Signal Attenuation (capacitive pickup)

[John Davidson]

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...

 

[berkeman]

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?

 

[John Davidson]

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...

 

[John Davidson]

image

 

[berkeman]

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

 

[Tom.G]

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)