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wannabegeekTC

Fri Aug 06 2010, 08:10PM

Registered Member #3076 Joined: Fri Aug 06 2010, 07:45PM
Location: San Francisco CA
Posts: 18

Hi all....it's great to be on this site, I had a lot of trouble registering...as my vacation slips away I'm trying to
finish my first flyback tesla coil...

I've been reading deepfriedneon site and have some questions since that site assumes use of a NST or such device.

I haven't been able to get my secondary to spark. However I just re-read something that may help me a lot.
I need to find the resonant freq of my primary and tank cap (MMC) first, then compare to my secondary.

To test the freq of the spark gap, I tried using a single loop of wire going to a 10x Oscope probe and carefully got near the spark gap....
it seemed like I was getting pulses at 200 Hz only from my spark gap. I do not entirely trust this determination of freq as well.

gonna go and find res. freq today and report back and develop better questions.
thanks in advance,

wbgTC

wannabegeekTC

Sat Aug 07 2010, 03:28AM

Registered Member #3076 Joined: Fri Aug 06 2010, 07:45PM
Location: San Francisco CA
Posts: 18

I went to school lab and tried some stuff and took some measurements...

I have a 7555 driven flyback at 24KHz producing a nice arc. This arc is connected in parallel
with an adjustable spark gap.

The primary has been several different geometries, at the moment its a single diameter loop of 15 turns, about 80 mm dia.
Tank cap is an MMC equal to 25pF. Secondary is 500 turns ~60 mm diameter and 140 mm. This secondary
is an instructional lab model. I plan to wrap my own, since I cannot keep this one. Am considering using a 6:1 ratio when I wrap my own secondary.

I get no spark at sec. termials, but I did overload my crappy DMM on high Voltage setting-500 VAC, was maxed when connected to secondary output.

Is the geometry of my secondary bad ?
Is 25 pF not discaharging enough current?
I am not using a top load yet...I thought i would get a spark first then tune it more carefully.

Any advice is appreciated....
tia
wbgTC

Arcstarter

Sat Aug 07 2010, 03:33AM

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

How is an arc hooked in parallel with something?

Are you sure the tank cap is only 50pf? There's your problem.

wannabegeekTC

Sat Aug 07 2010, 04:07AM

Registered Member #3076 Joined: Fri Aug 06 2010, 07:45PM
Location: San Francisco CA
Posts: 18

hi Arcstarter...

I edited my post, so its actually 24pF....

As for spark gap connections: Sparkgap is in parallel with flyback outputs and the MMC and secondary are serial with the sparkgap...
I thought this was the standard connection ...?

I started to add up my cap values then decided to hook it up the the DMM and read it off...
40 caps in serial (guessing a 500V b/c 20 in serial fried quickly and 40 has not) each are 0.047 uF:
[1/0.047uF * 40]^-1 = 1.175 nF ~ 0.002 uF

so that is not 25pF ....what do you think ?

EDIT: found this page

I was also looking for the resonant freq wrong...close but not right ! With this forums help I hope to get this thing running before class starts gain and its all theory again....

wannabegeekTC

Sun Aug 08 2010, 11:33PM

Registered Member #3076 Joined: Fri Aug 06 2010, 07:45PM
Location: San Francisco CA
Posts: 18

Hi all,

hopefully someone feels like helping a newbie...

I remade my tank cap with a double row of 40, 0.047uF cap @ 500V ea

the DMM said I have btw 3-2 nF which seems correct with calculation.
The resonant freq of my primary looks like 310KHz. It is about 15 turns of a single diameter coil.
I calculate the inductance of this coil to be 87uH. I tried a bridge circuit as well and it was defintely less than 1.9mH.
I couldn't get a great match on the scope.

The res freq of my secondary, 50KHz.....

I tried adding a top load in the form of a sphere of aluminum foil diameter about 1 inch.
(over wooden ball) but that did not seem to make any difference in the res frq of the sec coil.

I am really confused, because I am wondering even if i get my primary and secondary at the same magnitude,
what about the spark gap ?

Any help[ is appreciated as i am running out of time for this project...
wbgTC

EDIT: I found Mads first tesla page using a flyback...I think my secondary has way too big of a guage wire. I didn't
wrap this myself, it was a loan, a classroom solenoid demonstration thing...

Waiting for 50 0.1uF caps @2KV in the mail....I am going to copy Mads design....hope you don't mind...

dmg

Sun Aug 08 2010, 11:58PM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
Location:
Posts: 627

well, yeah, you will not get any decent results with capacitance in the picofarad range, also, if you have not said alraedy and I missed it, what are your capacitors made out of? (ceramic? polypropylene?)

wannabegeekTC

Mon Aug 09 2010, 12:11AM

Registered Member #3076 Joined: Fri Aug 06 2010, 07:45PM
Location: San Francisco CA
Posts: 18

hi Gated...

I am using ceramic 0.047 uF 40 in serial and 2 in parallel...

i ordered 50 0.1uF 2KV ceramic coming Monday....

dmg

Mon Aug 09 2010, 01:51AM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
Location:
Posts: 627

Oh boy, Ive never had any luck personally with ceramics, upon some of my first coil (also flyback), Ive used both TV style ceramic capacitors and disk, all with explosive results, and when they didn't blow up, they where pretty lossy and had poor performance.

perhaps can you provide us a picture of these kind of capacitors (search web for a picture of these capacitors) please?

Thank you sir.

Xplorer

Mon Aug 09 2010, 08:55AM

Registered Member #2416 Joined: Sun Oct 04 2009, 04:23AM
Location: Oceanside, CA
Posts: 91

Are your Primary and secondary coils wound in the same direction?

Yeah it's likely the capacitor. Shoot for at least .01uf, but it's hard to say for sure if you don't know the voltage and current of your flyback.

I know mylar caps arent reccommended. But I've had fairly good results with these. If it doesn't work, well at least they're cheap.

-Tony

quicksilver

Mon Aug 09 2010, 04:27PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

The Mylar "gum-drops" can work.....you should try to get something a bit bigger that 600v but that's not my point. Those Gum-drop caps are actually free with a wee bit of hunting & scrounging. - IF you're going to spend any money; get the the good ones.

The caps that WILL pay off (and are worth spending money on) are in the enclosed PDF.
Check these out and then try to get a good deal on enough to make a nice MMC. I've had some last for a great length of time and total cost was about as much as a decent lunch.
]best-deal-cap.pdf[/file]

Actually I have had one MMC that is still going strong & it's 3 yrs old and been somewhat abused (with the quality caps). I have a small coil that I made with the Mylar caps that has given me no trouble but I don't over-work it: keeping the operating time low & paying attention to specs of what voltage should be, etc, etc.
Just my opinion, but if you're going to try Mylar caps, over-build the MMC.

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