Re: [TCML] Measuring secondary current

[Steve Ward <steve.ward@xxxxxxxxx>]

Hi,

On Thu, Aug 30, 2012 at 4:47 PM, Andreas Hahn <andreas.hahn@xxxxxxxxxxx> wrote:
> How do you accurately measure the current in the secondary's ground wire? My
> current method suggests my secondary is putting 2800 amps into my RF ground,
> which seems a tad high for a coil powered by a two amp transformer.
>
> Here's what I'm doing right now.
> I wound ~110 turns on an FT 50-77 ferrite toroid (rated to a few megahertz)

Sounds good!

> and calibrated it at 2mV/mA with 14.7k ohms of resistance between the output
> leads, using my signal generator to put some current into a 1 ohm resistor
> via a wire through the center of the toroid.

Sounds BAD!

First, 14.7k? thats wayyyyy too big of a resistance to burden
something like this with, and your maths are just wrong.  Lets say you
put 1A through one loop on the toroid, this will produce 9mA on the
output, so with a 14.7k resistor this is then 9mA*14.7K = 132.3V.  So
the ratio as designed was 132V/A.

>
> Feeding the ground wire from my secondary through the center of the
> newly-minted CT and turning on the coil, I put my oscilloscope probe across
> the 14.7k ohm resistor (actually a 10k and 4.7k in series) and fired up the
> coil.
>
> The result is a nice 5.6V peak-to-peak sine wave at the ~500kHz resonant
> frequency of my secondary...

Suggests 42mA, also not realistic for any SSTC that makes sparks.  The
14.7k burden resistance is all wrong in practice.

My advice, start over.  Ive made many many CTs that calibrate against
professional CTs up to a few MHZ just fine.  No tweaking, just do it
right.  Here's what i recommend:

For measuring 1A and less, i use 10 turns on a ferrite toroid (type 77
is good, anything with mu around 2000-10000 is usually good, dont use
EMI suppression types).  Burden it with 10 ohms to yield 1V/A, or 1ohm
for 100mV/A.

For medium currents (up to say, 100A), id suggest 50 turns on the
core.  With a 5 ohm burden, this is 100mV/A, or a .5 ohm burden gives
10mV/A.

For real big currents (up to say... 2000A), i wound 200 turns on a
pretty big core (1" ID), and burden it with say 2 ohms for 10mV/A, or
.2 ohms for 1mV/A.

Also, you MUST take care to keep leakage inductance down on the output
side of the CT.  I always terminate my CTs with the shortest
connections possible, especially between the burden resistor and your
RF connector of choice (i use BNCs so it connects straight to an
oscilloscope).  If you leave big loops of wire exposed, they can
easily pick up signal, which will contribute not only magnitude error,
but phase error to the measurement.

If you must use a loooong BNC cable, its wise to try and match the
output impedance from the CT with the 50 ohm impedance of the cable.
You can do this quite easily by putting a 50 ohm resistor in series
with the CT output before the BNC connector.  You might also need a 50
ohm terminator at the scope, which will change your CT output ratio
(should generally cut it in half).

Hope that helps!

Steve

>
> (I just calibrated my 'scope this morning, too...)
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