Why Rectifiers Fail Heat! That was an easy answer, and fairly concise. Of course, if you’ve been rebuild- ing over six months you already knew that, didn’t you? All of us have seen the extreme failures and those more sublime. Was heat the only cause in every case, or have other factors con- tributed to the failure? And what caused all that the heat anyway? While that first answer to “why recti- fiers fail” was over simplified, in reality there are many circumstances that can contribute to the chain of events leading up to a diode’s demise. To understand these circumstances, we need to look at how a diode is manufactured and how it performs the task of rectifying alternat- ing current to direct current. This story begins with semiconduc- tors, which are elements that possess unique electrical properties. Silicon is the primary semiconductor used in our industry, mainly because of its low cost and the ability to perform at higher temperatures. So the diodes we see every day begin as a batch of raw sili- con. The silicon is heated to a very high temperature and a crystal is grown. In this crystal form, the silicon atoms form a lattice structure that allows them to share their outer ring of electrons. It is actually the number of electrons (four) in this outer ring, known as valance electrons, that gives a semiconductor its unique ability. Also during this process, a small per- centage of a foreign atom is introduced into the crystal lattice, producing another dramatic change. This is known as “doping.” The introduction of antimony or phosphorous adds free electrons, making the silicon N-type. The addition of boron creates electron “holes” and makes the silicon P-type. The crystals are cut into very thin slices and one each of the P-type and N-type are placed together to form the die, the heart of your automotive diode. This die is very small, less than .2 inches in diameter, and only a few thou- sandths of an inch thick. But because of the unique properties of the two differ- ently doped slices, current can flow from the positive side to the negative, but not in reverse. There are some exceptions, which will be discussed later, but a die is basically a one-way conductor. Before we can use it, the die must have leads attached, a process that dif- fers depending upon the application. The ceramic button diodes that were widely used in automotive rectifiers in the 1980s simply encased the die and leads in ceramic. The ceramic case sealed out air and contaminants, held the die and leads together tightly, and also conducted heat away from the die. The result was a fairly rugged diode, nearly bulletproof. They were also economical to manufacture. However, as output requirements for automotive alternators rose to 100 amps and more, the heat generated by the button diodes exceeded what the ceramic could dissipate from airflow alone. The excess heat was conducted through the solder connection and onto the heat sink, but once the solder reached its melting point, there was nothing left to hold the diode in place. We have all witnessed alternators with button diodes rolling around loose inside or missing completely. Most of the rectifiers utilized in vehi- cles today incorporate press fit diodes, which have been around since Chrysler introduced the first automotive alterna- tor around 1961. New technology has brought them back, more rugged than ever. Although their cost is higher, they can be pressed into the heat sink and spot-welded to the lead frame. The only solder required is used to attach the die to the cup and the stem, and these con- nections are made with a high-temp sol- der under strictly controlled conditions. The die and connections are then sealed with a potting compound, also under “clean room” conditions. The high temp potting compound both holds the inter- nal pieces structurally and provides heat dissipation. If the temperature does rise to the point where the solder begins to melt, the potting compound holds everything together. Most of the heat generated by the die is conducted away by the cup and onto the heat sink it is pressed into. But where does this heat come from? While the diode is able to conduct cur- rent in only one direction, this unique property comes at a high price. The cost is a voltage drop as the current crosses the P-N junction, the point at which the two slices of doped silicon are joined. This loss is .7 volts minimum with sili- con diodes and normally runs over | volt under heavy loads. The lost electri- cal energy is converted to thermal energy, and with some quick math, you can see that a CS-130 at full output will gen- erate 100 watts of heat, before it even warms up. As the heat in the die builds, the voltage drop increases, exacerbating the problem. If you have ever run a 100- amp alternator at full output for a few minutes and then touched the rectifier, you know what I mean. The size of this forward voltage drop is a predictor of how hot a diode will get. While internal de