[sdiy] more about ceramic disc caps

[jh.]

> This is really dumb, but how do I read the values on ceramic disc caps?  I
have a slew that are marked
> differently from anything I have seen before -- "56J" "680J" -- I
understand the J is the tolerance rating, in
> this case 5% -- but is this 56 pf?  56 uf?  I'm confused on the units, I
guess.
>
> Also I have some taht are marked "120GMV ; 1-3MEG ; L" and "0015M ;
Z5F" -- I'm confused on units
> and just this general mess.

Normally expect ceramics to be in the pF and nF range.
I guess somebody will bring up a good link to a web site that explains it
*all*.
However, I think that "680" can be both, 68pF or 680pF, so you have to
measure
it. There is a convention that "xyz" means xy * 10**z pF, but if z is a "0"
it can also be just the number in pF. So 221 and 220 can both mean 220pF,
but 22pF can be coded as 22 or as 220.

As for capacitance measurements, in the days before affordable multimeters
with C range, I had this diode tester that worked with 1kHz or so
(CMOS oscillator, 2 LEDs, LED shows anode. Works with extra resistor
and two switches for transistors as well.)
When I had a capacitor and was in doubt about the range, I used the diode
tester and checked how bright the LEDs would glow. I remember this
was possible down to 100nF in daylight, but taking the diode tester under
the blanket, I could indeed tell a 220pF from a 22pF. I thought that was
a great "discovery" back then (;->) - but what remains until the present is
a deep respect for the dynamic range of Vactrols !

JH.