In article ,
Doug White wrote:

Joseph Gwinn wrote in
:

In article ,
Winston wrote:

Joseph Gwinn wrote:
In ,
wrote:

(...)

That all looks inductively reactive to me.

Then there is problem with the spice model. More below.

Joe, I did the experiment in the real world just now.

You are right. With a resistive load on the secondary
I saw undetectable phase shift between voltage and
current on the primary. With an inductively reactive
load on the secondary, I saw the expected significant
lag in the current peak in relation to the voltage peak
on the primary.

That will be what I learned today and I thank you.

(...)

War story: Many years ago, I was interested in the physics of
xenon flash lamps. The physical model is pretty simple, a charged
capacitor discharging into an arc, which arc heated the xenon to
incandescence, the resulting light and heat radiation carrying the
energy away. This yields a set of coupled ordinary differential
equations that one solves numerically, time step by time step. I
got it all working, and all was well. Then I changed the duration
of the time steps, and more energy came out as light and heat than
was stored in the capacitor. Oops. Violated the conservation of
energy.

Turns out I had made two mathematical mistakes, which mistakes
cancelled one another only for the original step duration.

Wow!
That one must have had you scratching your head for a while!

It did. If only I could patent it, I could be rich and famous.
Boundless power source. I would have settled for rich.

More seriousy, models can be dangerous.

What's worse is a model in the hands of a freshly minted graduate who has
no common sense. Designs for things like high gain omnidirectional
antennas with 150% efficiency is what you get. They tend to get more
excited than suspicious, which is very depressing. They've been taught
that anything that comes out of an expensive computer program MUST be the
truth, and they have no horse sense to tell them otherwise.

I spend a lot of my time at work raining on kid's parades...

Same here, although I have young engineers not necessarily fresh minted.
Some people are born with common sense, some learn it (prompted by the
pain of accumulating scars), and some never acquire it.

The most effective approach I have found is to ask people to do simple
crosscheck analyses based largely on first principles. Like
conservation of energy.


Which elicits yet another war story: A now retired optical engineering
colleague of mine would evaluate requests for proposals by applying
conservation of (optical) energy to the requested optical system.
Typically, the minimum scene brightness, maximum allowed entrance
aperture size, and minimum image brightness would be specified, often
indirectly. Optical systems are passive (unless there is an image
intensifier), so the total energy in the image cannot exceed that
falling on the aperture. In many cases, the requested system was
physically impossible. The art is writing the proposal to explain and
solve this without tipping the competition off or embarrassing the
customer.


Joe Gwinn