Re: Resonance The hook up
Tesla List wrote:
> Original Poster: RWB355-at-aol-dot-com
> Hi Philip, all
> You wrote
> " I used my scope and funtion gen. Fed sine wave intp base of coil and
> hooked scope to other end of coil. I increased frequency until wave form
> grew in voltage. That I assumed was the resonance point. Is this wrong?"
> YesNo. The idea was okay. However your setup is incorrect. First of all, you
> do not want to "connect" your scope to the coil. Doing so will load it down
> (and give you the results you saw: 234kHz vs. ~400khz should be). Try
> this way:
> a.) Build yourself a pickup antenne. A 50 cm piece of wire is fine. Connect
> this wire to a 50ohm coax cable (to the hot wire). The ground wire stays
> Place this pickup about 1 yard away from your coil. I justed taped mine
> ceiling. Worked great!! If you do tape it to the ceiling, be sure to stay
> from electrical wiring in/above the ceiling, as this will give you false
> results. Connect a 50 ohm "trap" across the scopes input to prevent false
> readings as a further measure and use BNC connectors throughout, too.
> b.) Place your secondary (no primary, tank circuit, etc.) coil on
> raise it off the floor. Also keep the area (at least a 3-4ft circle) around
> your coil clear of everything (esp. metal parts). You will be amazed as
> sensitive your coil is to anything near it. Esp. if it is a high Q coil.
> During my Q-Testing coming any nearer than about 2 ft to the coil let the
> scope trace (amplitude) fall flat on its face. Try it on yours by just
> your hand near the coil, once you have found FRes and you will see what I
> c.) Connect the bottom of your coil to the hot wire of the signal generator
> via a 50 ohm coax cable. The sig gen ground stays disconnect, too.
> d.) Adjust your sig gen (by trying) until you get roughly near FRes. You
> notice a large jump in output voltage on your scope. Now readjust your
> V/div and time base until you have a single waveform on the screen. The
> amplitude should be slightly below the last divisions on your screen. This
> step isn´t really necessary, but it lets you see changes easier and "back
> calculating" the FRes from the scope picture is easier, too.
> e.) Very slowly increase the frequency until you have max amplitude. If you
> have a f-counter you can read it directly, otherwise you will have to "read
> it" from the scope.
> f.) Now you can add toploads and see what they do to your FRes.
> The above setup works for the secondary coil ONLY. If you are interested in
> finding the primary´s FRes, let me know.
> Really measuring your FRes (yeah, I´m a "hands on" guy) will aid you in
> your coil, because you will be closer to the sweet point, even before you
> start sparking. I.e. it is easier and quicker knowing you are in the "range"
> than to wildly start at some tap point on your primary and having to go
> down several turns until you hit it on the nose. :o)
> Hope this will get you more accurate results.
> Coiler greets from germany,
I'd add a few things. First of all, unless your signal generator has
very low output impedance, you may very well see a dip in the generator
output voltage as you tune through resonance. I generally use an RF
transformer with a 10:1 stepdown ratio between the generator to reduce
it's output impedance, in order to make the measurement of Q by the
Fo/BW method. Second, you can probably use a much smaller pickup
device. I use a small plate, of the order of 2" square, and place it
near the bottom of the coil. Plenty of reading with the stuff I use.
Third, to measure the resonant frequency of the primary circuit, connect
the capacitor and coil in parallel, and feed them from the generator
through a resistance which is a few hundred times the square root of
L/C. You'll get a peak of voltage at resonance. In this test you can
hook the scope directly across the LC circuit, as the impedance is very
low compared to the input impedance of the scope.