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.
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.
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).
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.