What Is a Diode? By Larry Wilhelm Diodes today have many forms and uses. Two names used for this device are “diode” and rectifier.” If we look back into the past, most of us first heard ofa solid state diode in a crystal radio set. If you had a good antenna and ground, one could have a simple radio. A coil of wire, a crystal (diode) with a cat whisker, and a set of headphones would allow a person to listen to a local radio station. All you had to do was establish the proper turns of the coil and find that special spot on the crystal with a sharp pointed wire (cat whisker) and you had music. The vacuum tube was then very prominent. Heating an element in the tube would emit electrons to a collec- tor (plate) and would only allow cur- rent to flow in one direction. These were used for many years in radios and televisions. Some may even remem- ber the gas filled rectifier tube used in the vibrator type auto radios. Some applications still use the vacuum tube or the gas filled rectifier. The selenium rectifier was used in some radios and televisions. Alarge plate for cooling was used for each rectifier. Most selenium rectifiers stacked the diodes (and plates) for higher voltage. Earlier battery charg- ers used this type rectifier. These had high internal losses and had a history of progressively increasing those losses. Germanium diodes came into exist- ence along with the silicon diode. The germanium diode is more efficient but not as rugged. The silicon diode has become the most common. This sili- con type is the diode used in the alter- nators today. It is much smaller, more rugged and very efficient for most ap- plications. Diodes effectively allow current to flow in one direction. One main func- tion is to change alternating current into direct current. Alternating current sources reverse the “+” and ‘-” termi- nals back and forth ina repeating cycle. Alternating current (AC) is no prob- lem for non-polar load devices such as light bulbs and many other devices. However, a car battery cannot receive this alternating current. First, the battery would not charge. Second, the battery would overheat and probably blow. Placing a diode in one of the two power sup- ply wires will only allow current to flow when the wires are of the proper po- larity (proper “+” and “-’’). Adding other diodes can cause the output load current to flow in only one direction while the power source is switching. The auto alternator basically has three power sources or three phase. Diodes are placed such that all phases conduct current to the alterna- tor output in one direction. Different diodes pass current as each phase switches polarity. Direct current is supplied to supplement the load in the auto and replenish the battery. The avalanche diode is also a sili- con diode but modified. The diode basically functions the same as the ear- lier diodes. The added feature is that the diodes will pass current in the re- verse direction only when the designed reverse voltage is exceeded. This helps to protect the computers and other electronic devices in the auto. Basic tests for silicon diodes are as follows: Forward Voltage Drop. This is the voltage loss in the diode as the cur- rent passes through it. Usually one can measure from .8 to 2 volts DC drop across the diode depending on the cur- MARCH 2001 rent being passed. Reverse current leakage. This is measured with a reverse voltage ap- plied to the diode and the current is measured in micro amps. Most diodes will not exceed .000010 amps or 1000 micro amps. Most are much lower than this. If excessive reverse current (above .0001 amps) is observed, the diode die may be cracked and it may progress into destruction at a later date. Avalanche voltage. This is mea- sured with a reverse voltage applied to the diode. The voltage must be from a special tester with current limiting. The diode will pull the voltage reading down to the rated avalanche voltage. This is between 24 and 32 volts for the new alternators employing the ava- lanche diode. Larry Wilhelm was formerly Senior Project Engineer for ACE Electric, Colum- bus, KS. Page 1 of 1