Why Avalanche Diodes? By Larry Wilhelm The main concept of a diode is to allow current to flow in one direction and block current in the opposite di- rection. There are many types of di- odes in use today. Only a few will be mentioned. Some diodes will emit light when conducting current (LED). Some diodes will allow current to pass if they receive light. Others will switch, or tum on much faster. Some diodes vary in capacitance and are used in tuners for televisions and radios for digital chan- nel selection. Zener diodes are another family type. These block current flow in one direction and allow current flow in the other direction just as many other di- odes. The unique difference is that these diodes will pass reverse current when the reverse rated voltage is ex- ceeded. Other diodes are destroyed if in this mode. The zener diode has many uses for voltage control. They come in many voltage ratings and sizes to carry different amounts of reverse (zener) current. Avalanche diode is in the family of zener diodes. The avalanche diode conducts current (or responds) much faster to reverse voltage than the nor- mal zener diode. Avalanche diodes are used in newer alternators to suppress high volt- age spikes from and around the alter- nator. They “short” the voltage spike to ground. These diodes function the same as the transient suppressor that you plug your computer into hoping to protect it from the high voltage shocks. Most electronics can be damaged by high voltage. These pulses last for a very short duration but it is enough to damage the circuits. A shock you give someone after walking across a car- pet only lasts a very short instant but it also has enough power to damage electronics. This instantaneous voltage pulse is called “electrical noise.” It can be heard in the auto radio as static. Where does this electrical noise come from? Every time a coil is turned off, there is a high voltage surge. We are most familiar with the spark from the ignition coil. Each time the points open (or power is removed from the coil), we release a high voltage pulse to the spark plug. This same type of energy comes from all coils, solenoids, motors, buzzers, and other devices that have a coil of wire. This energy arcs across switches as power is removed. Some of this energy travels along other wires dissipating energy in any other device that is connected to the same original power source. This transient voltage destroys electronic circuits. Mostelectronic devices have their own protection but it is not always enough. The avalanche diode (in the alter- nator) will turn on and dissipate these micro short voltage pulses within itself, thus removing the damaging blow to other components. These type diodes are located primarily in the negative diode assembly. Ifa high voltage pulse is present, the energy is passed to ground through one or more of these diodes. This path is only turned on if the voltage exceeds the avalanche rat- ing of the diode. In alternators, this voltage is between 26 and 32 volts. As soon as the voltage drops below the voltage rating, the diode again blocks the reverse current just as in a conventional diode. Avalanche diodes can be damaged. If they pass too much current or over- heating occurs, they can be destroyed just as other diodes. These diodes can also be destroyed if the avalanche volt- AUGUST, 2001 age is exceeded for too long of a du- ration. The diode is designed to only create a path for those very short volt- age pulses. Some will pass excessive current pulses from 40 to 25,000 amps. These pulses are expected to be much, much less than a micro sec- ond (.000001 seconds). Repeated pulses over a short duration or a con- tinuous avalanche current can destroy these diodes. Special care must be taken in testing alternators with avalanche di- odes. Many repairmen, in the past, have bypassed the regulator to test an alternator. This has been done on the vehicle and on the test bench. This test method can possibly cause the voltage of the alternator to exceed the avalanche voltage. These diodes pass or “short” all voltages above 26 to 32 volts to ground. This excessive continuous current and heat will de- stroy the diodes. Even one amp con- tinuous, at 30 volts, equals 30 watts dissipation across a diode. This watt- age is close to the maximum for most diodes used in alternators. Care must also be taken in test- ing the avalanche voltage of each di- ode. A voltage supply greater than 35 volts is required. A resistor (1,000 ohms or greater) should be in series with the supply to limit the avalanche (reverse) current. The negative sup- ply lead connects to the heat sink of the negative diode assembly. The positive lead (with resistor) connects at the stator terminal of the diode. Then voltage is measured across the avalanche diode. Testing is easier and more safe using an avalanche diode tester. We have only begun with devices like this. There will soon be avalanche diodes for 24-volt systems. Who kn