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Re: [TCML] Measuring secondary current



On 8/30/12 2:47 PM, Andreas Hahn 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.

Just a bit...

to a first order, look at what the peak current is in the primary.. The absolute highest it can be is determined by the energy stored in the capacitor.. So Lpri*I^2 = Cpri*V^2

And then, as a worst case (highest current), ALL that energy gets transferred to the secondary, so

Lsec*Isec^2 = Cpri * Vpri^2...

Isec = sqrt(Cpri/Lsec) * Vpri

Here's what I'm doing right now.
I wound ~110 turns on an FT 50-77 ferrite toroid (rated to a few
megahertz) 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.

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...

So 5.6/14.7k = 0.38 mA

110:1 turns ratio implies about 408mA secondary current...

How does that compare with an energy calculation

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