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question about oscilloscope and basic terms

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LAN_403

Physics Junkie

Sun Nov 11 2012, 03:47AM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

Hi all. I am currently working on building my second tesla coil and so I have a few elementary questions. First questions are regarding the driver. The coil I am building now is a drsstc and I'm using Steve Ward's controller 1.3b. I also bought a 100mhz 2channel oscilloscope to help me with this project and all future stuff. So my question about the driver is: what are some important parts to probe on the driver?. I've started with probing the interrupter signal at both the interrupter side and pin 5 of 74hc14 and everything is nice there. Would just like to know what other parts of the controller should be probed and what I might expect to see.

Second question is regarding some basic terminology that I see everywhere which I'm confused about and cannot seem to find any explanations for. Forgive my ignorance as this is only my second tesla coil build and I am still learning a lot and trying to understand everything. The term "cycles" is what I am confused about. I see the term "cycle", primary cycles", "RF cycle" used a lot. Do all of these terms have the same or similar meaning? Or how might they be different?. When I look up the term RF cycle, I only find information of duty cycle.. is RF cycle and duty cycle the same thing?. Next regarding primary cycles and cycles, I often see phrases like "the coil operates best at 6 primary cycles" or "had MMC failure pushing 10 cycles". All of the information I come across has lead me to believe that these terms are related to BPS. If anyone can enlighten me on this It is much appreciated.

Thanks,
Harry

Ben Solon

Sun Nov 11 2012, 04:02AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

all of those terms mean the same thing. one cycle is one period of oscillation in the primary circuit. one period is 1/coil frequency, so if your coil runs 100khz, one cycle will be 10uS long. the number of cycles run is dependent of how long your interrupter's on time is set to. the more cycles you run, the larger the ring up in current is based on the primary impedance. take a look at the characteristic rf envelope for a drsstc's primary current. on the rising edge of the envelope you can see that each cycles amplitude is greater than the last.

Steve Conner

Sun Nov 11 2012, 08:02AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Back in the old days, hertz were called "cycles per second".

Duty cycle is something different. It refers to the percentage of time that a pulse waveform spends in the "on" condition. For example, a square wave has a duty cycle of 50%.

Physics Junkie

Sun Nov 11 2012, 11:09PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

The first coil I built, a sstc mini runs cw or duty cycle of 1(100%). Still learning about lower duty cycles for pulsed systems. This might also sound like dumb question , can a drsstc run in CW? From most of what I have read I see all sorts of duty cycles used, most of which are <20%, but most 5-10%.

Ben, thanks for the clarification. That explanation was great and helped me understand it better, much appreciated. I had a feeling they were all similar.

HV Enthusiast

Sun Nov 11 2012, 11:23PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

For your typical DRSSTC design, you cannot run a DRSSTC in CW. You would certaintly blow it up as thats a tremendous amount of power you are running. For example, if you're coil was operating at 10kW and 5% duty cycle, if you were to run that at CW you would be exceeding 200kW!!!!

Plus, recall that DRSSTCs have a large storage capacitance. During the ON time, almost all the energy being driven into the DRSSTC primary is coming from the electrolytic dc bus capacitors. During the OFF time, these capacitors are then charged up by the line voltage. If you were to run at very large duty cycles or CW, you would simply need an incredible amount of capacitance and/or a very low impedance line input which just isn't practical.

SSTCs which are designed for CW operation generally have much higher impedance tank circuits and therefore don't require such a large input capacitance (energy storage) or low impedance input drive.

Physics Junkie

Mon Nov 12 2012, 12:25AM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

Ahh okay yes that makes sense, thanks for reply. Seeing as there are many settings you can make for an interrupter (pulse width, pulse repetition frequency, burst, one shot, etc...) are some settings more necessary than others, a must have, or just personal preference?

Ben Solon

Mon Nov 12 2012, 02:05AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

it really just depends of how elaborate you want the spark display/sound to be. the simplest controller will have a bps and pw control, while the most complex will be midi enabled with mode switching between settings. i'd get comfortable with a simple controller first,but it depends on your comfort level. you can do bps and pw with 2 555's and adding one more will give you burst mode.

Physics Junkie

Mon Nov 12 2012, 03:30AM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

Working with 555's is where I am at now. The audio and MIDI interface stuff is beyond me at the moment, but something I hope to accomplish down the road. The interrupter I made for my mini sstc works well (same interrupter I used to check that my drsstc controller is receiving a proper signal via fiber optic) but it is only a single 555 timer and the mini sstc coil is really sensitive to it because the value for the potentiometers I used were excissive, although the spark effects are really cool

. For the drsstc I'm building now I plan on going with Steve Ward's interrupter with burst mode as it looks simple enough for me at my experience level, and easy to build.

Regarding my first question about scoping areas on the controller, for example, the out pins on the UCC and the N and P channel mosfets. I'd think there would be an importance in scoping these areas, yes? So far as I can tell, the controller powers up just fine, everything's receiving proper voltage where it should be, no heating in any areas, except the 5v regulator which I can fix with a resistor at the Vin. Fiber optics work as mentioned before, the LED brightness changes as I mess with the interrupter. Also, I am unsure as how to check that the OCD will work? This is an area that I am still learning to understand as to how it works, I get the concept of what it is supposed to do, but still learning HOW. Maybe something that is testing once I am ready to fire up the coil. Since I am unsure as how to check the controller, I was going to just wait until I put together the bridge, mmc, and primary, then just probe the igbt gates, with controller attached of course. Still havn't decided if I want to wind up CT's for probing the bridge or just buy some (lazy). Any suggested methods for scoping controller's?

Thanks for replies, been very helpful.
Harry

Physics Junkie

Sat Nov 17 2012, 10:08PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

So I just got done constructing Steve's Interrupter with Burst Mode and I checked everything out on the scope. BPS and Pulse Control both work perfectly. I was having problems with the Burst Mode last night. The 555 was getting incredibly hot. It wasnt working at all either, scope was telling me that there was no signal from the Burst 555. I found that there was continuity between some pins on the Burst mode 555 where there shouldnt be, so I fixed that but that didnt change anything, still no signal. So I decided this morning to redo the whole Burst mode part, and this time I put an LED between the Out pin and the switch (555Out->resistor->cathode-anode->switch). The 555 no longer gets hot at all. But I get some odd waveforms that I have no idea if they are normal or not. It might just be my noobishly putting an LED Between the Burst 555 and switch.

This probe is clipped to the cathode, waveform seems normal enough to me.. Same as just probing the Out pin..

The probe is now clipped to the anode of the LED, switch is turned off.

This is same waveform as above, zoomed in.

When I turn the switch on, this is what happens (I have to zoom WAY out to see this).

The only indication that I have is the LED I put before the switch, It blinks when I turn the switch on, and by turning the potentiometers I can change the rate of the strobe and the brightness.

Since the LED blinks, I'm assuming that that burst mode works properly. I only put it there in the first place because I though that when the switch was turned on it was causing power to flow through to the 555 and making it get hot, and I liked the idea of a blinking LED indicator. I turned out to be wrong when I removed the LED, no heating occured, and no more ugly waveforms. The LED must not allow the signal to travel from cathode to anode properly, which I didnt know but I tried it anyways... I'll try and see if I can use the blinking LED indicator somehow. Not quite sure what to think of the Flatline when I turn the switch on but I can only assume it works just fine now.

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