[R-390] Replacement Capacitors

[Charles P. Steinmetz]

Bob wrote:

>There is an interesting tradeoff with the otherwise "really really 
>good idea" ceramic caps. Even if you go to the X7R dielectric (which 
>you should), their leakage is higher than most plastic parts. It's 
>not something I would worry about, there's also leakage from dust 
>and that spider web there. I don't believe the insulation resistance 
>leakage is going to mess anything up, either with the ceramics, or 
>between varieties of plastic.

As you say, nothing to worry about.  The starting leakage of ceramic 
caps is much too high to have any practical effect in bypass 
applications.  As long as the leakage does not increase over time, as 
it does with paper caps, there is no problem.  All "Class 2" disk 
ceramics (even those with lesser specifications than X7R, e.g. Z5U) 
have a well-proven track record in this regard.  If a radio has been 
underwater for a long time (months to years), the ceramic caps may 
get leaky enough to pose a problem.  Otherwise, they are very reliable.

I agree that X7R is the dielectric of choice for supply bypass 
capacitors, given a choice.  The reason to prefer X7R over other 
Class 2 ceramic caps is that they hold their capacitance value better 
at high temperature, over time ("aging effect"), with higher applied 
voltage, and at higher frequencies.  For example, an X7R cap is 
specified to be within 15% of its 25C value from -55C to 125C, while 
a Z5U cap is specified to be within +22% and -56% of its 25C value 
from 10C to 85C.  An X7R cap may decrease in value by ~10% over time, 
while a Z5U cap may decrease in value by ~20% over time.  However, 
for real-world use as a supply bypass in boatanchors, one can offset 
these effects by using two Z5U caps in parallel in place of an 
X7R.  (One Z5U of double the value should also work, but will bring 
the self-resonant frequency ("SRF") down by about an octave compared 
to the two smaller caps in parallel.)  The leakage of X7R and Z5U 
caps of a given value and voltage rating is approximately the same, 
and is entirely negligible in supply bypass applications.

Best regards,

Charles