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DRSSTC performance/behavior

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LAN_403

Killa-X

Fri May 27 2011, 01:50PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

The DRSSTC system is still new to me, meaning, this is my first DRSSTC. Though, many may think it's stupid my first DRSSTC is a 1' x 4' secondary, with a 3' topload. Anyways. Few days ago I did my first test after finishing the topload. I have my wires maxing out the primary, going from the start to end of the tube. But, I want to know if the behavior im seeing, is because its not in tune, in which case, I might need larger primary if things dont improve by going down.

Right now, when I turn my DRSSTC variac on, i get to 40V, and its all tiny non visable arcs. Then once i hit 50V, it kicks suddenly to 8" sparks, and with duty i can make it draw 15A. (Yes i have duty cycle limiting). Is this normal behavior? That it stays small then suddenly kick on? or is the whole coil not in tune and the streamers are pulling it into resonance?

What is the normal behavior I SHOULD expect from DRSSTC when ramping the voltage?

Primary wire -- Yes i know its thin pipe but until I get real power i dont plan to upgrade to 1/2" yet.

The whole coil, company that bent my tubes on a DIY bender, failed to get the bends right, but it's still decent, this picture just shows a bad view. And yes, I plan to redo the poly coating as it turned out horrible. The topload isnt done, I just quickly stuck it together for testing, and I plan to go back and clean it up, use real connection not foil, etc.

And yes, my wires are temp too, they are thin, as I made those 14awg wires get so hot that smoke was spilling out all over =D

Goodchild

Fri May 27 2011, 06:28PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

I hate to tell you this but that toroid is never going to work with the way you have those rings spaced. Think about how a toroid stores charge on the outside surface. You have large gaps in this surface that are going to led to decreased C and shielding.

You should space them symmetrically around the circuit with even spacing.

Eric

StevenCaton

Fri May 27 2011, 07:10PM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

I don't know what your tank capacitance is but you are mostly likely way out of tune.
You will probably need more primary turns. In fact, you will probably need near twice the amount of turns you have now.

Arcstarter

Fri May 27 2011, 08:56PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

SteveCaton wrote ...

I don't know what your tank capacitance is but you are mostly likely way out of tune.
You will probably need more primary turns. In fact, you will probably need near twice the amount of turns you have now.

Twice the turns would be near 8 turns, which is quite a few. The more turns, the higher impedance, which means more voltage is required to get the same current as a low impedance primary. If he hasn't enough primary turns he could just use less capacitance for the tank cap.

But to more directly answer Killa's question, a slight increase in voltage equals a much larger increase in current, and a small increase in current equals a much larger increase in spark length. For example, a drsstc may have 8 inch sparks at 80v input and 250 amps primary current, but at 120v input, it could be 4 foot sparks and 500 amps primary current. This is something i have noticed with every drsstc setup i have ever made, including my... current one which is similar to yours (driver and bridge wise).

When i put 80v into my drsstc, it would peak at around 250 amps, and at 110v, it was 600 amps. That would probably be around 5 foot arcs if i had a secondary hooked up at the time. Then on mains it was something like 650-700 amps, which blew my 100 amp IGBT brick.

However, im not so sure it should draw 15 amps at 50v unless you where running like 25 primary cycles and the primary tapped for really low impedance. If your bridge isn't heating, though, then it is probably fine.

I know i have showed you this video, but it may put things into perspective:

This was 80vac input, about 6 RF primary cycles max, a few amps input, about 4 turns on the primary with a .68uf tank cap, and around 250 amps primary current.

Goodchild

Fri May 27 2011, 11:44PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Arcstarter wrote ...

Twice the turns would be near 8 turns, which is quite a few. The more turns, the higher impedance, which means more voltage is required to get the same current as a low impedance primary. If he hasn't enough primary turns he could just use less capacitance for the tank cap.

Well 8 is not that many, most of my DRs use close to that and those are the ones I consider to have a low Z. My big DRSSTC is tuned at 9 turns with a rather small tank. This gives it a high impedance tank circuit that runs much less primary current <1kA and is still able to produce 12+ feet of spark. The difference is that you have to drive the primary for a long period of time. You don't necessarily need more voltage or more current, just more drive cycles.

