# Re: NST question 30ma or 60ma not sure?

```Hi Max,

Original Poster: Max Erhard <max.erhard-at-softhome-dot-net>
>Couldn't you measure the open circuit primary current
>too and take it away from the shorted current, in order to
>exclude any losses ?

You COULD, but there is always the question if the xformer
acts in a linear fashion, when going from an open circuit to
a dead short. A MOT will certainly NOT act linearly. A NST
should, tho. However, this will tell you the V * A value of
the xformer, but it canīt tell you either the V or the A value
itself. You would need to know either the secondary(!) V or
A value and then back calculate the other value from there.

For coiling work (matching the primary cap), it is important
to know what current your array of NSTs will deliver. As
pointed out, a 15kV xformer may only deliver 14.5kV (and
a sign; i.e: in normal usage; doesnīt care). If you parallel
a fictive 14.5kV/30mA xformer to another (fictive) 15.5kV/
30mA xformer, your resulting output will neither be 15kV
nor will you achieve 60mA, simply because the NSTs will
be *fighting* each other. As they are current limited, they
will not be harmed, but you will loose some power in this
process.

Leslieīs question was how to determine if the NSTs are 12
or 15kV units (and if they are 30 or 60mA units) as they
had all labeling removed (warranty case; itīs cheaper to
ship back 100 tags, than it is to ship 100 NSTs back to the
manufacturer ;o} ). The problem is that 12 and 15kV NSTs
look pretty similar (sizewise), although it has been some 17
years since I last saw a U.S. NST ;o)). The method Scott
suggested will not help in determining if they are 12 or 15kV
units.

3kV difference is about the max I would combine in an
xformer array. This is still okay, because the psu in a
coiling unit should be (I think) viewed as a current source
and not as a voltage source. In coiling, you have two different
*states* of operation. One being the open circuit (cap not or
only partially charged and the gap not conducting), the other
being a short circuit (or almost at least. The gap does posses
some resistance). During the short circuit condition, we have
very little voltage. This is why we can indeed parallel (current
limited only !!) xformers of different voltage classes.

The only true way of finding out if they are 12 or 15kV units
(and what current rating they have) would be to measure
them, using a high voltage probe for the voltage, which Leslie
may be able to find at school, university, or at the local TV
repair shop (it must be an AC probe tho!). If you ask the
local repairman nicely during a time when traffic in his shop
is low, he would probably do it for nil.

Coiler greets from Germany,
Reinhard

```