Changing the plug direction affects capactive touch sensor

Thread Starter

HoodedEEE

Joined Jan 13, 2020
2
Hi there. I am a student in an European country and in my capacitive touch sensor circuit that I designed is not working properly when I reversely plug in the power cable. I tried to find an answer but I couldn't find it. If you can help me I would be very happy.

Thank you.
 

ci139

Joined Jul 11, 2016
1,666
EMI / interference -- from ... options :
• the Line(/Neutral) passing nearby conductive case / PCB traces
• the non homogeneous layout or asymmetry of "rectifier"/PSU components results in less more noisy PSU output
the power plug contacts differ by their resistance ??? shouldn't affect anything
• the input EMI FLITER is asymmetrical -- e.g. -- as if Line has a resistor in series --or-- as if the Neutral has the resistor in series
 

Thread Starter

HoodedEEE

Joined Jan 13, 2020
2
Schematic is kinda big for me to be able to share here, so I am sorry.

thanks for the answers, do you think there could be anything rather then EMI?
 

LesJones

Joined Jan 8, 2017
2,687
What method does the capacitive sensor use ? If it relies on the common of the circuit being at mains live potential then reversing the plug polarity would mean that the common was now at the neutral potential which is almost the same as earth potential, So when an something at earth potential came close to the sensor it would not see any voltage pickup. We are all only guessing as you will not post the schematic.

Les.
 

Sensacell

Joined Jun 19, 2012
2,582
The thing about capacitive sensors that you need to understand is that the sensing current from the electrode MUST return via another path, it's always a closed loop.
This is counter-intuitive, the return path is usually not explicit, but it's an essential part of the circuit.
The return path can also be entirely capacitive, but it must be there.

In your system, the return path is via the mains.

One leg of the power input has a different impedance to the sensing circuit ground than the other, reversing the plug alters the system impedance.

Solutions:

Use an additional earth ground return connected to the sensing circuit ground - or a direct connection to a large metal object - this will couple to "ground" via mutual capacitance.
Place a mains-rated capacitor across the power input - this will effectively "short" the input leads at high frequencies, which will make it plug polarity insensitive.
 

MisterBill2

Joined Jan 23, 2018
6,659
Hi there. I am a student in an European country and in my capacitive touch sensor circuit that I designed is not working properly when I reversely plug in the power cable. I tried to find an answer but I couldn't find it. If you can help me I would be very happy.

Thank you.
The answer to why the performance is not adequate is that the sensor is detecting the capacitance between it's probe and one of the mains connections, but not the other. That is the cause. The cheap fix is to add a capacitor between the two power connections. But that capacitor must be rated for AC voltage at your mains service voltage.
 
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