Water doesn’t like capacitive touch :(

One of the factors that affect the amount of capacitance generated is the characteristics of the insulating material. This is quantified in the form of a material dependence constant known as Permittivity (e); the higher the permittivity of the insulating material, the higher the capacitance of the resulting capacitor. Permittivity is usually expressed as a function of the permittivity of free space(e₀), and a relative permittivity number for the specific material(e_r). Glass and plastic have relative permittivity values in the range of 3 – 8, so they create capacitances that are 3 – 8 times greater than capacitors that use air as an insulator. Water, on the other hand, has a relative permittivity of 78.

This means that if water is covering a capacitive touch panel, and the user touches a sensor covered by the water, then the user’s finger will couple to every sensor covered by the water, almost as well as it couples to the sensor beneath the user’s finger. In addition, if the water also covers a ground, then the parasitic capacitance of the sensor will also significantly increase and couple potentially cause a false touch trigger.

When I touch a capacitive touch sensor, why do adjacent touch sensors react?

When you touch a sensor, you form a capacitance to ground that is a function of the area of the sensor, the characteristics of the insulating cover over the sensor, and the thickness of the insulating cover. You also form a capacitance to ground with any adjacent sensors, through the insulating cover. However, the effective area of the sensor is smaller because the path to the adjacent button is at an angle, and the distance between your finger and the adjacent button is greater as well.

This means that the amount of affect that a press will have, on an adjacent buttons, is determined the distance between the buttons, and the thickness of the insulating cover. To minimize this cross talk effect, separate the buttons as much as possible, and use the thinnest insulating cover possible.

capacitor basics

In a recent webinar, a number of questions came up concerning capacitive touch. I thought I would cover some of the questions and provide some insight into the physics of capacitive touch along the way in some upcoming posts. Likewise, if you have questions or topics that you’d like us to address, drop us a note.

Let’s start by reviewing a few capacitor basics. A capacitor is basically any two conductors separated by an insulator; the area of the conductors, the spacing between the conductors, and the characteristics of the insulating material, all combine to determine the amount of capacitance created. So, any two pieces of metal, which are not connected to one another, exhibit capacitance. The two pieces of metal can be two plates in a tuning capacitor, two cars sitting in the parking lot, or two bits of iron in adjacent red blood cells.

hat this means for capacitive touch is that every surface of an average human being is capacitively couple every other surface. So, if I touch a capacitive switch sensor, I am actually capacitively coupling the touch sensor, through my body, to ground beneath my feet. Or, in the case of a hand held device, I am capacitively coupling the sensor, to the ground of the circuit through my other hand that is holding the device.