NST current metering surprise

On Tue, 29 Jun 1999, Tesla List wrote:

> Original Poster: "Lau, Gary" <Gary.Lau-at-compaq-dot-com> 
> I finally got around to measuring my NST primary and secondary currents,
> with the help of a hamfest 20A AC ammeter and a borrowed 250mA AC ammeter.
> Briefly, my coil uses a 15KV/60mA NST.  Until recently, I had  been using a
> Today I metered the NST secondary current.  This is an unmodified NST whose
> short-circuit current measures 72mA (-at-140VAC).  With the .022uF cap, it was
> pushing a whopping 230mA!  Excluding power due to bypass cap discharging,
> that accounts for over 84 Watts, finally enough to explain the hot
> resistors.  When I went back to my .012uF cap, the current fell to 130mA.
> Regards, Gary Lau
> Waltham, MA USA

Hi Gary,

The 230mA sounds very high,  I tried various simulations and could not get
near this.  I did however get about 130mA with a 32kv gap setting with
12nF,  so then again the meter could just be right !

For a 110:15kv transformer,  230mA sec current would translate to over 30
amps at the primary.  I would expect that to take out a 20A breaker in
under 10 seconds.

You really need to scope the static gap to see what voltage it is firing
at, (carefully)  This will give you an insight into what is happening, as
it seems to be the only unknown factor ?  (You have measured everything

I saw your earlier post about performing scope measurements across a
centre tapped tranformer.  I was going to reply before,  but here goes...

I use a pair of HV dividers,  on the Y1 and Y2 inputs of the scope.  They
have a common ground,  and each one goes to one HV bushing of the supply.
The scope is set to show Y1-Y2 on the display.  I found I could get away
with one HV probe for Neon supplies but needed two when I started using
power transformers, to get a balanced trace.

Keep us informed.

						- Richie,

						- In sunny Newcastle