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Re: [TCML] Unexpected NST behavior


Most Acme NSTs (and most other manufacturer's as well) had the center tap of the secondary connected to the case.

Best way to rectify them is to use one single diode on both of the HV terminals, and the other connection to the case.  You only get 1.414 x 1/2 the rated nameplate voltage, but double the current capability.


On 7/26/2019 11:06 PM, 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
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