In the product I analyse (an optical fork sensor, rated 10V-35V), there is a sizewise big capacitor between ground and chassis.
I measured its value with an LCR meter, it is 60nF. I also broke one by accident, which revealed a liquid from inside. Looking at its size and considering the liquid inside, I think it is a film capacitor.
Questions:
1) Why is there a capacitor between ground and chassis? For EMI purposes? (Note: the cables used for this product are usually not shielded - chassis is floating if there is no capacitor)
2) Why is the capacitor a metallized film type? (if it is) It seemed to me like a ceramic capacitor may also have been used in this scenario - it has a smaller footprint.
Answer
The sensor is a PNP output with 2x potentiometers, I presume as coarse and fine with act as some optical comparator with a load resistor to 0V for measuring an optical gap. In noisy environments a metallize film cap shunts Stray RF noise between 0Vdc and the metal body to improve SNR from injecting noise with your hand.
The hands can inject high impedance line noise picked up easily by the body. thus cap shunts this line noise to 0Vdc.
Film is often the best choice for very low ESR and big enough to reducing errors from handling.
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