A fried xformer

SSNSanders-at-aol-dot-com  Writes: 

> Tell me someone: I hooked my 15kv neon { new } transformer leads to
each lead 
> of a  36000v .05uf capacitor for a little while, until I heard the frying 
> noise then removed it. The plates are seperate; It wasent a dead short, I 
> could understand it then but I was just charging the cap and timing the 
> duration of  time to the degree of loss to see how much charge was lost over 
> different time periods. Why would this burn up a transformer, why; why why 
> why why why!     Stephen Sanders.              


A few transformer basics are in order, here. First of all, all transformers 
have some leakage flux, that is, some magnetic flux from one winding that 
doesn't get through the other winding, but instead loops around through 
empty space. This manifests itself as series inductance in the windings. 
In ordinary power transformers, this is a Bad Thing, but neon transformers 
make use of this property to get some big inductance, almost for free,  
that's useful in neon signs where these things are intended to be used. 

Neon signs need a big voltage at almost no current to break down the  
gas in the tube, then a sustained current at a much much lower voltage 
to cause the gas to emit light. This can be done by putting a series  
impedance in the secondary of a high voltage transformer. The series 
impedance produces a voltage drop that allows the neon tube to find 
its proper operating voltage. If that series 
impedance were a resistor, it would dissipate extra power, reducing the 
efficiency of the system. But if that impedance were inductive, we could 
get the extra voltage drop without the power dissipation. This requires  
a very big inductor; much larger than the series inductance that normally 
appears in a power transformer due to leakage flux. 

To accomplish what's needed, the designers of the transformer simply  
increase the leakage flux with a carefully designed "magnetic short  
circuit". An extra leakage path is built into the core of the  
transformer which causes a suitably large inductance to appear in 
series with the secondary winding. This leakage path is typically part 
iron and part air gap, precisely dimensioned to produce the series 
inductance required to drive a neon tube. Actually, there's also a  
fairly large series resistance due to the resistance of the secondary  
winding, but the series inductance is many times greater. Together,  
they (usually) limit the current that can be obtained from the  
transformer, and (usually) make it short-circuit proof. You've found 
the circumstance where the protection doesn't work. 

The current rating for the neon transformer, typically a multiple of 
30 mA, is the current you get when you short the secondary. This  
should be able to be drawn continuously without damaging the  
transformer. But a problem happens when you hook a capacitor across 
the secondary. 

The AC impedance of a capacitor is 180 degrees out of phase with the 
AC impedance of an inductor. This means that they will at least  
partially cancel one another when placed in series. (At resonance, 
they will completely cancel, leaving only the winding resistance,  
but you've most likely just canceled out a fraction of the inductive 
impedance.) The consequence of this is that by connecting in that 
capacitor, you've decreased the effective inductance that would have 
protected the transformer, and drawn more current from the secondary 
than would have been possible with just a dead short. The maximum 
allowable secondary current was likely exceeded, and the transformer  
probably just overheated. 

I'm sorry to hear that you damaged a new neon transformer; that's  
gotta hurt the wallet. If you want to replace it in a more economical  
fashion, visit the neon sign shops in your town and ask them to  
save you their used transformers. When a sign is damaged, they  
usually replace the whole thing, and the transformer is not the  
weak link in a sign made of glass. The transformers often end up in 
the dumpster, even though they still work fine. I tend to visit on 
Saturdays, when only the "worker bees" are around, and negotiate 
with them. While some owners may be reluctant to sell high voltage 
equipment to some stranger off the street who wants to do bizarrre 
"Frankenstein Experiments" with them, the workers will be glad to  
sell you their trash and make a few extra bucks on the side. I get 
my transformers for 5 bucks apiece.  

Take care, & I hope this helps. 

Wes B.