Page 426 - Robot Builder's Bonanza

DIODES 395

Body of
diode Cathode Figure 31- 14 Component outline and
band schematic symbols for a diode. Diodes are
polarized, as noted by a colored band. The
Diode schematic band denotes the cathode, which is the
symbol negative (–) connection.

• Light- emitting. These diodes emit infrared or visible light when current is applied.
• Laser. The now- common penlight lasers use a specially constructed diode that emits
single- color laser light. They can be used for visual effects and some sensor applications.

HOW DIODES ARE RATED
Diodes carry three important ratings: peak inverse voltage, current, and forward voltage drop
(there are others, but these are the main ones).

The peak inverse voltage (PIV) rating roughly indicates the maximum working voltage
for the diode. In the case of the common rectifier diodes, this maximum voltage is 50,
100, even 1000 volts, depending on the component. These voltages are well beyond
what you’ll typically find in electronics for a robot, giving you a wide variety of compo-
nents to choose from.
The current rating is the maximum amount of current that can be passed through the
diode without risk of overheating and eventual self- destruction. Current ratings are in
amps or, in the case of small diodes, milliamps. This rating is very critical in robotics
applications, where it’s common to have circuits that require more current than what
the average diode can handle.
Forward voltage drop is the amount of voltage that is essentially lost when it passes
through the diode. The voltage drop affects the overall behavior of the diode; for exam-
ple, with a very low drop, the diode is faster- acting.

UNDERSTANDING DIODE POLARIZATION
All diodes are polarized, and they have positive and negative terminals. The positive terminal
is called the anode, and the negative terminal is called the cathode.
You can readily identify the cathode end of a diode by looking for a colored band or stripe
near one of the leads. Figure 31- 14 shows a diode that has a band at the cathode end. Note
how the band corresponds with the heavy line in the schematic symbol for the diode.

EXPLORING THE COMPOSITION OF DIODES
Among other variations, diodes are available in two basic flavors, germanium and silicon,
which indicate the material used to manufacture the active junction (the part that conducts
current) within the diode. The two types of materials also have an effect on the forward volt-
age drop of the device: about 0.7 volts for silicon and 0.3 volts for germanium.

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