So What’s The Mystery? By Peter DePetris To help you better understand my article in the October issue of the Ex- change (see “Ford IAR Fire Mystery Solved”), I will explain the “mystery” of how rectifier semiconductor failures in alternators can cause catastrophic underhood fires. Figure 1 is a typical Ford IAR alternator charging system and Figure 2 illustrates a simulated set of semiconductors in any rectifier, which will short the automobile’s bat- tery directly to ground. One only has to observe the OEM’s alternator charging schematic (Fig. 1) to understand that installing the alternator completes 12 possible reverse current short circuits to ground, any of which can cause an en- gine compartment fire. It’s like setting a time bomb in your customer’s vehicle and waiting for “T” minus zero. “T” being a function of the heat generated by the semi- conductors, rectifying the al- rows simulate only one of the possible paths to ground, but it must be under- stood that when any two diodes in se- ries fail, they can switch the “B+” bat- tery power on directly to ground or through the stator coils, voltage regu- lator or the electric choke if used. As illustrated in Figure 2 by semi- conductors (104 and 108), the cur- rent flows from the “B+” battery (114), in the reverse direction, through the fusible link (113)— an illusion of safety—and up into the wiring harness (115) which plugs into the alternator. The reverse current continues through diode (104), the stator coils (101 and 102) and continues through the sec- ond failed diode (108), completing the short circuit to battery ground. A re- verse current path is also provided through the “‘S” lead (116) to the volt- A Wwvyvr r O—~ YOU -O- r U ternating current for automo- tive use. The hour at which it will “go off’ can’t be pre- dicted, but be forewarned, it will happen. Pray it’s not parked in an enclosed area when it does. Figure 2 has two semi- conductor diodes high- lighted to illustrate a failure. The arrows indicate the cur- rent flow in the reverse di- rection to ground through the alternator field coils. The highlighted diodes and ar- TTERY TERMINAL NOVEMBER, 1999 age regulator and/or electric choke if used. The above rectifier (200) also il- lustrates a unique system of thermal disconnects (201 through 208), to dis- connect runaway semiconductors, pre- venting catastrophic engine compart- ment fires. Avalanche diode symbols are used illustrating the disconnects, which can also be used on GM alter- nators. This unique system was de- signed because the wiring harness fus- ible link will not protect the auto-mo- bile or its charging circuits. The reason the fusible link will not offer protection is because semi-con- ductors do not short out or blow a fuse like most electronic devices, instead they allow current to flow in the re- verse direction, overheating the chip’s silicon, causing its resistance to de- crease. The battery, being an inlimited power source con- rolled only by the silicon’s hot esistance, causes them to be- ome high wattage electric eaters, melting everything ex- ept the fuse link, which was esigned to allow full alterna- or current to flow through it in he forward direction to re- harge the battery. The melt- own continues until the battery is discharged. The circuit cur- rent in the reverse direction, controlled by the silicon’s re- sistance, isn’t high enough to melt open the fuse link. It is high enough to melt down the rectifier’s diodes, for which there hasn’t been adequate prevention against the exces- sive heat or fire. In conclusion, the diode is asemiconductor chip and a semiconductor is a semi- conductor, whether it’s ina Page 1 of 2