Re: Success Re[TCML] Measuring secondary current

[paul <tcml88@xxxxxxxxxxx>]

Andreas wrote:

> JavaTC gives 53 ohms for the secondary DC resistance ...

> (V^2 / R = very high).

> Come to think of it, that can't possibly be right.
> V^2 / R = 8300V^2 * 53 ohms = 1.3MW

The top V is applied across the series combination of the
resistance and the 48k reactance, so the loss resistance only
'sees' a small fraction of V.

Simply calculate dissipation using I_rms^2 * R, then you have

   140mA^2 * 53 ~= 1.04 Watts

Really, you should use the AC resistance instead of the
DC resistance, which takes account of the skin effect and
proximity losses.  These arise because the magnetic field
squeezes the current into an area somewhat less than the the
cross-section of the wire.   The AC resistance might be 3 or
5 or more times the DC resistance, so you're probably warming
up the wire with at least 3 to 5 Watts.

There will be other losses too - through the magnetic and
electric field coupling the resonator to its surroundings.
From your Q measurement you can determine the total effective
loss resistance,

 R_loss = 2 * pi * F * L / Q_factor

R_loss is the series combination of the coil's AC resistance,
any other resistance in the circuit such as shunts, bulbs, and
an effective resistance representing losses in the surroundings.

It is this total loss resistance that limits the secondary
Q, and thus sets the top voltage that you can achieve for a
given input power.    This contrasts with impulsive TCs which
start with a fixed 'bang' energy and the voltage is ultimately
limited to V_pri * sqrt(L_sec/L_pri) and there, secondary Q
is only a concern if it is seriously low.

To the CW coiler, Q factor is a major consideration and
the secondary design comes out quite different to that of a
traditional TC:  a smaller height/diameter ratio, thicker wire,
and fewer turns, maybe with a little spacing between turns to
reduce proximity loss.

--
Paul Nicholson
--
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