Why is the capacitance changing while measuring?

Question

I have 2 different 100nF capacitors. Let's call them A and B. My dad told me that B is "better" than A. I've tried measuring their capacitance using a Metex multimeter which has the option of measuring capacitors. I connected A and it showed 80nF and then it started slowly rising until it reached about 130nF. What's more, I've tried touching the capacitor (I was careful not to touch the terminals, I've touched only the coating). It influenced the result of the measurement - capacitance decreased. I thought that maybe it is related to the temperature but then I blowed warm air on it and it didn't change anything.

And then I connected capacitor B. It showed 100.5nF. I could touch it as much as I wanted and it didn't influence the measured capacitance. It was rock solid.

At first I thought that A is faulty but I have more capacitors of this type and all of them behave in the same manner. Why is this happening? Can I still use them e.g. as a Vcc buffer in an IC (like Atmega, etc.)?

Answer 1

Type A might be a Y5V

or a Z5U "semiconductor" capacitor. They're not only bad, they're horrible. Try to avoid ever using them. They're sort of acceptable sometimes as crappy bypass capacitors, but note that they pretty much disappear from the circuit if the temperature reaches extremes.

Answer 2

The high value ceramic capacitors are far from being ideal capacitors. Their capacitance varies with among other things temperature and level of charge. They are also "microphonic", that is sound waves can cause the voltage on a charged ceramic to change.

As Spehro says Y5V and Z5U are horrible and I would avoid them completely unless I was really trying to penny pinch.

X5R and X7R are bit better but still really only suitable for power bypass capacitors and the like, not for any application where stable and predictable capacitance is important.

For stable predictable capacitances in this capacitance range you want film capacitors or possibly C0G/NP0 ceramics. The downside is larger physical size and higher cost.