ELECTRONIC PRINCIPLES - OD1647 - LESSON 1/TASK 1
dielectric. A highvoltage capacitor that has a thick
dielectric must have a relatively large area in order to have
the same capacitance as a similar lowvoltage capacitor having a
thin dielectric. The working voltage also depends on the
applied frequency because the losses, and the resultant heating
effect, increase as the frequency increases.
A capacitor that may be safely charged to 500 volts dc cannot be
safely subjected to an alternating voltage or pulsating direct
voltage having an effective value of 500 volts. Since an
alternating voltage of 500 volts (root mean square, rms) has a
peak value of 707 volts, a capacitor to which it is applied
should have a working voltage of at least 750 volts. In
practice, a capacitor should be selected so that its working
voltage is at least 50 .percent greater than the highest
effective voltage to be applied to it.
f. Capacitor Losses. Power loss in a capacitor may be
attributed to dielectric hysteresis and dielectric leakage.
Dielectric hysteresis may be defined as an effect in a
dielectric material similar to the hysteresis found in a
magnetic material. It is the result of changes in orientation
of electron orbits in the dielectric because of the rapid
reversals of the polarity of the line voltage. The amount of
power loss due to dielectric hysteresis depends upon the type of
dielectric used. A vacuum dielectric has the smallest power
loss.
Dielectric leakage occurs in a capacitor as the result of
leakage of current through the dielectric. Normally, it is
assumed that the dielectric will effectively prevent the flow of
current through the capacitor.Although the resistance of the
dielectric is extremely high, a minute amount of current does
flow. Ordinarily this current is so small that, for all
practical purposes, it is ignored. However, if the leakage
through the dielectric is abnormally high, there will be a rapid
loss of charge and an overheating of the capacitor.
The power loss of a capacitor is determined by loss in the
dielectric. If the loss is negligible and the capacitor returns
the total charge to the circuit, it is considered to be a
perfect capacitor with a power loss of zero.
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