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Distinguishing ceramic from film capacitors electrically?
mcovington:
I see that several microcontroller data sheets recommend ceramic bypass capacitors, presumably because of their lower ESR.
I have a few dozen 0.1-uF capacitors and don't know which kind they are. They are blue and plastic-encased. Obviously the usual green or red film capacitors are easy to distinguish from tan ceramic ones, but these aren't either one.
Is there a practical electrical test to tell me? (Do ESR meters read low enough?) Or should I cut one open and look at it under the microscope?
Kleinstein:
There are quite different types of ceramic and film caps. The good thing here is that the main ceramic type used for decoupling is class 2 ceramics like X7R and similar. These capacitors are relatively easy to identify: there capacitance is quite a bit voltage dependent. With film caps the capacitance is essentially not effected by a DC voltage.
So one could build a simple oscillator (e.g. 74HC14 + resistors + cap) and see if the frequency changes with voltage at the cap:
It should be something like <0.01% change for a film cap and some >1% change for X7R if a reasonable voltage is applied. 2 Caps in series and a high resistor to apply the voltage (e.g. 0 V or 1/2 the supply) should do the trick.
The main reason they suggest ceramic is that these are small and cheap - if small enough it would be OK to use a film cap too.
Yansi:
You are not probably going to distinguish C0G/NP0 ceramic from a decent foil one - easily. ???
But what might give, is if you'd stick the cap to a VNA, to measure the impedance. Foil type shall have higher parasitic inductance. But still - in some cases such difference may not be that high.
//EDIT: Also, comparing the leakage current may lead somwhere (ceramic should have higher) and dielectric absorption. Polystyrene shall be superior.
nfmax:
Even simpler - most ceramic capacitors around the 100nF mark will use a dielectric like X7R that is strongly temperature sensitive, i.e. not C0G Pop one in the capacitance meter and monitor the value (not the ESR) as you warm it up. If it decreases significantly as the temperature rises, it's ceramic!
bob91343:
One of the distinguishing features of ceramic disc capacitors is the low ESL. This results in a high self resonance frequency, important when working with high speed components.
A dis ceramic capacitor is basically a slice of ceramic with leads bonded on the faces. If done well, there is very little opportunity to create inductance.
One should also realize that ceramic can be piezoelectric, causing circuit noise when subjected to movement.
Multilayer ceramic capacitors are a different animal.
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