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signalautoparts.com ALL ABOUT ELECTRONICS

The fundamental difference between an electrical system circuit and an electronic circuit is the amount of electrons that flow through the circuit. Electrical currents are generally measured in amps while electronic currents are measured in milliamps (.001 amps).

Electronic circuits are also based on "solid state" components such as "transistors" and "diodes." They're called solid state components because they have no moving parts. Solid state electronics have revolutionized the world, and along with it automotive ignition, fuel and emissions control systems. Electronics are now used to regulate everything from antilock braking and traction control to automatic transmissions and "smart" suspensions.

To understand how solid state electronic devices work, we have to go down to the atomic level. The electrical properties of a material are determined by the number of electrons in the outermost shell (valence ring) of its atoms. Metal atoms have three or less electrons in the outer shell, making them good conductors of electricity. Insulators such as glass, rubber and most plastics, have five or more electrons in the outer shell which inhibits the flow of electricity. But some materials have exactly four electrons in the outermost shell. This gives them unique electrical properties that allow the material to either act like an insulator or a conductor under certain conditions (as when doped with other atoms). These materials are called semiconductors, and are the basis of solid state electronic devices.electronics and sensors

TRANSISTORS
The age of solid state electronics actually began with the invention of the vacuum tube, not the transistor. The glass vacuum tubes that were once used in radios and later televisions were really the first electronic switching devices that used electronics rather than mechanical contacts to reroute the flow of electrons through a circuit. Vacuum tubes are no longer used today because they consume too much power, generate too much heat, are bulky and fragile. They also tended to fail rather frequently. But they laid the groundwork for the solid state revolution that was to follow.

In 1948, Bell Laboratories made what was to become one of the key inventions of this century: the transistor. A transistor is essentially a switch that reroutes an electrical current according to a voltage input. A transistor performs the same switching function as a vacuum tube except that it does not require a heated filament or magnetic field to switch the circuit path. The switching function is accomplished by changing the conductivity of the junction inside the transistor.

A transistor is made by sandwiching together three layers of silicone or germanium crystal that have been doped to create opposite electrical properties. When certain trace impurities are added to either silicone or germanium crystals during their manufacture, it alters the material's electrical properties. Remember what we said about semiconductors having four electrons in the outermost ring? In a pure crystal of either material, the outer rings of the atoms link up in such a way that there aren't any "holes" left over for extra electrons to move about (a hole is a space in an outer orbit for an electron to orbit). Pure silicone and germanium are actually good insulators. But when the crystals are doped with trace "impurities" the neat arrangement of electrons is upset. The trace impurities (which only require about one atom in 10 million!) add just enough extra electrons or create just enough empty holes that the material will conduct a current when a voltage is applied. This transforms them into semiconductors.

Doping creates one of two basic types of semiconductor materials. One is "Negative" or "N-type." To create a N-type semiconductor material, the crystal is doped with atoms (such as phosphorus) that have five or more electrons in their outer valence ring. The extra electron doesn't fit into the neat arrangement within the crystal, so a surplus of electrons is created that gives the crystal a negative charge. The other type of semiconductor material is the "Positive" or "P-type."

Positive semiconductors are doped with atoms (such as boron, indium or aluminum) that have three or fewer electrons in the outer valence ring. The missing electrons leave holes in the crystal lattice into which electrons can move when a voltage is applied. Thus P-type semiconductors have a positive charge. So how does all this work together to make a rear parking sensor transistor act like a switch? The transistor consists of three layers of alternating semiconductor material: either N-P-N or P-N-P. Each layer is connected to its own electrical lead. The "in" lead is called the "emitter," the "out" lead is called the "collector," and the center control lead is called the "base." The middle base layer is the one that performs the switching function by either allowing current to pass from the emitter or in lead to the collector or out lead.

Current cannot move from the emitter side to the collector side within the transistor unless voltage is also supplied to the middle base layer. The opposite charge of the center base layer creates a boundary across which current can't pass because there aren't enough extra electrons (N-type) or holes (P-type) to carry it. But when an outside voltage is applied to the base layer, it reverses the charge and makes it conductive. The outside voltage provides the extra electrons or holes depending on the type of material (N-type or P-type) that are needed to carry the current. The switch "closes" and the transistor passes current to the opposite side.

Thus the on/off switching function of a transistor is controlled by the application of voltage rather than opening a set of mechanical contacts as in a conventional switch. The same type of transistor can also be made to act like a variable resistor by varying the current to the middle base layer. As the current increases to the base lead, the transistor passes more and more current to the collector or out lead. A typical application for a variable resistor transistor might be the power transistor for the blower motor in an automatic climate control system.

