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Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
Hi. I've run into a problem with my DRSSTC build. I suspect that my primary is not oscillating at all. To start with, here is a few details of my build:
-Secondary + toroid is 75 khz approx 1915 turns 30awg on 6" OD PVC. Toroid is 20" x 6.5" -Helical Primary consist's of approx 7.125 turns of quarter inch copper tube, 11" diameter form, about .22" spacing between turns. -MMC is a total of .375uF 4kV. So For resonance I've calculated that the primary should be tapped at 5.5 turns for 75khz, but to take into account for streamers, tap between 6-6.5 turns for 60-70khz. -coupling factor is about .19 -Full bridge of ixys ixgn60n60c2d1 -Using primary feedback and OCD. My CT's are the Triad CST206-1A -For interruption i have a 555 timer based interrupter as well as MIDI. -Power supply to bridge is rectified mains (117Vac) then doubled by two lytics in series for a total of 6000uF 400V, but measured with multimeter it is roughly 340-360VDC to the bridge.
To eliminate other possible faults I have: -Rebuilt the drivers/controller circuit twice, eliminated a few problems I made from the first controller, and each time checking solder connections and other connection's, proper voltage to all IC's, etc.. (The controller I'm using now I have not found a single problem with anything, checked the circuit many times..) -Check all my IC's are good -Rebuilt the Bridge board using double sided copper board etching my own connections, I never found any issue with the components, traces, IGBT's are all good, etc... -Made sure my GDT's are properly phased, I've even tried switching them for the off chance that maybe they were not phased correctly -When my calculated tap points gave me no sparks, I tried EVERY possible tap point -Tried different coupling factor between .16 and .2 -scoped my IGBT gates and primary current. -Checked and re-do all of my ground connections
At this point I was stumped... My first thought was that my whole system was completely out of tune and there must be a miscalculation somewhere. So I sat down for an hour or two and re-calculated my MMC, primary, secondary, toroid parameters. I even set up a different table of purposely miscalculated parameters (for example, slightly different toroid capacitance's etc..) But everything checked out to be the same or really close that a miscalculation would be negligible. So for now I've ruled out that my system was not in tune. So then I decided to place a fluorescent tube right next to my primary and repeat everything I previously tried as mentioned above to see if the tube lit up. And of course the tube did not excite at all so I've come to the conclusion that my Primary is not oscillating.
My next steps are to do the following and/or what I have not yet tried: -Switch the primary in the opposite direction (but my secondary winding's and my primary winding's are already in the same direction) -switch the phasing of my CT's - QUESTION: Does that even matter?? Could phasing of my CT's be the cause of this?? I didnt think this mattered but I dont know any better. -I dont have a signal generator to troubleshoot my primary feedback circuit, so I am getting some parts to make a simple 100khz sqaure wave signal -Gather my pennies and buy a darned signal generator since (a) I could really use one (b)properly tune my system (c) test my feedback circuit. -Build a whole new bridge circuit AGAIN but try smaller IGBT's, with redesigned power supply, MMC, Primary, pretty much build a whole new system but a smaller scale. But I dont want to do this -Ask the merry folks of 4hv -dump napalm all over my build and start over
So what I am ACTUALLY going to do is find me a signal generator, but in the mean time I'm going to put together a 100khz square wave generator just to double check my primary feedback circuit.
And again, the only reason I have to believe that my primary is not oscillating is because, after checking everything many times and trying many things, i could not get a fluorescent tube to light up. Other than that I dont know what possible problems I could be having. If theres something you guys are noticing that I am not, please do tell! I am open to any suggestions and advice. If anyone would like any more specific details of my build and/or scope shots just let me know and I'll add them.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
A simple RF Sniffer will detect any oscillation in your primary. More elaborate versions can be built with an op amp, and used as a field strength meter, but this is all you will need to detect activity in a TC. Observe the meter from a distance lest the co-ax cable connect you to a spark!
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Physics Junkie wrote ...
Hey thanks for idea Mary, I hadn't thought of that! So a "more complex version" would be something that measure flux density like a tesla/gauss meter?
