[Elecraft] CAPACITOR VALUES

Sun Nov 23 08:54:04 2003

Hi John,

All capacitance meters have an offset in their reading that depends on the
test fixture and the leads.
On some meters, the offset reading can be set to zero by pushing a button,
otherwise you can subtract it from each measurement.  This stray capacitance
will be included with the large cap values too, but it will be less
significant.

It looks like your stray capacitance is about 20pf, which is a reasonable
value and it can be subtracted from any cap reading to get an accurate
result.

After subtracting 20 from each value in your data table, there is still a
scale factor error of 1.5X.   Since you are getting good results in the
1000pf to 10000pf range, maybe it's only the scale of the lowest range that
is affected.

You can still measure small caps pretty accurately by taking the offset and
scale factor into account.

73/ Bob - AG5Q

----- Original Message ----- 
From: "john.pam" <[email protected]>
To: <[email protected]>
Sent: Sunday, November 23, 2003 1:43 AM
Subject: [Elecraft] CAPACITOR VALUES

Encouraged by the book to do so, I checked all the capacitor values.   I had
just purchased an autoranging DMM with a capacitance measuring feature (the
first time I have ever owned such a device) and began to go through the
packets.   All the monolithic caps checked out fine, so did the
electrolytics but, when I turned to the NPO caps, I began to get very
strange readings from most of them.

For example:

    1 pf    read     21 pf
    10pf    read    35PF
    12pf    read     38pf
    20pf    read    51pf
    27pf    read    62pf
    33pf    read    70pf
    39pf    read    77pf
    47pf    read    90pf

    and so on up the scale ad nauseum.

My first inclination was to think that either my DMM was faulty (this was
its first job, after all) or that Elecraft had tracked down a supply of dud
parts (unthinkable).   However, a little reflection reveals that all the
capacitors of a any given value gave the same meter reading and that, as the
marked values increased, so did the indicated values,albeit by a factor of
around 2:1.   All of which leads me to suspect either the DMM or the way I
am using it.

Can anyone help this greenhorn?

73

John   ZL3LI