Re: [TCML] Permanent Magnet GDT(?)

[Michael Twieg <mdt24@xxxxxxxx>]

On Tue, May 31, 2011 at 12:03 AM, Christopher Karr
<chriskarr4@xxxxxxxxxxx>wrote:

>
> Mike, all,
>
>
> I understand that the magnetic material is already close to saturation
> when fully magnetized, as in a permanent magnet, but can that nearness
> to saturation not be reversed by applying an opposition to the current
> magnetic field?
>
Yes, but I don't see how that helps you when designing a GDT.  In inductors
used for current in one direction, they are sometimes biased with permanent
magnets near saturation so that when current is applied it is pushed towards
saturation in the opposite direction, allowing the full BH curve to be used
(thus decreasing the necessary size of the inductor).  That's the only
application I've heard of in which permanent magnets are used, and it's
completely different from what happens in a GDT.

>
> Here's a small example to illustrate what's in my mind:
>
> If you bias a core in one direction with a constant DC voltage of 9V (let's
> neglect current, for our purposes), you get what is akin to a permanent
> magnet. If you add two windings, one a primary and one a secondary, you will
> have a total of three windings - 1 = Bias; 2 = Primary; 3 = Secondary
>
Actually you would want a current source to define the bias flux.  Biasing
with a DC voltage doesn't make sense, since that doesn't define any level of
flux (unless you account for winding ESR, which just complicates things).

>
> These are the theoretical voltages on each winding, with (2) as the
> independent variable:
>
> Zero degrees:
> 1 = 9V+               2 = 0V               3 = 9V+
>
> Ninety degrees:
> 1 = 9V+               2 = 0V-               3 = V0
>
> One-hundred-eighty degrees:
>
> 1 = 9V+               2 = 18V-               3 = 9V-
>
> Two-hundred-seventy degrees:
>
> 1 = 9V+               2 = 18V-               3 = 9V-
>
> (This is assuming a perfect square-wave on winding 2, no losses and no
> noise created)
>
I don't understand what you mean by "degrees".  Try describing operation
simply by defining the flux in the core as a function of time.  That, along
with the primary and secondary turns, should be enough to describe the
voltages that appear on the windings.

>
>
> I understand that I'm overlooking the fact that current is induced by
> change in flux and at 270 degrees there hasn't been a change in flux for an
> entire quarter-cycle, but my thought is that, given sufficient protection to
> reversals on winding 2, winding 3 would effectively average out the
> difference.
>
> Again I'm having trouble parsing what you're saying.  Maybe a picture would
be better?

-Mike
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