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Re: Beginner Measurements

On Wed, 23 Oct 1996, Tesla List wrote:

> My questions are... Now hungry for more actual readings (instead of
> computer projections or forumlae) I want to know if there is a way to
> measure primary resonance with a VOM, Frequency Counter and Frequency
> Generator (old Heathkit 1Hz - 1MHz, 10V)? 

You can use the signal generator to drive your primary inductor and cap in
parallel and tune for the voltage peak.  Generally, no rewiring is
necessary other than possibly shorting your gap.  This works because the
primary impedence is maximum at the parallel resonant frequency (compared
to the impedence minimum you would look for while testing the secondary as
a series resonant circuit).

> Can one use measurement
> techniques to pinpoint the tap point? Is ther a way to measure the mH of
> a particular primary coil. Since inductance increases as one moves a tap
> along the primary and the resistance of the primary conductor increases
> too, wouldn't it be possible to somehow relate the two in an effort to
> measure resistance to infer inductance?

If you have a cap with a known value (Cp), you can use the above technique
to find the primary resonant frequency (Fp) at different tap points, and
then simply calculate the inductance given Fp and Cp.  The resistance and
inductance would be correlated (i.e. - R and L would increase at the same
time), but other than that, there would be no general purpose formula you
could use for any coil since the inductance would not necessarily increase
linearly as a function of primary "length" as would the resistance.

It would be much easier to simply measure the inductance as you go along
every turn or half turn on your primary and actually write the values on
your primary supports right where it was measured (tip from R. Hull -
thanks!). That way, you will have your primary coil all characterized and
ready to go.  Changing capacitors will be as simple as calculating the
inductance needed to resonate the primary at the secondary frequency, and
tapping your primary at that point (actually you would want a little less
inductance to account for capacitive loading on the secondary, but the
computed value will get you close enough to start tweaking).

An even simpler solution is to buy a handheld RLC meter.  They're nice and
simple, and used ones can ocassionally be found at hamfests in the $20 -
$30 range (or new for $50 to $70). I got mine for $20 by posting a
"wanted" ad on one of the internet newsgroups (rec.radio.swap I think).

> Therefore, the first major hunka' test gear for the coiler after a good 
> meter is a good signal gen.  A scope should be way down on the buying 
> list for a beginner.
> 
> Richard Hull, TCBOR

I would put a frequency counter right under a signal generator in terms
of usefulness and bang-for-your-buck.  That way, you can see that your
signal generator is really running at 437kHz when the dial says 500kHz.  I
picked up an old HP frequency counter (neat nixie tube display and all) at
a hamfest for $3!!!  It works up to a little over 1.5 mHz, then the
display goes to 0's.  I figure that it will work as-is for about 95% of
the coils that I'll ever build. I will eventually get around to dinking
with it to try to see what the problem is, but for what it cost me, I'm
more than happy.

Steve Roys.