Re: Secondary Q's and magnets
Subject: Re: Secondary Q's and magnets
From: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Date: Wed, 13 Dec 1995 13:38:03 +1200
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Steve Roys writes.....
> 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?
My reading of this is that minimum loading is obtained by the
"dangling probe" technique. I am a champion of this since it
introduces the lowest loading of any sort on the system. Using
resistance in series with the coil is going to introduce a error
that increases with resistance because the "standard" resonance
formula (1/2PISQRTLC) is actually an approximation that assumes
coil resistance to be negligible. The probe pickup is the method I
am using for the measurements I am currently doing as I have reason
to think it is the most accurate. HOWEVER, it is IMPORTANT that the
generator used have a very low output impedance and that this
impedance is resistive. A reactance in the generator is also going
to introduce errors by making the coil look more inductive/capacitive
than it really is. The generator I am using has clocked in at 7+j0.
The smaller a coil is, the tighter the test gear parameters have to
be because of the low inductance/capacitance of these systems.
> What, if anything, should the probe ground be tied to when
> monitoring the secondary voltage?
I use a foot or so of wire and suspend it as far away as allows me to
get meaningful readings. Fluoro lamps, computers etc. in the vicinity
should be switched off and the Y amp should be cranked for minimum
sensitivity as far as getting accurate readings allows. It also helps
to have a generator with a high output. The graticule lines you use
for measuring should be centred on the centre of the "fuzz" to get
the correct readings.
> 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.
A foot away is really too close. Try moving it another foot away and
check where the resonant frequency has shifted (it should be higher
reflecting reduced capacitive loading). Your Q should also improve
I settled on these methods after numerous experiments to
determine the most accurate that I could find.