DIODE:

As previously stated, the diode is a current rectifying device. It serves as a one way electrical check valve which permits current flow readily in one direction but stops its flow in a reverse direction. Silicon die or Wafer in the diode possesses this electrical characteristic by virtue of the molecular construction of the metal.

The diode symbol is an arrow indicating the direction of current flow allowed by the diode. The bar indicates a one-way "gate" or block to current flowing in the opposite direction.

The cross-sectional view illustrates the position of the Silicon wafer in the bottom of the diode case. The case is made of rather heavy metal to serve both as a protection for the rather brittle silicon wafer and to effectively dissipate the heat, induced by the current flow through the diode. The case is tightly sealed during manufacture to prevent the entrance of moisture into the diode which would result in short circuiting of the unit. Moisture is readily drawn into any unit that operates at a temperature since it "breathes" as it heats and cools.

In all negative ground alternator charging systems the negative (case) diodes are pressed into the alternator grounded end-frame and the positive (case) diodes are pressed into a holder called a heat sink. The heat sink is usually made of die-cast aluminium because it possesses high heat dissipating qualities. It is mounted in, but electrically insulated from, the end-frame. The end-frame also serves to absorb the heat developed by the passage of current through the diodes.

A negative diode is properly identified as a negative case diode meaning the diode case is negative polarity and the diode lead is positive polarity. A positive case diode will have a positive polarity case and a negative polarity lead. Diodes are color coded for popularity identification. The part number of the diode is printed with the red dye on the positive diode and with the black dye for the negative diode. When part numbers are not used, a dab of red or black dye on the diode case identifies its polarity. Diodes used in heavy duty application are identified with a + (plus) or a - (minus) sign.

 The Zener diode is used in many transistorized ignition systems and voltage regulators. Its basic function is to protect the transistors in the circuit from the harmful effects of high voltage. When a predetermined voltage is reached, the Zener diode " breaks down" and permits the passage of current in the opposite direction by providing a shunt circuit for the high voltage current. This breakdown voltage does not harm the Zener diode since it is designed to perform in this manner. When the voltage drops below the predetermined voltage, the Zener diode again blocks current.

Since the breakdown voltage of the Zener diode is lower than the voltage value to that would damage the transistor, the transistor is there by protected. The Zener diode can be constructed to "breakdown" at varying voltages.

DIODE SYMBOL

DIODE CROSS SECTION