B Signal Attenuation (capacitive pickup)

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The discussion revolves around detecting an electrical current through a conductor using a capacitive sensor that is not in direct contact with the circuit. The students observe that the signal is stronger when the conductor is connected to only one electrode, but it diminishes or disappears when the circuit is completed. They are attempting to sense a short impulse of 2 volts over 0.45 milliseconds from a battery-powered generator. Suggestions include using a current-sensing transformer for AC or pulsed currents and a Hall sensor for DC components, as well as considering the grounding and capacitance effects in their setup. The importance of relative voltage measurement and connection adjustments is emphasized to troubleshoot the signal detection issue.
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We have a question we cannot understand…

We are university students working on a pet project, we are trying to pick up an electrical current passing through a conductor via a capacitive sensor (i.e. attached to a charge amplifier with filters) that is not in direct contact with the circuit.

The signal we detect seems to be larger when the conductor is in contact with one electrode only.

As soon as the circuit is completed (we connect the conductor at both ends) the signal we detect on our sensor is either attenuated or disappears altogether…

The signal source is a battery powered generator with no direct connection to the sensing circuit. We are simply trying to detect an electric field.

The signal is about 2 volts over 0.45ms

Does anyone know why this is or what is happening?

Any help really appreciated...
 
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Can you post a picture or a diagram? Given what you have said, it doesn't sound to me like it will work. Is this an AC current or a DC current that you are trying to detect capacitively? And are you really just trying to capacitively detect a voltage (change) and infer that a current is flowing?
 
HI, I am really new to all this so forgive my lack of knowledge but we are trying to capacitively sense the occurrence of a short impulse (0.5ms 1.5V pulse) through a conductor. The key principle is that we are trying to pick up the pulse with a sensor that is not in any way part of the circuit that the pulse is present. Instead this is a proximity capacitive sensor.
I know I have probably not explained clearly this image is a real basic image of the problem...
 
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Well, if you want a non-contact way to sense an AC or pulsed current, you can use a current-sensing transformer. If you want a non-contact way to detect currents that include a DC component, you would usually do that with a Hall Sensor based probe.

If you want to sense an AC or pulsed voltage, you can do that with capacitive pickups. You can either use a differential capacitive pickup (like with one probe pad on each side of the load that is dropping the voltage), or you could do it with a single-ended probe pad with respect to the ground in the circuit (assuming one side of the load is grounded and you are probing the non-grounded side of the load).

https://en.wikipedia.org/wiki/Current_transformer

https://en.wikipedia.org/wiki/Hall_effect_sensor#Hall_probe

https://en.wikipedia.org/wiki/Test_probe
 
Try swapping the output leads of the Generator; i.e. the Generator lead you are disconnecting from "Conductor" should remain connected and the lead that is currently remaining connected gets disconnected. Do not change the positions of any parts of the experiment, change only the connection and and disconnection operations.

Remember that a voltage at a point is always a value measured relative to another point. I suspect that the Charge Amplifier "Ground" reference has some stray capacitance to some part of the Battery - Generator - Conductor system. (maybe even if someone is touching or near some part of the experiment)
 
So I know that electrons are fundamental, there's no 'material' that makes them up, it's like talking about a colour itself rather than a car or a flower. Now protons and neutrons and quarks and whatever other stuff is there fundamentally, I want someone to kind of teach me these, I have a lot of questions that books might not give the answer in the way I understand. Thanks
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