Page 427 - Robot Builder's Bonanza

396 COMMON ELECTRONIC COMPONENTS FOR ROBOTICS

Since the voltage drop can change the behavior of the circuit, you should always be careful
which of these two kinds you use. If the circuit doesn’t say, a silicon diode is probably fine; if
a germanium component is called for, be sure not to substitute a silicon type.

COMMON APPLICATIONS FOR DIODES

Like all other electronic components, diodes are used for a fantastically wide array of applica-
tions. Two serve as good examples of the beneficial role diodes play in circuits for robotics
control: incremental voltage drop and reverse polarity protection.
In both of these examples, no attention is given to ensuring that the diodes selected for the
task can adequately handle the current demands of the circuits. In an actual circuit you’ll need
to determine the total current draw through the diode and make sure you pick one that is
rated for that current.

Incremental Voltage Drop
Recall that diodes exhibit a forward voltage drop, whereby a certain amount of voltage is lost
as current passes through the device. With silicon diodes, this drop is about 0.7 volts. You can
use this “feature” of a diode to incrementally decrease a voltage in specific steps. Note that
this isn’t real voltage regulation; if the voltage from the power supply goes up or down, so will
the voltage on the other side of the diodes.

Reverse Polarity Protection
Connecting a battery backward to an electronic circuit can easily damage the circuit. You can
prevent such damage by putting a diode in series with the positive power supply connection.
This technique works because the diode passes current in one direction only: from anode to
cathode. It won’t allow current to flow the other way.

Light- Emitting Diodes (LEDs)

All semiconductors emit light when an electric current is applied to them. This light is gener-
ally very dim and only in the infrared region of the electromagnetic spectrum. The light- emitting
diode (LED; see Figure 31- 15) is a special type of semiconductor that is expressly designed to
emit copious amounts of light. Most LEDs are engineered to produce a specific color of light,
as well as infrared and ultraviolet. Red, yellow, and green LEDs are among the most common,
but blue, violet, and even white light (all- color) LEDs are available.
LEDs carry the same specifications as any other diode. The typical LED has a maximum
current rating of 30 milliamps or less, though this varies greatly, and depends on size, type,

Figure 31- 15 Component outline and
schematic symbols for a light- emitting diode
LED Anode Flattened edge
schematic (+ positive) (LED). Diodes are polarized, and, depending
on the component, the polarization may be
Shorter lead
noted by a flattened edge and/or a shorter
Cathode lead. These denote the cathode, which is the
(- negative) negative (–) connection.

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