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On 04/01/2015 01:52 PM, Timothy Gilmore wrote:
> Jon,
>
> I bought 15 of the fast blow 250vac 20A fuses online as they were not found
> locally in 30mm sizes. I like the idea of the slow blow and possibly going
> up to 25 or 30 amp. I did also notice the blown fuses I had were pretty hot
> when removed which makes sense. I would also assume that raising the fuse
> rating might also blow the 15A (I'm assuming) outlet the variac is plugged
> into. Don't know.
> Sounds a bit scary to short together the 15 kv outputs of the NST to
> measure the current of the 120vac input. Sounds like I should put a 10M ohm
> resistor inline with the input to read the current from also.
NSTs are output current limited devices that are designed to run a
low-resistance arc 24/7. The high voltage of a NST secondary is only
used for the initiation of the arc; once the arc is establish, the NST
functions as a current source to keep the arc going. It is actually
healthier for an NST to run with the outputs shorted than it is with the
outputs open (open outputs = high voltage = corona and other breakdown
effects).
When you short the outputs, they are no longer at 15kV, they will be at
0V (well, close to 0V, wire resistance notwithstanding), but there will
be a 60mA current flowing through them (or 120mA with both of your NSTs
in parallel). This current is limited as a function of the design of
the transformer, by including inductive shunts in the building of the
transformer; some manufacturers incorporate these shunts as part of the
E/I laminations, some use separate shunts that can be removed when the
transformer is unpotted.
Here is a picture of a transformer with the shunts labeled; in a typical
NST, the shunts would go from coil to coil:
https://i.stack.imgur.com/Ziakg.jpg
Jon
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