3-32
Figure 3-34.—Photocell.
The photovoltaic cell, or solar cell, is a device which converts light energy into electrical energy. An
example of a solar cell and its schematic symbol are shown in figure 3-35. The symbol is similar to that of
a battery. The device itself acts much like a battery when exposed to light and produces about .45 volt
across its terminals, with current capacity determined by its size. As with batteries, solar cells may be
connected in series or parallel to produce higher voltages and currents. The device is finding widespread
application in communications satellites and solar-powered homes.
Figure 3-35.—Solar cell.
When it is necessary to block the voltage between one electronic circuit and another, and transfer the
signal at the same time, an amplifier coupling capacitor is often used as shown in figure 3-36. Although
this method of coupling does block dc between the circuits, voltage isolation is not complete. A newer
method, making use of optoelectronic devices to achieve electrical isolation, is the optical coupler, shown
in figure 3-37. The coupler is composed of an LED and a photodiode contained in a light-conducting
medium. As the polarity signs in figure 3-37 show, the LED is forward biased, while the photodiode is
reverse biased. When the input signal causes current through the LED to increase, the light produce by the
LED increases. This increased light intensity causes current flow through the photodiode to increase. In
this way, changes in input current produce proportional changes in the output, even though the two
circuits are electrically isolated.
