Avionics News October 2019 - 68

series
B Y

T H O M A S

I N M A N

ICE
L

Figure 1

ast month in this column,
we examined inductors in an
alternating current environment.
In March, we looked at capacitors in a
direct current environment. You may recall
our mnemonic: ELI the ICE man. For
capacitors, the principles were relatively
simple and can be summarized as follows:
1.	 When voltage is applied, current
will flow before the capacitor
will begin to charge and drop
voltage (ICE).
2.	 If connected in a circuit, current could flow for
a moment after voltage is removed.
3.	 Capacitors in parallel add to a total
capacitance.
4.	 Capacitors in series equal the reciprocal of the
sum of the reciprocals.
When a capacitor is placed into an AC circuit, one
and two above are of particular interest. As discussed
in June, the voltage in an AC circuit is changing
constantly. Like inductors, AC applied to a circuit with
capacitors complicates matters.
Capacitors are constructed with plates, which have
no direct contact; therefore, capacitors have infinite
resistance. The application of voltage will cause the
capacitor to charge, which means current will flow. In a
DC circuit, the current flows only until the capacitor is
charged. At that point, no more current flows, and the
circuit behaves as if it is open.
68

avionics news

*

october

2019

As stated in No. 1 above, any time voltage is
applied to a capacitor, there is a lag before the device
begins to drop voltage. In June, we examined how
an AC waveform can be divided into 360 degrees.
Figure 1 shows a voltage curve and a current curve
in a capacitive circuit. Notice there is a lag of 90
degrees before a voltage drop is shown. As shown in
Figure 1, after the 180-degree point, the current flow
is in the negative direction, while the voltage is still
on the positive side of the scale. Furthermore, at the
270-degree point, the current is zero, but the current is
at peak negative.
Figure 2 shows the net result of the offset between
current and voltage. As the capacitor discharges and
recharges in the opposite direction, the current flow
will induce a voltage, which is out of synchronization
with the voltage applied. As in inductive circuits, we
call this counter electromotive force, or counter EMF.
The capacitor is actively impeding the current flow by



Avionics News October 2019

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