Radio hams use RF sniffers as field strength meters for tuning antennas. The basic sniffer isn't calibrated in absolute units, but gives a relative indication of more, or less, signal strength.
The same basic design can also be used to detect microwave oven leakage with a 2.3 GHz quarter wave antenna - a short piece of wire - to pick up the signal.
To make it more sensitive, an op amp can be added.
To turn it into an absorption wavemeter, add a parallel LC tuned circuit at the antenna end, with a variable capacitor and plug in coils to make band switching easier. This is a good way of detecting unwanted harmonics and spurious oscillation.
You'll find lots of ideas and circuits around this if you google RF sniffer.
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
I have seen problem like this before. It was caused by a design flaw in the driver. When interrupter line turns high, the driver should output drive pulse to the gate drive in order to get the primary oscillations going. Do you see this pulse at your gates?
Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
Proud Mary wrote ...
You'll find lots of ideas and circuits around this if you google RF sniffer.
Will do. Thanks!
Conundrum wrote ...
Wonder if its worth making a miniature version using LED tuning to cover the most common TC frequencies?
That's a good idea! I'm going to do some googling and see what I can come up with for an RF sniffer. Maybe try to implement some LED's in there. Were you thinking along the lines of those "LED tuners" where you pump a signal into your TC and the LED lights up at its resonant frequency or harmonic? More or less just an LED for indication that you've hit the fundamental frequency or harmonic...
Kizmo wrote ...
When interrupter line turns high,
I might be misunderstanding you, what do you mean by this? The pulse width?
Kizmo wrote ...
the driver should output drive pulse to the gate drive in order to get the primary oscillations going. Do you see this pulse at your gates?
If I am understanding you correctly, I am not seeing this. Now I am thinking that there could be a problem in my primary feedback circuit. And again I dont have a proper way to troubleshoot this yet, I might have some extra CT's lying around in my junk pile I'll try to scope primary current with. I've traced the signal from my interrupter all the way to the gate drive, but what im having a problem seeing now is the primary feedback signal. My CT's are placed between where the bridge connects to the MMC.. this is correct placement right?
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
It doesnt matter where the current transformers are placed in your primary circuit.
This start up pulse i was referring to doesn't need feedback to be there. If you see nothing at your igbt gates when your interrupter is running, then you have some sort of problem in your driver.
Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
Just a little update here one some progress.. It turns out that my 74hc14 somehow got fried when the pulse width of my MIDI got too high on a song I was trying to play When the pulse width went high the 5v LED indicator flickered off, I found that to be odd, rechecked the circuit and the 74hc14 was shorted on just about every other pin. So I swapped that out and I reconfigured the pulse width limit on the MIDI controller. I still find that odd though, it is my first MIDI controller but has anyone experienced something like that with high pulse width? Also I got a hold of a signal generator so I was able to properly tune my secondary and toroid as well as give the feedback circuit a test. The approximate resonant frequency is now 73khz, very close to the calculated 75. And with streamer it was 60khz, so again it was just as i had estimated.
Here are some scope shots of the waveform on the igbt gates: Wave from signal generator is 100khz sqaure wave Wave from interrupter has a set pulse width of 250us and PRF of 100hz Both blue and yellow was to confirm identical phasing of igbt 1 and 4 (Q1 and Q4) not important though.
Notice that small rise inbetween pulses, Is this anything to be concerned about? I cant imagine why that would be a problem but just asking to make sure.
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Primary oscillation
When the pulse width went high the 5v LED indicator flickered off, I found that to be odd, rechecked the circuit and the 74hc14 was shorted on just about every other pin. So I swapped that out and I reconfigured the pulse width limit on the MIDI controller. I still find that odd though, it is my first MIDI controller but has anyone experienced something like that with high pulse width? Also I got a hold of a signal generator so I was able to properly tune my secondary and toroid as well as give the feedback circuit a test. The approximate resonant frequency is now 73khz, very close to the calculated 75. And with streamer it was 60khz, so again it was just as i had estimated.
