Know thy Transforer

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Gerry Reynolds by way of Terry Fritz <teslalist-at-qwest-dot-net>" <gerryreynolds-at-earthlink-dot-net>

Hi All,

I have a wimpy transformer 3000V and 35ma that I've been using to learn 
more about coiling.  TC designers calculate the impedance of the 
transformer by computing the thevenin impedance Vout(oc)/Iout(ss).  For my 
transformer this works out to 85.7K ohms of impedance at 60 Hertz.  I 
figured that most of this is due to inductive reactance and some is due to 
winding resistance.  I measured the resistance to be 22.1K ohms.  I 
extracted the inductive reactance from the total impedance as follows:

      Zt  =  SQRT (XL^^2 + R^^2),  therefore

      XL = SQRT ( Zt^^2 - R^^2) ,  also

      XL  = 2 * Pi * Freq * L

where Zt is the total transformer impedance from above.  R is the winding 
resistance.  The inductive reactance calculated to be 82.8 K ohms (which 
accounts for most of the impedance).  This corresponds to 220 henerys of 
inductance at 60 Hertz.  The designer program predicted a resonant charging 
capacitance (Cres) of .031uf which, btw, simulated to be right on.

I set up a spice simulation representing the transformer with its 
inductance and resistance charging a TC primary capcitance Cp.  I wanted to 
find out what range of Cp would overvolt the transformer.  The range (with 
reduced Q due to the winding resistance) was much larger than expected and 
I thought this posting might help keep someone from destroying their 
NST.  I arbitrarily defined overvolt as larger than 1.5X rating.  To stay 
under this limit, Cp had to either be less than .01 uf or larger than .048 
uf, quite a large range considering that Cres = .031 uf.   I have no idea 
what the design margins for my transformer are,  but found that at the .013 
uf that I had been running it at, I could have been operating it at 2X 
rating (apparently it survived).

Conclusion:  determine the range of BAD capacitances for your transformer 
and avoid this range, or be very careful with your spark gaps.

Gerry R
Ft. Collins, CO