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Registered Member #32
Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549
wrote ... Speaking of experiments, I notice that as I move my probe away the voltage I pick up drops. Theoretically speaking, is the drop I see with increased distance do to a resistance element that has to be added to the transmission model or is the capacitance between the HV line, the air and my pickup plate frought with voltage leakage?
You're talking about the capacitive coupling, right? When you move the probe away, the capacitance between the probe and the source goes down. Remember that capacitive reactance goes up as capacitance decreases. Your measuring device is some sort of load so the theoretical model is just your usual voltage divider: as the impedance of one part goes up, the voltage across the others goes down.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Test Screwdrivers of the type that need batteries and use a led, not the neon type, make excellent field-strength meters!
My Ignition Coil invertor wire will light the led in my test pen from about 1m My TC on low power sinewave testing can also light it from about the same distance, surprisingly sensitive.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Simon wrote ...
wrote ... Speaking of experiments, I notice that as I move my probe away the voltage I pick up drops. Theoretically speaking, is the drop I see with increased distance do to a resistance element that has to be added to the transmission model or is the capacitance between the HV line, the air and my pickup plate frought with voltage leakage?
Remember that capacitive reactance goes up as capacitance decreases. Your measuring device is some sort of load so the theoretical model is just your usual voltage divider: as the impedance of one part goes up, the voltage across the others goes down.
However, my measurement is across the resistor. The capacitor is made up of a plate that I move away from the HV line. Connected to the place is a resistor. My measurment is across the resistor. So the question is, how come the further I move away from the HV line does the voltage go down? Is it due to the field dropping as 1/r^2, a resistive element added in series to the existing capacitor/resistor, or something else?
Registered Member #32
Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549
It's still what I was talking about. You have a capacitor in series with a resistor. That's a voltage divider, albeit a frequency dependent one. Just like any other voltage divider, when the impedance of one element goes up, the voltage across the other goes down. Just as I said.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I've been walking around my basement with a makeshift, voltage-sensing device based on the capacitative coupling. I connected a piece of foil to a digital voltmetter and set it to AC. When I approach a live wire I can register up to 1-1.5v. If I ground one end of the probe I can pick up almost 5v. A dead wire picks up just about nothing.
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high voltage induction (capacitive coupling)
