TE>Subject:  Tuning

TE>From wesb-at-VNET.IBM.COM Mon Mar 27 12:46 MST 1995

TE>  If you're wanting to measure AC current through a wire, then the lines of
TE>magnetic flux from the current must pass through the loops of wire in your
TE>sensing coil. Since flux lines are concentric around a wire, they are not
TE>passing through your loops, but are running parallel to them. If you wrap yo

TE>sense coil around a plastic toroid, and then run the ground wire through the
TE>toroid opening, you'll have the geometry you're looking for. You may still
TE>need some sort of amplifier, and wether or not your meter's frequency respon

TE>is limited or not may just depend on the meter, but rewinding the coil would
TE>seem to be the place to start.

TE>Wes B.

I've done this sort of thing. It does work. I wired in a diode
bridge using 4 IN4148 diodes and a smoothing capacitor. It
probably wasn't truly RMS but it gives an indicator of relative
magnitude. I needed to use the 50 microamp meter scale to get a
measurable reading. My pickup coil was many turns (1000's) of
fine wire.

The HF currents are very fickle when you begin to measure them,
there are many blind alleys which you can go up in search of a
peak just to find that if the wires move a bit everything
changes. I still reckon that length/strength of spark is the best
indicator of tune. Thats assuming you have designed the whole
thing properly.

The tuning peaks can be fairly narrow as well, it's easy to
miss the peak tune by going across it. An oscilloscope and signal
generator is probably the best way, rather than tuning using a
"live" coil and meter.

Jim Oliver <jim.oliver-at-welcom.gen.nz> (3:771/370)

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