Re: Output of Neon Sign Transformers (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 18 Jun 2007 09:39:24 -0500
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Output of Neon Sign Transformers (fwd)

Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Mon, 18 Jun 2007 11:27:14 +0000
> From: Nicolas Alstrup <n_alstrup@xxxxxxxxxxx>
> To: tesla@xxxxxxxxxx
> Subject: Output of Neon Sign Transformers
> 
> Hello,
> 
> I have found two identical neon sign transformers, that I am trying to build 
> a Tesla Coil Around, but I am experiencing some difficulties..... My main 
> problem is that I can't figure out my transformer output, and my MMC 
> capacitors keeps blowing.....
> 
> The Transformers are from Neoncomp, www.neoncomp.fi, type "N 50/8 RE" and 
> here is what it says on them regarding the secondary:
> 
> Us = 4000V - E - 4000V
> Is = 50 mA
> Pp = 520VA
> 
> On the homepage it is described as a 2x4000V transformer, but does that 
> means it gives out 8000V?

Yes.

> 
> I have parallel connected them, and beleive that should give a output of 
> 4000V and 100mA. Right?

No - you'll get 8 kV at 100 mA

> I have used 40 x 0.047uF 1000V capacitors, 5 in string, 8 strings 
> parralell.. This should give me 0,0752uF, and 5000V potential, but the 
> capacitors keep blowing... Any Ideas, thoughts regarding the transformers?

You are definitely overvolting the caps. Your NST's will output 8 kV 
RMS, which means that they peak output is is about 11.3 kV. However, 
when your main gap fires, the voltage will then reverse as the tank 
circuit "rings". To the capacitor, the rapid voltage reversal severely 
stresses the dielectric-foil boundary. The voltage reversal stress is 
almost like applying 2X the peak voltage, or 22.6 kV to the foil-film 
boundary. To avoid damaging or destroying your caps, you need to 
significantly increase the number you are using in each chain.

The number of caps in series per chain will depend on the type of MMC 
capacitor you're using. If your caps use foil-film construction, you'll 
want to design your MMC to have a DC rating that's 3X the transformer's 
RMS voltage (or, 24 kVDC for your case). The 3X safety factor takes into 
account the AC mains peak voltage, RF/ringing stresses, and the fact 
that foil-film caps don't gracefully self-heal if you overvolt them.

If you are using metalized polypropylene film snubber caps (such as 
recommended CDC 942C, WIMA FKP1, or Panasonic ECWH series caps), you can 
reduce the factor to between 1.5 - 2X the RMS voltage. This is because 
the caps will "self-heal", and dielectric failure won't cause an 
immediate failure of the entire cap. However, the lower the safety 
factor, the fewer hours of operating life you'll see before the caps 
begin to fail from progressive dielectric failures and corresponding 
reduced capacitance.

Also, make sure that your capacitors have a polypropylene dielectric 
system, and not Mylar or Polyester film. If the capacitor manufacturer 
says that they are simply film capacitors, they are most likely Mylar 
and they'll quickly overheat and die.

> 
> Thank you very much, sincerely Gryphon
> 
> _________________________________________________________________
> F� 250 MB gratis lagerplads p� MSN Hotmail:  http://www.hotmail.com
> 

Good luck and play safely,

Bert
-- 
***************************************************
We specialize in UNIQUE items! Coins shrunk by huge
magnetic fields, Lichtenberg Figures (our "Captured
Lightning") and out of print technical Books. Visit
Stoneridge Engineering at http://www.teslamania.com
***************************************************