Page 85 - Understanding Automotive Electronics

2735 | CH 3 Page 72 Tuesday, March 10, 1998 11:03 AM

3 ELECTRONICS FUNDAMENTALS

Diodes

A diode acts much like a A diode is a two-terminal electrical device having one electrode that is
one-way valve, allowing called the anode (a p-type semiconductor) and another that is called the cathode
current to flow in only (an n-type semiconductor). A solid-state diode is formed by the junction
one direction. between the anode and the cathode. Diodes can be thought of as one-way
resistors or current check valves because they allow current to flow through
them in only one direction, depending on the polarity of voltage (bias) across
the anode and cathode. When current flows in the forward (conducting)
direction with a plus voltage on the anode, diodes have low resistance (typically
a few ohms). This is called the forward biased condition. (The conventional
current flow direction of positive to negative is used in this book.) When the
current flows in the reverse (nonconducting) direction with a plus voltage on
the cathode, diodes have a very high resistance (typically a few million ohms).
This is called the reverse biased condition.
Figure 3.1a shows the schematic symbol for a diode, and Figure 3.1b is a
graph of the actual and ideal voltage and current transfer characteristics for a
typical diode. Notice on the ideal curve that the diode doesn’t start conducting
until the voltage across it exceeds V volts; then, for small increases in voltage,
d
the current increases very rapidly. For silicon diodes, V is about 0.7 volt. For
d
germanium diodes, V is about 0.3 volt. Even for the actual curve, the change
d
in current is quite steep for 0.1 volt changes in the voltage across the diode after
V has been exceeded.
d
When designing or analyzing circuits, V is often ignored in relatively
d
high-voltage circuits in which V is a very small percentage of the total voltage;
d
however, in low-voltage and low-level signal circuits, V may be a significant
d
factor.
Rectifier Circuit
A diode has low resis- The circuit in Figure 3.1c, a very common diode circuit, is called a half-
tance when forward wave rectifier circuit because it effectively cuts the ac (alternating current)
biased and high resis- waveform in half. Consider the circuit first without the dotted-in capacitor.
tance when reverse The alternating current voltage source is a sine wave with a peak-to-peak
biased. amplitude of 100 volts (50 volt positive swing and 50 volt negative swing).
Waveforms of the input voltage and output voltage plotted against time are
shown as the solid lines in Figure 3.1d. Notice that the output never drops
below 0 volts. The diode is reverse biased and blocks current flow when the
input voltage is negative, but when the input voltage is positive, the diode is
forward biased and permits current flow. If the diode direction is reversed in
the circuit, current flow will be permitted when the input voltage is negative
and blocked when the input voltage is positive.
The half-wave rectifier is used to convert an ac voltage into a dc (direct
current) voltage that stays either above 0 volts or below 0 volts, depending on
which way the diode is installed. Rectifier circuits are commonly used to

72 UNDERSTANDING AUTOMOTIVE ELECTRONICS