading by the secondary can cause failure in the mosfet/igbt stage amplification via over voltage. this is at least what i have read in the past, it is not my personal experience to my knowledge.
I believe this only occurs in DRSSTCs where primary feedback is used. Using secondary base or antenna feedback, or running open loop doesn't have this problem.
Any FM modulation of the drive signal will have capacitive and inductive going excursions by the nature of FM. Setting the bias point so that the frequency lowering excursions stay above resonance prevents a capacitive impedance being reflected to the bridge, thus preventing high peak currents from flowing as a result of current leading.
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
Welcome to the forum, always good to see another Aussie (greetings from the Yarra Valley).
I'm also in the middle of designing a small SSTC (albeit very slowly, not much time on my hands recently and I'm dividing it between a bunch of projects). I'm probably going to use a CT over the secondary's ground lead for feedback rather than an antenna. It looks neater and I have a bunch of Epcos N30 cores sitting around. Apart from that and the use of IGBT's, the bridge itself is very similar to Steve's Mini SSTC design. My secondary is around 465 kHz (50mm x 285mm coil with a 150mm x 40mm toroid) and I'm using 0.68 uF DC blocking caps, which gives an effective impedance of around 0.25 Ohms. With a 325 VDC bus this is negligible. The DC blocking caps are the flat terminal snubber types rated at 20A RMS and the IGBT's are FGH40N60SMD's (600V 80A, 137 nanosecond total switching delays), which is probably excessive for the power level I hope to achieve.
I haven't yet decided what feedback scheme to use, I've been looking at both the simple Schmitt trigger design as seen in the Mini SSTC, and a PLL design based on a 4046 IC.
Looking forward to seeing how your design progresses. Hopefully one day there will be enough of us for an Aussie Teslathon :)
GrantX, if you do decide to use the 4046 be sure to read up here on the forum about the problems related to the phase comparators (type 1 vs type 2) intrinsic to the 4046. Long story short it is a pain in the arse to get right unless you get lucky. I made four TCs with that ic, and only one ever ran right, even though there were no deviations in design other than secondary size and resultant frequency. I eventually moved on to other drive topologies.
That high of DC bus does indeed negate the impedance of the dc blocking caps, but do remember that a half bridge only sees half the bus voltage at its output. Running off 177V bus like we do here in the states, that quarter ohm is more important.
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
Sigurthr wrote ...
GrantX, if you do decide to use the 4046 be sure to read up here on the forum about the problems related to the phase comparators (type 1 vs type 2) intrinsic to the 4046. Long story short it is a pain in the arse to get right unless you get lucky. I made four TCs with that ic, and only one ever ran right, even though there were no deviations in design other than secondary size and resultant frequency. I eventually moved on to other drive topologies.
That high of DC bus does indeed negate the impedance of the dc blocking caps, but do remember that a half bridge only sees half the bus voltage at its output. Running off 177V bus like we do here in the states, that quarter ohm is more important.
Thank you for the reply Sig, that helps a lot. I do understand that a half bridge will only give me +/- ~160 VDC across the primary, but I got lucky and scored a huge box of snubber capacitors. Since the snubbers are all screw terminal, I can increase or decrease capacitance without having to change any of the layout.
I was considering the 4046 scheme because I foolishly assumed it would be more reliable and stable than the Schmitt trigger, but I see now that people have had some serious difficulties with those IC's. Thanks for the warning, I'll start looking into some alternatives.
I don't want to clutter up Greg's thread, so I'll make a new thread soon to continue procrastinating about the feedback circuit ;)
Registered Member #9039
Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117
GrantX wrote ...
Welcome to the forum, always good to see another Aussie (greetings from the Yarra Valley).
I'm also in the middle of designing a small SSTC (albeit very slowly, not much time on my hands recently and I'm dividing it between a bunch of projects). I'm probably going to use a CT over the secondary's ground lead for feedback rather than an antenna. It looks neater and I have a bunch of Epcos N30 cores sitting around. Apart from that and the use of IGBT's, the bridge itself is very similar to Steve's Mini SSTC design. My secondary is around 465 kHz (50mm x 285mm coil with a 150mm x 40mm toroid) and I'm using 0.68 uF DC blocking caps, which gives an effective impedance of around 0.25 Ohms. With a 325 VDC bus this is negligible. The DC blocking caps are the flat terminal snubber types rated at 20A RMS and the IGBT's are FGH40N60SMD's (600V 80A, 137 nanosecond total switching delays), which is probably excessive for the power level I hope to achieve.
I haven't yet decided what feedback scheme to use, I've been looking at both the simple Schmitt trigger design as seen in the Mini SSTC, and a PLL design based on a 4046 IC.
Looking forward to seeing how your design progresses. Hopefully one day there will be enough of us for an Aussie Teslathon :)
Gidday Grant. You don't live all that far way from me then.
I am a volunteer for the Science Talent Search run by the Science Teachers Association of Victoria. If I can get my SSTC working, plus a few other devices, I was going to see if they would let me demo them on their presentation day at Latrobe Uni.
I was also toying with the idea of displaying them as alternative Christmas lights one year.
Perhaps even a display at Epping Plaza or Westfield South Morang if they let me.
I am currently doing a kitchen renovation, as well as run my landscaping and mail order nursery business and maintain my business website. So my time is also some what divided.
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
Gregary Boyles wrote ...
Gidday Grant. You don't live all that far way from me then.
I am a volunteer for the Science Talent Search run by the Science Teachers Association of Victoria. If I can get my SSTC working, plus a few other devices, I was going to see if they would let me demo them on their presentation day at Latrobe Uni.
I was also toying with the idea of displaying them as alternative Christmas lights one year.
Perhaps even a display at Epping Plaza or Westfield South Morang if they let me.
I am currently doing a kitchen renovation, as well as run my landscaping and mail order nursery business and maintain my business website. So my time is also some what divided.
I'm often in Melbourne's northern suburbs since most of my friends live in that area, seems to be a nice place.
What's involved in volunteering for the Science Talent Search? Seems like it could be entertaining. Once I have a few projects finished to a high enough quality I'd love to take part in some kind of science/tech demonstrations.
Registered Member #9039
Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117
GrantX wrote ...
Gregary Boyles wrote ...
Gidday Grant. You don't live all that far way from me then.
I am a volunteer for the Science Talent Search run by the Science Teachers Association of Victoria. If I can get my SSTC working, plus a few other devices, I was going to see if they would let me demo them on their presentation day at Latrobe Uni.
I was also toying with the idea of displaying them as alternative Christmas lights one year.
Perhaps even a display at Epping Plaza or Westfield South Morang if they let me.
I am currently doing a kitchen renovation, as well as run my landscaping and mail order nursery business and maintain my business website. So my time is also some what divided.
I'm often in Melbourne's northern suburbs since most of my friends live in that area, seems to be a nice place.
What's involved in volunteering for the Science Talent Search? Seems like it could be entertaining. Once I have a few projects finished to a high enough quality I'd love to take part in some kind of science/tech demonstrations.
I attend the STS meetings, do what I can to help with their judging day and presentation day - for models specifically. It is a lot of fund playing with the in models and inventions the kids come up with.
The first year one kid created a small high voltage coil gun - it was very well done. Last year another kid made a model of 3D laser scanner. I.E. He manually and randomly scans a 3D object with a small laser and then he used a standard USB webcam, and some software he wrote, to construct a 3D model on his laptop.
If you are interested in getting involved then check this out:
Call Janice in the office and let hew know you want to be involved.
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Question for Steve Ward re his mini SSTC
