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Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
I made a simple frequency generator with a 74HC14 to tune various circuits with. When tuning secondaries for example, I connect it like so;
And when I sweep through I get an expected sine wave with massive amplitude, but another wave afterwards with a much lower amplitude. The sine wave seems a likely candidate, with resonant rise and all. But at resonance a series resonant circuit has the lowest impedance, so the signal should in theory be shorted to ground at resonance, therefor no signal.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
I've been playing with metering and detectors and a bunch of stuff for a few months in an attempt to come up with something that everyone can build. I've been looking for ideas and things, and my design is coming soon, but in the meantime I saw this which is really interesting:
I'm trying to get real numbers so I know exactly where the coil is in tune insted of coarse tuning methods like LED's. That will be coming hopefully in a few weeks if finals don't kill me.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Connecting it to the top of the secondary means that the probe capacitance will screw up the result. Also, the series resistor should be smaller than 50k.
If you're looking at the voltage at the bottom, the point of *minimum* voltage is the series resonance that you use in a Tesla coil. The voltage maximum is a parallel resonance that does nothing useful.
If you're holding a scope probe unconnected a few feet away to pick up the electric field from the top (the method I prefer) then you're looking for the voltage maximum.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Connecting it to the top of the secondary means that the probe capacitance will screw up the result. Also, the series resistor should be smaller than 50k.
Matt meant just to bring the probe nearby, so it won't disturb the secondary any more that it would normally be... and this worked for me well enough. One interesting thing you are mentioning, about parallel resonance...
If I tried to current-feed the secondary between it's ends, would parallel resonance manifest (due to inter-winding capacitance)? I somehow never thought about that.
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Interesting idea for you guys to try -- and no signal generator needed!
Plonk a secondary on a sheet of copper, and connect the base and the copper to the ground of your oscilloscope. Now connect the probe to the top of the secondary, and adjust the ranges on the scope until it triggers and you can see a filled sinewave on screen. (There should be enough EM noise around you for the coil to pick up some of it and act as a filter)
What frequency is it? On most of my coils, it tends to be around 1/3 to 1/4 of the resonant frequency.
It's not completely arbitrary, because A) it's always a fairly stable frequency, and B) seems proportional to the signal-gen measured Fres of the coil.
Can anybody care to suggest what this number actually represents?
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Resonance tuning - which waveform means actual resonance?
