Dan Michaels wrote:
>
> Roman wrote:
> .......
> >> 150 = ???? [guess]
> >> 220 = ???? [guess]
> >>
> >> Anyone who guesses 15 and 22 pF for the last two, I'll send you
> >> the caps, and you can measure them.
> >>
> >> - danM
> >
> >
> >I guess 150pf and 220pf for the last two, it is common
> >in very small caps (<999pf) just to label the pf value,
> >hence the 10=10pf etc you show above. Did I mention
> >our workshop has 3 capacitance meters, and two ESR
> >capacitor testers? Who trusts the written label? ;o)
>
> Yeah, who/what can be trusted at all anymore? I put my sine
> generator to the cap marked 220 thru a 10K [5%] resistor, and
> adjusted freq until the voltage across the cap dropped to 50%
> on the scope. Using Xc = 1/(2*pi*F*C) gave me a value of 130 pf
> --> :). I was hoping for something ~250 = 220+scopeprobe_cap.
>
> I then took another cap marked 270+/-10% and did the same. This
> time I got a value of 160pF --> :):).


Hi Dan, do you have a frequency meter, you know,
usually about 7 digits and can switch for
freq/period?

My favorite cap tester is home made, its more
accurate for very small caps, I just used a
hex inverter oscillator (very simple!) and run
the freq meter on "period" and adjusted the
resistor so that it reads correct. I made it
when I was about 15 and it has worked well
for 20 years. It's inside the freq meter box,
(which was a kit one), with it's own switch and
two short leads to keep lead capacitance down.
Since then I have purchased dedicated units,
but nothing measure sub 100pf parts like mine,
and with 7 digit accuracy, good for "seeing"
caps drift off value with heat, time, etc. :o)
The commercial ones have leads too long and
are never good for very small caps. I let the
apprentices use them.
-Roman

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads