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FIGURE 36-50
Transistors. A. The NPN transistor has a P-type semiconductor between two N-type semiconductors. B. A PNP transistor has an N-type semiconductor between two P-type semiconductors.

A transistor is a semiconductor device used as a switch and to amplify currents. A transistor is a key component in almost any electronic device. There are two kinds: NPN and PNP. The NPN transistor has a P-type semiconductor between two N-type semiconductors. A PNP transistor has an N-type semiconductor between two P-types Figure 36-50.

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FIGURE 36-51
NPN versus PNP transistors. A. NPN transistor connected to power, switch (with resistor), and load. B. PNP transistor connected to power, switch (with resistor), and load.

Each of the three regions has a terminal. The center region is always called the base. The outer regions are the collector and the emitter. In the symbol for a transistor, the emitter is the terminal with the arrow. It always points toward the negative material.

In a circuit, transistors can act as an electronic switch. If an open control switch is connected to the base, the depletion layer at one PN junction will block current from flowing through the transistor and powering the load.

When the control switch is closed, a small current flows through the emitter–base PN junction. The base has only a limited number of charge carriers, so extra ones flow across the emitter–collector PN junction, letting current operate the load. The transistor then operates as a low-resistance conductor. Most transistors have about 0.5 volts of voltage drop across the emitter–collector junction while current is flowing. A small current through the base lets larger current flow across the emitter–collector junction. The transistor is then said to be turned on.

One difference between PNP and NPN transistors is whether the base is activated by positive current flow or negative current flow Figure 36-51. In the case of an NPN transistor, positive current, through a resistor (to limit current flow to the base), is applied to the base. In a PNP transistor, a ground (through a resistor) is applied to the base. Transistors can be placed in the feed side of a circuit or on the ground side of a circuit.

AS-78: Magnetic Fields/Forces: The technician can explain the effect of magnetic fields on unshielded circuits in control modules.

Shielding is applied to electrical circuits to prevent unwanted electromagnetic induction, where errant voltages may be generated in unshielded circuits due to electromagnetic interference. Some systems within control modules, particularly sensor inputs, work with high-sensitivity and very low voltages. In these cases, even a small induced voltage, known as “noise,” can have serious effects on the running of the vehicle or the functionality of safety systems. The control unit may effectively respond to the noise as if it is actually a signal from a sensor.

AS-79: Semiconductor Devices: The technician can demonstrate an understanding of the capability of semiconductor devices to rapidly modify engine operation parameters.

Semiconductor devices are the foundation of modern electronics, having replaced vacuum tubes in nearly all applications. Common semiconductor devices include transistors and many types of diodes. They are used in all types of electronic devices, from radios and televisions to mobile phones and computers, including those used in automotive applications.

Semiconductor devices have enabled the development of automotive engine management systems due to their small size, high resilience to shock and vibration, and high reliability. They can also operate at very high speed, which allows increasingly accurate control of an engine’s running parameters. ECUs in modern vehicles may now run at sampling rates exceeding 1.5 million times per second.

AS-81: Photocells: The technician can demonstrate an understanding of the purpose of photocells.

An example of a photocell is a sun load sensor which is usually mounted on top of the dashboard. The sensor is a part of the heating, ventilation and air-conditioning (HVAC) system. Its purpose is to send a signal to the programmer based upon the amount of sunlight that is striking it. The signal information will cause the programmer to adjust the system output according to sensor output voltage.

AS-82: Photocells: The technician can demonstrate the measurement processes used to determine the output of photocells.

A technician is diagnosing an electrical concern with a vehicle’s heating, ventilation, and air-conditioning (HAVC) system. Several steps have been taken in the initial testing of the system. At this point, the service information refers the technician to the testing of the sun load sensor which is a photovoltaic device. A digital multimeter will be used to measure the output of the photocell in DC volts in most cases. A scan tool can sometimes be utilized, if so this would be the preferred method. For the exact procedure, consult the manufacturer’s service information.