RE: Dimensions of my flat spiral coil

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "David Thomson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dave-at-volantis-dot-org>

Hi Jim,

>Can you measure changes of <0.5% reliably?  I am reminded of the problem
with inexpensive DMMs... It may have 3.5 digits on the display, but only be
accurate to 5%.

I'm using a BK Precision model 875A LCR Meter.  According to the spec sheet
at the 20mH setting that I used for the coil, the test condition is 1KHz,
100uA and the accuracy is +/-(2%rdg + 1dgt).

>Off hand, the relative values between windings are probably more accurate
than the absolute value, especially since they are all about the same value.

I believe the difference is due to the condition I stated earlier.  The
three outer wires terminate at 120 degrees from each other so this means
their lengths are slightly different.  Had I terminated all three wires at
the same point, I'm confident I would have three identical inductances.
This is why I brought the point up, btw.  Just in case someone else builds a
three wire coil and needs three identical inductances, then they should
terminate all three wires together.

>What frequency are the measurements being made at?

Noted above.

>What sort of surroundings and what was the test lead configuration (the
test leads themselves could easily have 10 uH of inductance, or more
important, the uncertainty in the test lead inductance is on that order.

The surroundings were clear of any obstruction for a four feet radius.  Then
there are some electronics equipment such as TV and VCR, 10 feet away is my
computer.  The walls and ceiling in my work area are covered with space
blanket, more for the insulation value than for the Faraday value.  But
there is an aluminum coating in the space blanket.  The insulated test leads
are 10" long and came with the unit.  This said, none of these factors would
account for consistent differences in readings.  I have also measured the
exact same readings while the coil was in different areas of the room.  I'm
sure the difference are due to wire length differences.

>A measurement uncertainty of 10-20 uH would be on the order of the
differences you are seeing.

Only if the measurements produced random results among the wires.  But a
specific wire has a specific inductance and it is the same regardless of
where the reading is taken.

>Not to be nit picky here, but I've gotten "beat up" in reviews for
seemingly simple measurements where all these effects weren't at least
quantified, if not measured.  Precision metrology is much an art, and
remarkably painstaking, even for simple measurements.

In this case, it is clear about the accuracy.  And I believe the wire length
differences is an adequate explanation for the different inductance
readings.

>Interestingly, if you hook the windings in parallel, since they are very
tightly coupled, I'd ballpark the resulting inductance at 9 times that of
the single winding, or around 50-60 mH, just on the N^2 principle.

Well, let's try that and see what we get.  I hadn't thought of that.

Wow, the mystery deepens.  Since the three outer leads were not long enough
to reach a common point, I had to make a copper ring out of a 30.5" bare #12
wire.  At first, the ring was split, that is I didn't connect it such that
it was a complete loop.  I connected the positive lead to the terminal and
the negative lead near the closest outer lead, but on the copper ring.  The
inductance reading was 6.12mH.  Just to make sure there was not a problem
with inductance in the copper wire, I connected it to the other end of the
copper ring and the inductance was 6.18mH.  I double and triple checked,
removed all the wires and put them back on and I still got the same
readings.  Just for fun I connected the negative clip to the center of the
ring near the other coil lead.  The reading was 6.16mH.  I suppose this
means I can use the ring for tuning my primary circuit!

Just out of curiosity I made the copper ring a complete circle.  Starting in
the front of the meter and measuring the midpoint between outer leads and
then the adjacent outer lead, next midpoint, next outer lead, etc., I
received the inductances of 4.92mH, 4.90mH, 4.92mH, 4.90mH, 4.92mH, and
4.90mH successively.  The 4.92mH readings were on the midpoints.  After I
finished the readings and went back to double check (about 1 minute,) all of
the readings were 2mH lower than the first reading.

Taking readings again 2 minutes later, the measurements were a total of 4mH
lower than the first readings.  Taking readings 2 minutes later and the
readings were once again only 2mH lower than the first readings.

In all cases, the proportion of midpoint to lead readings remained near
constant while the actual readings as a whole varied.

To make sure there were no stray induced voltages I measured for DC and AC
between the terminal and outer leads, and between the outer leads and
midpoints.  There were no stray voltages of any kind.

When I measured for internal capacitance by using the same configuration as
the open ring above, I got the same capacitance measurements on where each
lead connected to the ring.  But the capacitance reading continually
fluxuated between 126.6pF and 128.0pF.  The same degree of fluxuation
occurred at each point.

When I measured for internal capacitance with the ring connected in a circle
the capacitance stabilized a bit more with a range of 127.2pF to 128.3pF.
When measuring the internal capacitance directly between each outer lead and
the terminal, the results are pretty much the same as being connected as a
group to the ring.  The capacitance of each individual lead fluxuated from
127.4pF to 128.9pF.

To sum it up, the self capacitance of the coil is the same whether all three
leads are used together or individually.  However, the inductance decreases
significantly when all the wires are connected together as opposed to being
used individually.  Further, the inductance varies along a wire that
interconnects the three outer leads.

These are unique properties that may be able to be exploited.  Essentially
this is implying that the resonant frequency of the coil can be changed on
the fly.  Using the inductance range of 4.90mH to 6.25mH and the constant
internal capacity of 128pF, the coil can be oscillated within a range of
200.96KHz to 177.94KHz.  Since you asked the question, I'll share this
discovery with you, Jim.

Dave