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Re: Secondary Q's and magnets

I was playing with my scope last night characterizing my latest 
secondary (900 turns of 24 gauge wire on a 4" diameter PVC tube) and I 
measured the steady-state Q to be 32 by finding the delta-f that gave 
.7x the maximum output voltage .  I also noticed that f0 measured from 
the maximum current into the base was 435.25kHz, but f0 measured by 
observing the secondary voltage peak using a scope probe pointed at 
the coil was 438.42kHz.  The ~3kHz difference was signficant (to me at 
least) and consistently reproduceable.  So what gives - why does the 
voltage peak occur at a frequency higher than the frequency that gives 
the highest base current?  Is one method more correct than the other?

What, if anything, should the probe ground be tied to when monitoring 
the secondary voltage?  I started out using an aluminum pie tin as an 
antenna hooked to the probe so I could monitor the secondary voltage, 
but I was picking up so much 60Hz noise that the trace looked like a 
solid band of fuzz.  By not using the plate and just pointing the scope 
probe at the coil from a distance of a foot or so, I got a reasonable 
trace, but it still had a small 60hZ modulation to it.

Finally, a few comments on large magnetic fields.  A typical MRI 
system that you would find in a hospital has a field on the order of 1.5 
Tesla, and a static magnetic field, even of that magnitude, is generally 
considered safe.  The problems that people have are more in the way 
of loose magnetic objects getting sucked into the bore of the magnet 
and damaging people simply by becoming projectiles, embedded 
magnetic objects being torqued around during a study (magnetic clips 
in the body, etc...), people having their credit cards wiped and their 
watches magnetized so they don't work any more, and things like that.  
I've heard that with higher fields (5 to 10T or more), you can shake your 
head and "taste" the eddy currents induced in your tongue, but I'm not 
aware of any inherent dangers of static magnetic fields of any strengths 
that you would be exposed to outside of some secret government lab.


Steven Roys (sroys-at-radiology.ab.umd.edu)