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Tank caps not doing there job. Should I fire them?

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Ubern

Wed Apr 08 2009, 06:01AM

Registered Member #2069 Joined: Wed Apr 08 2009, 04:21AM
Location:
Posts: 2

Hi, Im having a problem with my primary circuit and its led me to question something about this project. I am using an MMC made of .011uf 1600vDC 400vAC caps. Now when planning this project I read that you can us the DC rating of these CAPs instead of the AC rating (I could not find a reason why but did it just the same to save some money on the MMC).

Right now Im getting no spark at the spark gap when the MMC is connected (my NST is rated for 10kv 30ma). Each string in the MMC had 20 CAPs each for a breakdown voltage of 32000vDC, 8000vAC. With no spark at the spark gap and no voltage drop across the spark gap i naturally thought that the breakdown voltage of my CAPs was too low and they where shorting through.

*So my first question is: Can i use the DC rating for the caps? And, if i can, why?

*My second question will take another paragraph or so to explain, but the gist is "can you please make sense out of this"

To keep it simple i removed the NST, spark gap, and MMC and placed them all in parallel. We have tried a variety of configurations with the MMC but only 2 are relevent here.
1. MMC made of 60 CAPs in series to make the voltage breakdown as high as possible(remember, all i want at this point is to see the spark gap fire with the MMC in the circuit). the result was no spark at the spark gap but we did get about 100v across the spark gap.
No noticeable voltage stored on the MMC after tests

2.MMC made of 20 CAPs in parallel with the variac set to produce 1kv out of the NST. No spark out of spark gap and no voltage drop across spark gap. About 200 volts stored on MMC after tests (we later doubled the # of caps in parallel to see if this number would change....it did not)

The MMC NEVER got hot. Even if we just let it sit there running with no spark at spark gap for 10 mins. Also i chose the caps from a list of "good capacitors for MMCs", they are made by Philips Comp and the MFG P# is 222237682113

*so does "question" 2 make any sense to anyone? What am i missing?

Thank you for the help
Brandon

Plasma Lover

Wed Apr 08 2009, 06:07PM

Registered Member #1911 Joined: Mon Jan 05 2009, 06:30PM
Location: Salem, Oregon, USA
Posts: 165

Hello Brandon,

Did you try adjusting the spark gap?

You said that you read 100V across which, which makes no sense. First of all, why would you put a meter on a circuit with potentials reaching larger than 10KV? Second, why were you that close to an operating primary circuit to begin with? Are you trying to kill yourself? Remember - all it takes is ONCE.

Do you have resistors across your MMC? If so, what are their ratings, and is there one per capacitor in parallel, or some other arrangement?

Pictures would definitely help with a diagnosis.

Looking forward to your reply,

Christopher

Ubern

Wed Apr 08 2009, 07:08PM

Registered Member #2069 Joined: Wed Apr 08 2009, 04:21AM
Location:
Posts: 2

Hi Christopher.

First, thank you for your reply. I appreciate your concern for my safety, and i dont think its important for me to get into all the details so ill just say that Me and my project partner each have 6 years experiance with HV in a nuclear power plant (red tape and all) and have taken necessary precautions (and understand the danger of being too comfortable with this stuff).

We have tried adjusting the spark gap. The spark is only visible when the two contacts are almost touching and the spark is similar to what you see when you short out a walky-talky battery (basicly a quick fizzle). This is consistant with what i would expect to see when shorting something with a potential around 120v. Now here is where it is strange, we set it up so that we could disconnect the MMC manually while it was running. Doing so will result in a spark forming at the spark gap (the type of spark i would expect from a high voltage source). Reconnecting the MMC returns the voltage to 100v.

Keep in mind that the CAPs never even get a little warm. They are mounted on a square PC board and the resisters have been removed so we could see what kind of a charge they are holding (never more than 250 volts). We are 100% certain that we did not accedently wire in a short of some kind and have changed every component of the MMC out at least once (including the PC board). The results are always the same.

(original MMC resistor configuration was 10Mohm resistor across each CAP)

Thanks,
Brandon

Herr Zapp

Wed Apr 08 2009, 08:07PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Ubern -

Something is obviously VERY wrong here, so you need to methodically perform some very basic tests to isolate the root cause.

1. First, verify that your HV power transformer is truly an iron core transformer ("NST"), and not a high-frequency, solid-state, "gaseous tube power supply". A NST will be relatively heavy for its size, the nameplate will state "transformer" somewhere on it, and will also be marked "mid-point grounded" or some similar verbiage. A "solid state gaseous tube power supply" is NOT suitable for driving a conventional spark-gap Tesla coil.

2. Second, after you have confirmed that it is truly an iron-core NST, then use a multimeter to verify that full AC line voltage is present at the NST's input terminals when you apply power to it. This will confirm that your variac is connected correctly, and is delivering power to the NST's primary.

2. Third, disconnect your spark gap and MMC from the NST output terminals. Connect a short length of stiff, solid-conductor wire to each HV output terminal, and form the wires so there is about a 1/8" gap between the free end of each wire and the NST's metal case. Power up the transformer, and there should be a continuous solid blue arc between the end of each wire and the NST case. This will indicate if each "leg" of the NST secondary is generating full output voltage.

Let us know the results of these tests, and we'll proceed to the next steps.

Regards,
Herr Zapp

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