About Diodes?
There are different types of diodes for different
applications. For vehicles, we are mostly interested in rectifier
diodes. Rectifier diodes, in simple terms, are "valves" that allow current
to flow in one direction only.
Uses
Different circuits can be isolated to avoid undesired
interactions. Diodes are also used to protect circuits. In car
applications, diodes are used to isolate switches and sensors in security
systems. Diodes can also eliminate current transients in inductive
components such as relays.
Purchasing the Correct Diode
 The
top image on the left is the symbol commonly used in circuit diagrams to
represent diodes.
Most rectifier diodes are small black tubes with a white
line at one end and look like the bottom image on the left. The white
line is commonly called the "negative" side (or cathode). The other end
of a diode is called the anode.
Diodes can be obtained at any electronic parts store such
as Radio Shack. Low current diodes (1 amp or less) are very cheap.
Several diodes can be purchased for a dollar. Higher current diodes do
cost more.
Current Ratings
For circuit isolation purposes (i.e. isolating switches or
alarm sensors), 1 amp diodes would suffice. Other applications, such as
isolating parking lights circuits in cars require diodes that can handle more
current, say 3 amps.
By hooking up diodes in parallel, larger current
capabilities can be obtained. For example, by connecting two 1 amp diodes in
parallel, a total of 2 amps can be passed through the
diodes.
Modes of Operation
 |
If you want a diode to conduct
current, hook it up in the "forward bias" mode.
In the forward bias mode, the diode will behave as a
short circuit (i.e. being replaced by a wire). |
 |
If the purpose of a diode in a
circuit is to block current, then try the "reversed bias"
operation.
In the reverse bias mode, it can be represented by
an open circuit (i.e., cutting the wire in the
circuit). |
Voltage Considerations
Diodes do have shortcomings. A typical diode has a voltage
drop of 0.7 volts. This is not critical for most car applications, but should
be taken into consideration for other applications.
Exceeding voltage limits of diodes is not a concern in car
applications because we deal with only 12 volts. For higher voltage circuits,
the diode's maximum voltage ratings on forward and reverse bias should be
observed.
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