Diode Trio Functions and Alternator Stator and Rotor Field Testing COLUMN BY LARRY WILHELM Diode Trio Functions An alternator (diode trio) dash light may be illuminated when the alternator is providing some output. This article will address this as well as some other functions of the diode trio. A diode trio is used in many alterna- tors. These are three small diodes in addition to the main alternator output diodes. Some are in a package with four leads. Some are three separate diodes and may be mounted in a main rectifier assembly. Others may be part of the integral voltage regulator. These diodes are usually the axial diode type. “Axial” means that a wire protrudes at each end of a barrel package. This style package has a band around one end of the barrel. The banded end is the negative end of the diode. In electronics, the banded end is called the cathode and the non-banded end is called the anode. In most applications, the diode trio supplies field current. The voltage regu- lator, being part of the field circuit, is also powered from the diode trio. Some voltage regulators sense the output volt- age from the trio voltage. The unbanded (anode) end of each diode connects to a stator lead. The three banded (cathode) ends connect to the diode trio terminal. This is the positive potential for field circuit. Field current is supplied by the diode trio and the main negative diodes of the alternator. One terminal of the alternator dash light connects to the ignition switch and the other terminal connects to the diode trio terminal. A resistor is usually added across this light. When the ignition switch is on, the current through the light and resistor supplies minimal field current. The light will be on, in this con- dition, indicating that the alternator is not charging. This small current helps the alternator to turn on when the alter- nator starts turning. When the alternator is charging, the diode trio terminal should have the same voltage as the pos- itive battery terminal. We can consider the diode trio a low current positive diodes assembly. The ignition side of the dash light is the same potential as the output battery post of the alternator. This connects to the positive side of the main rectifier assembly. The other side of the light connects to the positive terminal of the diode trio. Before the alternator starts charging, the light is on. The trio termi- nal connects and has a ground path through the field. When the alternator is running, the trio terminal and the battery terminal have the same voltage. The alternator light is out, being there is no voltage across it. If any diode, in the trio, becomes open or shorted, the light will have a voltage across it for the duration when that diode is supposed to function. The alternator may provide some charge but the light will be dim. Voltage meas- ured between the trio terminal and the battery terminal should be less than 1/2 volt. If a main diode is defective, the dash light may not turn on. The light will have minimal voltage across it. This voltage will only be the differences between the battery voltage and the trio voltage (1 to 2.5 volts). When the alternator is turning or not turning: A bad diode will cause the light to glow dim. The diode trio ter- minal (or the light terminal) is often accessible. A good diode trio (while charging) will have zero (or less than 1/2) volts between the trio terminal and the battery terminal. If a near zero volt reading is observed, the dash light should be off. Some alternators, with diode trios, do not use a dash light. Sometimes only a resistor is used to supply the initial low current. Some one wire alternators have a diode trio but do not require a small amount of current to the field to start charging. These start with the magnetism that remains in the rotor. The trio still supplies the field current for charging. When replacing individual diodes of a diode trio, be assured that the amperage rating exceeds 1/3 the amperage draw of the field. Make sure the unbanded end connects to the stator terminal. Alternator Stator and Rotor Field Testing Some functions of stator and rotor field coils cannot be tested with conventional test equipment. When voltage is applied to a coil, the coil bucks the input amperage. When the voltage is removed, the coil attempts to continue the current flow. This is called inductive reactance. There are coil testers for coils that have many turns of wire and have little or no magnetic iron in the area. These testers apply a signal to the coil and observe the ‘ringing’ of the coil when the power is removed. This ringing is a collapse of the input signal that looks like a diminishing AC signal. The more inductance, the longer this ringing will occur. This type of test will detect shorted windings in even one or two turns. Another test for a coil is to apply a sharp rising voltage to the coil. The rise of the voltage is compared to the delayed rise of current through the coil. This is a