above the corner. Thus, the resistive divider sees
mainly the fundamental, while the capacitive divider
sees the higher frequency harmonics.
To generate the perfect square wave, the fundamental
and all harmonics must remain in the correct amplitude
and phase. One simple adjustment of the probe’s 15 pF
capacitor makes this a simple calibration.
The circuit you presented as Figure 7 on page 80
could be simply modified to generate a 1 kHz square
wave by replacing the 1 MHz crystal with an RC
These are fun things to do ... I really enjoy your
Harvey Mudd College
Typical Oscilloscope Probe
I believe you dropped a zero in answering the February
2004 question about the necessary filter capacitor size for
a 5 V power supply desired to have 100 mV ripple at 1 A
(“Power Supply Design 101”). The correct answer is
83,000 µF, not 8,300, as stated. Your formula is correct,
but your computation is in error.
You can do this in your head if you remember that, for
1 V of ripple at 1 A current, you need 8,300 µF. 1 V x 1 A
= 8,300 µF, which you can simply scale to other values. For
example, 1 V ripple at 0.5 A is 8,300/2 or 4,150 µF. The
constant 8,300 µF for 1 V x 1 A formula, of course, is only
for full-wave rectification at 60 Hz.
volt of potential. A capacitor of C farads with V volts
across its plates will contain Q coulombs of stored
charge (Q = CV).
The energy stored in a capacitor is equal to the
work done to charge it (U = 1/2 CV2). Where stored
energy U is in joules. When the plates are physically
separated, the stored energy is increased and so is the
potential (voltage), but not the charge (C), which
Where I tripped up was the energy of one electron
— which equals . 62 x 10-19 coulombs — is the reciprocal
of a coulomb charge. I’m sorry for the mix-up. NV
Your answer to the “Basic Electronics 101:
Capacitance” question in the March 2004 issue says that
the charge increases when you pull the plates apart. This
is not true.
The voltage increases, but the number of extra protons
on one plate and extra electrons on the other plate does
not change. When you pull the plates apart, you have
more potential energy, just like lifting a ball higher off the
ground gives it more potential energy. The charge on the
plates stays the same, as does the mass of the ball.
I fell into the same trap that many people do — I
unintentionally exchanged the term “potential” for
“charge.” By definition, charge is a number of electrons
measured in coulombs, where one coulomb (1C) equals
6. 24 quadrillion ( 6. 24 x 1018) electrons. Voltage — also
called electromotive force — is a quantitative expression
of the potential difference in charges between two
points in an electrical field. One coulomb equals one
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