DIODES
A diode is a type of "one way" switch or filter that allows current to flow in one direction only. A diode is made by sandwiching P-type and N-type silicone crystals back to back. The P-N junction will only flow electrons when an outside voltage pushes the holes in the P-material towards the extra electrons in the N-material. This happens when the applied voltage polarity is in the same direction as the P-N junction (positive on the P side, negative on the N side). This is called "forward bias." When this happens, the junction conducts current.automotive 02 emissions_sensor

But when current is applied in the opposite direction ("reverse bias"), the reversed polarity pulls the electrons away from the P-N junction on the N side, and the holes away from the junction on the P side, leaving nothing to carry the current across the junction boundary. So the diode effectively blocks the passage of current.

Of course, nothing is perfect and diodes are no exception. If enough reverse bias voltage is applied, it will punch through the diode and flow in the opposite direction destroying the diode in the process. That why alternator diodes that convert alternating current (AC) to direct current (DC) are usually damaged when the battery is hooked up backwards or someone uses a battery charger that puts out too much voltage. The voltage at which a diode breaks down is called the "peak inverse voltage." Exceed it and the diode is ruined.

The same precautions apply to transistors. All semiconductors are designed to handle limited current loads, even the large power transistors used in ignition modules and distributorless ignition systems. If the maximum current load is exceeded, it usually ruins the component.

ZENER DIODES
"Zener" diodes are a special type of diode that are designed to flow backwards under certain circumstances. The semiconductor material is more heavily doped so the P-N junction will allow reverse current flow without damage when a certain voltage level is exceeded. These are sometimes referred to as "avalanche" diodes because they don't pass reverse current until a certain voltage is achieved, then they open up all at once. Zener diodes are used in voltage regulators to provide voltage overload protection.

LED
Another special type of diode is the "Light Emitting Diode" (LED). The crystal in this type of diode glows red when current is applied much like the filament in a light bulb. But since there's no filament to burn out, LEDs tend to be long lived. LEDs also require less current than conventional bulbs. LEDs are used for digital displays in some test instruments, and in some CHMSL (center high mounted stop light) brake lights. LEDs are also used to trigger ignition and/or injector pulses in some engines (such as Chevrolet's Opti-Spark Ignition System or Nissan's ECCS system).

INTEGRATED CIRCUITS
As solid state electronics evolved, components were miniaturized so more and more components could be crammed onto smaller and smaller circuit boards. The "integrated circuit" (IC) allowed a complete electronic circuit consisting of transistors, diodes and resistors to be formed on a single silicon wafer or "chip."

Invented by Jack Kilby of Texas Instruments in 1958, ICs are made by plating and etching multiple layers of P-type and N-type material over one another to form interconnecting circuits on a tiny wafer that may be no larger than a small letter "o" on this page! IC technology has allowed engineers to pack the electronic equivalent of 6000 lbs. of spaghetti into a 1 lb. box! Under a microscope, these interconnecting circuit elements resemble a complex street map. Keep in mind, however, that it's a three-dimensional map. The little "roads" that crisscross each other are several layers deep with numerous underlying interconnections like the overpasses and access ramps on an intercity expressway. But the layers are extremely thin, only a few microns thick at most.

These integrated circuits may be combined like building blocks to create even larger more complicated chips. For example, "very large scale integration" (VLSI) refers to ICs that contains 50,000 or more such building blocks.

The silicon chip that contains the integrated circuits may be packaged several different ways. One is to mount the chip on a flat ceramic or metal plate so it can be installed on a printed circuit board. This is done by soldering the IC's leads to the circuit board.

Another type of packaging that's used to encapsulate the chip with plastic. The surrounding plastic not only supports the chip but protects it against corrosion. An example would be Ford's Thick Film Integrated (TFI) ignition module.

PROM CHIPS
Replaceable IC's that are found inside electronic control modules (ECM's) and other electronic devices are usually square or rectangular in shape, with anywhere from 16 to 28 pins in rows underneath like the legs of a caterpillar. The pins allow the IC to be plugged into a circuit board -- which eliminates the need for soldering and allows the IC to be removed and replaced should that be necessary. Examples of this type of IC include the "Program Read Only Memory" (PROM) chips and Electronically Erasable Program Read Only Memory (EEPROM) chips used in General Motors ECMs and other automotive computers. The PROMs or EEPROMs are used to program the engine's fuel and ignition curves as well as other emission functions. The PROM or EEPROM chip calibrates the engine control computer for a specific vehicle application. This is necessary because different accessories or drivetrain combinations can affect a vehicle's emission performance. An engine with an automatic transmission, for example, may require a different spark curve than a manual transmission.

