Zener Diodes. A Zener diode is a specially designed diode that is constructed to operate with a reverse bias current. Zener diodes are named after their inventor Clarence Zener who was an American professor of physics in 1934. A zener diode acts as any other diode because it blocks reverse bias current.
Zener diodes are different because this occurs only up to a specified voltage. Above this specific voltage ( called the zener region) a zener diode will conduct current without any damage or side effect to the diode itself. A zener diode is heavily doped, so that the reverse bias does not harm the material.
Thus, the voltage drop across a zener diode remains practically the same before and after the breakdown voltage, and this is what makes a zener diode perfect for regulating voltage. Zener diodes are constructed for a wide variety of breakdown voltages and are used in a wide variety of automotive and electronic applications, particularly in voltage regulators.
Clamping diodes: Diodes can be used as a high voltage clamping device when connected positive to the cathode ( – ) of the diode. When a coil is pulsed on and off, a dangerous high voltage spike is generated when the coil switches to “off ”.
To control or redirect this possibly damaging spike, the diode can be installed across the leads of the coil to absorb any transients that occur due to natural induction. This proves to be an invaluable use of diodes because it protects the rest of the automotive electrical system from voltages it was not designed to handle.
These designs can also be referred to as clamping or de spiking diodes. Diodes were first most predominantly used in the automotive market on A/C compressors since the early 70’s. When the clutch coils powered down, any component parallel to it would suffer high voltage spike exposure. Placing the clamping diode across the coils saved the rest of the system from failure.
Many early vehicles have been reported to have suffered damage to blower motors and switch controllers. Also climate control units are considered in jeopardy by a failed or removed clamping diode on the A/C unit.