Killa-X

Sat May 28 2011, 12:07AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Well I ran my numbers on javaTC for estimation on how many primary turns I would need with my coil and it's topload. With capacative reactance and all calculated, it came to 2.4 so I just went with 4 turns.

My MMC is made of 1200vdc 1.2uF capacitors. 8 series 4 parallel for a 0.6uF 9600v tank capacitor as I calculated it off a 1000A system, so my capacitors will survive the voltage spikes as it runs. I need to rescope and see how many cycles I'm rubbing but so far it works, but I think I may have too many because I can get it to draw 15A and have thick sparks.

I have thought about maybe adding some metal mesh on the topload. One that isn't too small so its not really noticeable from a distance, but still combine to add that extra capacitance.

Well, This is what i found when i scoped the system but only turned to like 5VAC so it had enough to show me a waveform. This was a CT on the primary.

Means no sparks of course, but, just shows how many cycles I am running.

Arcstarter

Sat May 28 2011, 04:20AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

You're totally right Goodchild

What made me say that was because i try to run no more than 5 primary cycles, for no real reason other than power consumption (and therefor heat).

Killa, 15 amps at 50vac input and 8 primary cycles does sound high judging by how much my drsstc draws. I *just* killed my CM400 (i am so pissed and disappointed at myself for killing a 400a brick with 600a primary current) and everything still oscillates, but it draws some 10 times more current, and the primary current is cut in half. However, 8 inch sparks sounds about right for 50vac input into a doubler and a halfbridge, so that probably isn't the case.

Goodchild

Sat May 28 2011, 08:50AM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Killa-x what is the fres of your coil as calculated with Java TC?

The reason I ask this is because with a secondary and topload of that size your fres should be in the 30KHz - 40KHz range if I'm not mistaken. And also with only a 0.6uF tank you should be using many more primary turns.

Just to give you a comparison my big DR is also a 12 x 45 secondary with a similar size tube topload and a 0.54uF tank and my primary is tuned out at 11 turns as I stated previously. With a fres of about 38KHz.

2.5 turns just seem way to small for the parameters you have given. If this is true then tuning is most likely your problem as Steve said before.

Killa-X

Sat May 28 2011, 12:34PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

If I recall correctly, it said WITH the topload it would be 42Khz resonance. And with the MMC and all entered in, it claimed i would be safe around 2-3 turns, with 3/4" spaced coupling on the primary. But, If I do a lot more turns, I mean, What is your spacing then? 1/4 of an inch? Obviously I dont want to go too far up the secondary else I'll have to lift it to prevent primary-secondary flashovers due to over coupling. Doing a new primary isnt an issue if needed, The acrylic peices are just pressed together so I would just need to simply cut longer ones.

But, The amount of primary cycles, I take is fine? I think I was told 5-10 is about common for DRSSTCs

Goodchild

Sat May 28 2011, 06:07PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

OK let me break it down a little better.

Your tank cap is 0.6uF
Your Resonant freq is 42KHz

Using this data that means you need at least 23uH to run the primary at 41Hz, even more is need so that you can detune the primary for more power.

Your primary from what I can see from the photo is about 3-4 inches from the secondary making it about 15 or 16 inches in diameter, 1/4 tube along with 3/4 spacing give a primary inductance of about 4uH. This primary inductance with a 0.6uH tank cap give a fres of about 102KHz. Now these calculation don't take into account coupling effects, but with this much of a spread that is tiny.

So as calculated your primary is running at 102KHz way to high! I would suggest checking the frequency on your current waveform you posted earlier to verify what frequency your primary is running at.

With all that being said, I would suggest a flat primary, it is mush less likely for flash overs and toroid to strike rail sparks to happen. I use 1/4 spacing on my primary with a spacing of about 1 inch from from the secondary.

It is also good to take note that with wide secondary coils like yours you can run much higher coupling with out flashover or racing sparks. I run up to 0.2k on my big coil.

Here is a photo of the primary on my coil with a 12 x 45 secondary.
5271137079 03d0c9b014

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