Hi Steve,All NST's that use two identical secondary windings connect them in series. Electrically, the pair of windings behave as a single center-tapped secondary. The "center tap" is always connected to the core and case, sometimes through a current-sensing Ground Fault Interrupter (GFI) circuit for newer NST's. Small coil and core NST's below 5 kV sometimes use a single secondary winding. In this case, the end of the secondary winding that's nearest to the core is electrically connected to the core.
However, the higher output voltage from the FWB is a bit surprising. The face plate output voltage is when the NST is open-circuited. Once the gas tube fires, the voltage drops to 1-2 kV. Are you certain you were powering the NST with the face plate primary voltage? There's always some voltage drop in a transformer due to winding resistance. Perhaps Acme compensated for this by boosting the number of secondary turns.
Also, you'll see higher output voltage from a bit of resonant rise if the HV outputs are directly connected to a small capacitive load. However, its not clear how resonant rise would occur if you have rectifiers in series with the NST outputs circuit. One of life's many mysteries I guess... :)
Bert Steve White wrote:
I am building a project using a NST. The NST is an old-style one with an iron core. It is an Acme Electric rated for 12 KV at 30 ma. What seems unusual to me is that I can draw an arc from either HV terminal to an earth ground. This is with the case floating so there doesn't appear to be a complete circuit to allow this to happen. Can anyone explain this? I want to connect one of the HV terminals to an earth ground (after rectification and filtering) and leave the case floating but this odd behavior is preventing this. When I try to do this, the NST appears to be shorted to ground even though the case is floating. I tried a different NST and got the same behavior. I also notice that when I measure the voltage with no load, I get a DC value of 20 KV. This is with a full wave bridge rectifier and a 400 pF filter capacitor. I have read that NSTs are designed to initially output a higher than normal voltage to start a neon tube. Is this what I am seeing? I expected to see about 17 KV DC (1.414 x 12000). Steve White Cedar Rapids, Iowa _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxxxxxx https://www.pupman.com/mailman/listinfo/tesla
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