[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: How to Tune a Flat Spiral Coil



Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

Dave,

Connect things up as

             Resistor
 [HP465A]----\/\/\/\/\/----------o> to outer coil lead.
     |        100 ohms         |
     |                    [AC V meter]
     |                         |
     ----------------------------o> to earth connection

leaving the center terminal unconnected - I'm assuming here
that you intend to ground the outer end(s) of the coil during
operation, so that the HV is developed at the center.

Start at a low frequency and sweep slowly upwards, monitoring
the AC voltage until you see a pronounced dip.  Note the
frequency at which the voltage dips lowest.  Then continue
sweeping upwards, to find the next resonance (won't be quite
such a low dip).  Continue until you've found the lowest
three resonances.

The primary should be in place during these tests, because its
presence affects the secondary self-capacitance, but the 
primary should be open-circuit, with one end grounded.

Make sure that there are no stray test leads, or anything else,
near to the center terminal.

The 100 ohms resistor is not a critical value, try anything from
a few tens of ohms up to a Kohm or two,  whatever gives a 
decent dip on the meter.  Too big a resistor and you won't get much
of a reading at all.  Too low and the dip will be very shallow.

You won't be able to rely on the 4.54kHz reading because the
flat coil's capacitance is being swamped by the added parallel
capacitance of the HP465A output and the voltmeter.  The above
configuration almost eliminates that effect and give a good measure
of the Fres to be expected in normal operation.

> what I'm saying is that if I have a 35mH coil

You've mentioned 35mH a couple of times recently - where does this
come from - I thought you had 6.23mH or thereabouts?  Are we talking
about the same coil? 

> I have found that the primary capacitance is directly coupled to
> the secondary inductance.

In a way, that could be a reasonable statement.  It all depends on 
the coupling coefficient (k-factor) between your primary and
secondary.  If it's very high, then both coils, along with their
various respective capacitances, will just combine their effects into
a single resonance, rather than the two needed for a conventional TC.

Tell us about your primary: dimensions, turns, etc, and its position
wrt the secondary.

> This is a significant discovery in flat spiral coils

Whoa, there's a long way to go before you can claim any such thing!
--
Paul Nicholson
--