Diode Rectification Worksheet

In each of the illustrations above, trace the path of current flow through the stator coils, the corresponding diodes and the DC circuit.

The arrows in the illustrations next to the stator coils show the direction of conventional current flow.

De-spiking—Diodes are used on some relay coils to suppress voltage spikes. These spikes can damage components such as transistors in the control circuit of the relay. The voltage spike is produced by the collapsing magnetic field in the relay coil which occurs whenever current flow through the coil is stopped suddenly. The voltage induced in the relay coil is similar to the way an ignition coil operates. The induced voltage in a relay coil can be several times more than the system voltage.

A de-spiking diode is connected in parallel with the relay coil. It is reverse biased when the relay is turned on, therefore no current

Redox Reaction ExamplesPrintable Cut Out Ring Size

Because some relays are located in very hot environments where de-spiking diodes can fail prematurely, resistors are sometimes used instead. The resistor is more durable and can suppress voltage spikes in much the same way as the diode, but the resistor will allow current to flow through it whenever the relay is on. Therefore resistance of the resistor must be fairly high (400 to 600 ohms) to prevent too much current flow in the circuit. Because of resistors' high resistance, they are not quite as efficient at suppressing a voltage spike as diodes.

will flow through the diode. When the relay control circuit is opened, current stops flowing through the coil, causing the magnetic field to collapse. The magnetic lines of force cut through the coil and induce a voltage. Since the circuit is open, no current flows. The voltage builds until it reaches about .7 volts, enough to forward bias the diode, completing the circuit to the other end of the coil. The current flows around in the diode and coil circuit until the voltage is dissipated.

Isolation—A diode can be used to separate two circuits. Diodes are used in this way on many Toyota models. The Electronic Load Sense (ELS) circuit used on a Camry is a good example. This system signals the ECU to increase the idle speed when certain electrical loads are turned on. It uses two diodes so two different circuits can provide a voltage signal to the same terminal on the ECU. Without diodes, whenever either of the systems were turned on, voltage would also be applied to the other circuit causing it to operate.

Zener Cathode


Anode —




Zener diode—A zener diode acts like an ordinary silicon diode when in the forward bias direction, but it has been specially doped to act very differently in reverse bias. A zener diode allows current to flow in reverse bias at a specific voltage without damage over and over again. The reverse bias voltage at which the zener will conduct, sometimes called the zener point, differs from one zener to another as each zener diode is doped to have a zener point at a specific voltage.

Zener Schottky Curve

A zener diode can be used to suppress spikes by connecting it between the circuit and ground with the diode reverse biased. When a voltage spike exceeds the zener point of the diode, it completes the circuit to ground and prevents the spike from damaging anything.

Led Betreiben

A more common use of a zener diode in an automobile is to sense the charging system voltage. By connecting the zener between the base of a transistor and the positive side of the charging system, the zener can allow current to flow to the base of the transistor when its zener point is reached. If the zener point is 14.5 volts and the transistor to which the zener is connected turns off alternator field current when the transistor is turned on, a constant charging system voltage can be maintained. As soon as the system voltage drops below the zener point, the diode stops conducting and the transistor turns off, allowing field current to flow.

Light emitting diodes (LEDs)—An LED is a diode that is specially designed to produce light. LEDs are made with a transparent epoxy case so they can emit the light they produce when forward biased. The color of the light given off by an LED can be red, green or infrared, depending on how the material is doped An LED, like a standard silicon diode, will conduct current in only one direction. The forward bias voltage drop of an LED (1.5 to 2 volts) is much higher than a silicon diode. The forward bias current through an LED must be controlled, as with any other semiconductor' or damage will result. LEDs have advantages over ordinary bulbs, such as longer life, cooler operation, lower voltage requirements and the ability to produce the same amount of light as an incandescent bulb while consuming less power.

Adjustable Objective Scope Fell Apart

In vehicles, LEDs are used in a variety of ways, including displays and indicators. LEDs are also used in conjunction with phototransistors, which convert light to electrical current. A vehicle speed sensor, known as a photo-coupler or light-activated switch, is a good example. In a speed sensor, the speedometer cable is connected to a slotted wheel which separates the LED from the phototransistor. As the wheel turns, it constantly breaks the beam of light emitted from the LED to the phototransistor, thereby turning the phototransistor on and off. The pulsed signal goes to the computer and is used to determine vehicle speed.

Taken with permission from the Toyota, Advanced Electrical Course#672

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