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Re: [TCML] Determining a transformer's voltage and current

Gary and all,

Part of the problem (with NST's at least) may be due to the internal current limiting shunts which reduce the effective coupling coefficient. This will cause the measured voltage ratio to be somewhat different than the actual turns ratio.

Vs/Vp = k*(Ns/Np)

However, I suspect that the root cause is actually a change in core permeability at low driving current. The permeability of silicon steel cores (and other ferromagnetic materials) is a function of the magnetomotive force (MMF) which is proportional to the winding excitation (in ampere-turns). At very low or very high driving currents, the core permeability drops off dramatically.

A low core permeability allows more flux to leak through the air instead of linking through the other winding. This dramatically reduces the effective coupling between primary and secondary windings at either very low and very high driving voltages. See Figure 2-2 to see the change in permeability versus MMF for ferromagnetic materials (from McLyman's Transformer and Inductor Design Handbook):


Most iron-core transformers are designed to operate in the region around peak core permeability. When the HV secondary is back-driven from a much lower voltage source, the MMF is a small fraction of its normal level. Since the effective coupling coefficient plunges versus its normal level, the measured voltage ratio is similarly reduced (per the above equation). This effect will occur in all transformers with ferromagnetic cores, including NST's, MOT's, plate transformers, and pigs.

A more accurate approach might be to drive the LV side from a small variac and then monitor the secondary voltage using the HV setting on a DVM or a cheap analog meter. Or, use a simple resistive or capacitive HV divider and a high-input impedance DVM. Driving the LV side at 10% of its normal operating voltage should provide sufficient MMF to make the core happy, while reducing the output to a lower, and safer, level.

Bert Hickman
Stoneridge Engineering, LLC
World's source for "Captured Lightning" Lichtenberg Figure sculptures,
magnetically "shrunken" coins, and scarce/out of print technical books

Gary Lau wrote:
I've had very misleading results when trying to determine the turns ratio
by energizing the secondary with a known line voltage.  This has been
discussed in the past and I'm not sure that an underlying cause or
workaround has been identified.  Anyone else?

Gary Lau

On Wed, Apr 20, 2016 at 7:27 PM, Ed <evp@xxxxxxxxxxx> wrote:

I strongly suggest a modification to this test, and that is to feed line
voltage into the secondary and measure the resulting primary voltage.
Safer and no chance of damaging the  voltmeter.


On 4/16/2016 2:52 PM, Kurt Schraner wrote:

There are just 2 fundamental tests, which allow the determination of most
importent transformer parameters: The open circuit test, and the short
circuit test. In Jim Lux' HV-book you find a nice description of all what
is needed; please click the following link:


Regards, Kurt

-----Ursprüngliche Nachricht----- From: Brian Hall
Sent: Friday, April 15, 2016 8:44 PM
To: Tesla Coil Mailing List
Subject: [TCML] Determining a transformer's voltage and current

I posted a question similar to this a couple weeks ago, but it was a bit
cluttered or may have just gotten lost in the shuffle - there were no

To simplify:
Given a 120v 60Hz primary high voltage transformer (looks like it could
have come from a bug zapper or similar device), but there is no plate or
markings indicating the HV output voltage, current, or manufacturer:

1) How could I determine the output voltage?
I got a cord and wire nuts, and hooked it up to 120v 60Hz AC to get a max
spark of 1.8mm between the sharp ends of two bolts. What is the scale in mm
for room temperature (20 degrees C) and still air on that max spark gap?

2) How could I determine the output current?

I have a variac and some decent power resistors, if putting in say 10 or
20 volts on the primary side, then a resistive load on the hv side would
help along with Ohms Law for finding the current on the HV output.
Just wondering if anyone else has encountered this and already knows how
to find such answers, and has advice on how to determine with some degree
of accuracy the HV side voltage and current values of an unmarked

Brian Hall

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