Tab Electronics Guide to Understanding Electricity ... - Sciences Club

Capacitance
151
Many of the more expensive DVMs are specified as measuring true
rms. If you are using this type of DVM to measure the ripple content,
the indication you’ll obtain will be the true rms value. The majority
of DVMs, however, will give an accurate rms voltage measurement of
sine-wave AC only. Referring back to Fig. 5-5, note that the ripple waveshape
is not a sine wave; it is more like a “sawtooth” (you’ll learn more
about differing waveshapes in succeeding chapters). The point here is
that there can be some error in the ripple measurement you just performed.
High accuracy is not important in this case, but you can
experience circumstances in the future, where you must consider the
type of AC waveshape that you are measuring with a DVM, and compensate
accordingly.
As an additional test, I used a 100-ohm, 25-watt resistor to apply a load
to the circuit. If you have a comparable resistor, you might want to try
this also, but be careful with the resistor; it gets hot.
The resistor is connected across each capacitor, and the subsequent AC
and DC voltage measurements are taken. The loading effect was practically
identical between the positive and negative supplies, which is to be
expected. The DC voltage dropped by about 3.1 volts, and the ripple voltage
increased to about 135 mV. These effects are typical.
Food for Thought
Throughout this chapter, I have followed a more traditional, and commonly
accepted, method of teaching and analyzing capacitor theory.
I suggest that you continue to comprehend capacitor operation from
this perspective. However, in the interest of accuracy, you will find the
following story to be of interest.
Michael Faraday, the great English chemist and physicist, had a theory
that more closely approaches the way a capacitor really works. His
theory stated that the charge is actually contained in the dielectric
material—not the capacitor’s plates. Inside the dielectric material are
tiny molecular dipoles arranged in a random fashion. Applying a voltage
to the plates of a capacitor stresses these dipoles causing them to line up
in rows, storing the energy by their alignment. In many ways, this is
similar to the physical change occurring in iron, when it becomes
a temporary magnet by being exposed to magnetic flux lines. When a
capacitor is discharged, the dipoles flex back like a spring, and their
energy is released.