PROM chips cannot be reprogrammed but they can be replaced with a new PROM that contains different or updated instructions. Replacing a PROM with one that contains updated information is sometimes necessary to cure a specific driveability or emissions problem. If a vehicle manufacturer believes a problem is due to the computer's calibration, it may issue a Technical Service Bulletin (TSB) describing the problem and announcing the availability of a new or updated replacement PROM. The PROM can then be obtained from the new car dealer and installed in the vehicle's computer.

With EEPROMS, physical replacement isn't necessary to recalibrate a computer. An EEPROM can be reprogrammed by downloading updated instructions through the computer's diagnostic hookup. To prevent "unauthorized" tampering with the EEPROM's instructions, special access codes are required -- which are only available to new car dealers at this time.

IC RELIABILITY
One of the major advantages of using ICs rather than conventional circuit boards with soldered individual circuit components is reduced power consumption. An IC can do the same job on a fraction of the amperage required to run a similar large-scale circuit. But because the circuit elements in an IC have been reduced to such tiny proportions, there's a limit as to how much voltage and current they can safely handle. The limit for most ICs is 20 volts or less, and current ratings are measured in milliamps.

The reason for the power limitation is heat. The more current an electronic circuit carries, the more heat it generates. If the heat is concentrated in a tiny device such as an IC, it can produce temperatures high enough to damage or destroy the chip. That's why the IC's role in most electronic devices is usually limited to information processing or controlling rather than switching or handling the voltages and currents that make things happen.

With an ignition module or a voltage regulator, the control output from the IC is used to switch a power transistor on and off. The power transistor can safely handle larger voltages and currents, but the IC cannot -- which brings us to the question of reliability. As long as an IC is operated within its voltage and current limits, it is generally more reliable than a large-scale electronic circuit with individual soldered components.

The main reason why most electrical and electronic circuits fail is because of breaks in connections between circuit components rather than the outright failure of individual components. The overlapping circuit elements within an IC are deposited on top of one another by a plating and etching process, so there's little chance of anything wiggling or vibrating loose. And if the IC is encapsulated in plastic, the circuits are protected against environmental contamination and corrosion as well. But that doesn't make ICs immune to trouble because the pin connectors that link the chip to the outside world are the weak point.

Soldered pin connections can and do break loose, causing opens that result in circuit failures. The pins on the push-in variety of ICs can also be bent or broken during installation. That's why GM recommends using a special tool to remove and install PROM chips in their ECMs. The IC pins must also fit tightly in the circuit board receptacle and be corrosion free. ICs are most vulnerable, however, to damaging voltage spikes. Because the overlapping circuit layers on the chip are so thin, it doesn't take much of an overload to destroy a connection. And once a chip is damaged, it's history.

Voltage spikes can occur when an electrical connection is broken while a circuit is still hot. An example would be unplugging an engine sensor or wiring connector while the ignition is on. The sudden break in the circuit causes the voltage to momentarily surge, creating a spike that may be as high as 50 or 60 volts in a circuit that normally sees only 5 or 12 volts. The surge occurs because the electrons want to keep flowing as the circuit is being broken. They pile up and try to push their way across the gap, creating a transient voltage spike that can fry a chip. The car makers "harden" their electronic circuits by building in voltage overload protection, but safeguards may not be enough to protect the chips under all circumstances. So never disconnect or connect any wiring connector or electronic component when the ignition is on because some components receive current directly from the battery.

Voltage surges and spikes can also be created by arc and MIG welding equipment. As a precaution, the battery should be disconnected prior to welding. If welds are being made in the close proximity of an electronic control module or other device, the module or device should also be unplugged for additional protection.

Potentially damaging voltage overloads can also be created by electrostatic discharges. Sliding across a vinyl seat can build up a static electrical charge of thousands of volts. The actual amperage of such charges isn't much, but the voltage can be high enough to produce a visible (and sometimes painful) shock when you touch a conductive surface. If the thing you touch happens to carry the voltage back to a computer chip, the spark can literally punch a hole right through the microscopic circuit layers of the IC leaving it permanently maimed.

When handling any type of chip based electronic module, precautions should be used to avoid electrostatic discharges that might damage the circuitry. You can minimize the danger by "grounding" yourself with an anti-static wrist strap, by placing modules and computers on non-conductive anti-static mats, and/or by wearing cotton fiber clothing.

TESTING & DIAGNOSIS
Since there's no practical way for a technician to test or repair faulty integrated circuit chips inside a control module, your only option is to replace the module if it isn't working correctly (unless, of course, it's a situation where the problem is really module calibration and there is an updated PROM chip for the module, or new programming for an EEPROM). But first you have to isolate the problem to the module by following a detailed step-by-step diagnostic procedure.

With transistors and diodes that are not part of an integrated circuit, it is possible to replace individual circuit components in some instances (the diodes in an alternator, for example). But in most instances, the labor to do so is too time-consuming so the entire unit is replaced if a circuit component fails.

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