Thanks Dave! It is an important lesson that needs to be said more often!
A good rule of thumb to choosing ceramic capacitors:
- Don't pick anything worse than X7R if you can help it,
- Look for minimum double the voltage rating you need, preferably triple,
- Design your circuit to tolerate twice the capacitor's rated tolerance, to account for manufacturing, temperature, voltage and aging (+/-30% or so for X7R?).
I did this plot some time ago, because I felt like it:
- Random "203Z" ceramic disc from the junk box, in series with a much larger cap; the common node was biased with a large resistor (>100k?) to the voltages shown.
- Capacitance was measured by resonating with a known inductor, I think it was around 100kHz (varying, of course) and 1V rms.
- The capacitor was sitting around for at least ten years, so likely had aged since its original salvage. I annealed it by heating each leg with the soldering iron (set for 350C) for 10 seconds, then measured again.
Would it be possible to do one of these videos for magnetic components as well? The effect is, after all, exactly the same -- ferroelectricity and ferromagnetism are two sides of the same electromagnetic coin!
My pet peeve: ferrite beads. I ask you: what's the point of a 10A ferrite bead when it saturates at 100mA? Nothing, that's what! Even if you dig through their database programs (Kemet, TDK, etc. all have their things), you rarely find bias data for ferrite beads. If you need DC filtering, don't look at ferrite beads, look at the "inductor" listing -- they make multilayer ferrite chip inductors, outwardly identical to ferrite beads, that are just as cheap and plentiful, but actually perform correctly.
Tim