LEARNING OBJECTIVE: Recall electronic control circuit components, how they function, and proper troubleshooting procedures.

Modem control circuits allow a single operator to control the entire operation of a gun system or GMLS from one electrical panel. Low-voltage components and wiring are used to control the flow of high-voltage power to drive motors and solenoids. Imagine the size of a switch or pushbutton that would be required to control 440 VAC three-phase power to a drive motor. Most mounts and GMLSs have three to five drive motors. This number of motors alone would require a second panel and operator just to start and stop the drive motors.

We will describe the newer solid-state logic circuits used to control the 5"/54 Mk 45 gun system. You will see how a circuit that looks fairly complicated at first glance actually is made up of individual one-function circuits. These smaller circuits are made up of just one or two simple devices (and their associated wiring) and do just one job. When properly connected to other one-function circuits, they can act automatically to start or stop current flow in a circuit, combine or divide these circuits, and act as safety devices to protect equipment and personnel. The transistor is the component that makes all this possible, so that is where we will begin. We will also describe the control circuits used in the newer 5"/54 Mk 45 gun system. All GMLSs control circuits are similar to the 5"/54 Mk 45.

TRANSISTORIZED CONTROL CIRCUITS

A transistor is a solid-state device constructed of semiconductor materials. Transistors are capable of performing many of the functions of different types of electronic and electrical components, such as electron tubes, interlock relays, switches, control relays, and current amplifiers. In gun system control circuits, transistors are used as electronic switches that control the flow of current.

In most cases, transistors are more desirable than tubes for ordnance equipment because they are smaller, require no warm-up power, and operate at low voltages with comparatively high efficiency.

Semiconductors are the basic components of a transistor. How these materials behave and the electrical conduction properties that give the transistor its basic characteristics are explained in NEETS, Module 7, Introduction to Solid-State Devices and Power Supplies, NAVEDTRA 172-07-00-82.

Transistors are classed as either PNP or NPN, according to the arrangement of impurities in the crystal. The schematic symbols for both types are similar but not identical, as illustrated in figure 5-16. A heavy straight line represents the base, and the two lines slanted toward the base represent emitter and collector terminals. An arrow head in the emitter line always points toward the N-type material. Thus, if the arrow points toward the base line, the base is of N-type material. Since the base is always of the opposite type of material from the emitter and collector, the transistor is of the PNP type. Similarly, if the arrow points away from the base line (toward the emitter), the transistor is of the NPN type. In other words, the arrow points toward the base in PNP transistors and away from the base in NPN transistors.

Recall that we said transistors are used as electronic switches to control the flow of current in a circuit. Let us now explore how this control is possible. Transistors used as switches are operated in one of two conditions: (1) the transistor is reverse bias (nonconducting) to open a circuit, or (2) the transistor is forward bias (conducting

Figure 5-16.-Transistor schematic symbols.

heavily) to close a circuit. Under these conditions, it acts as a simple ON-OFF toggle switch.

A transistor conducts when it is forward biased and stops conducting when it is reverse biased. An NPN transistor is forward biased when the electrical potential felt at the base is HIGH in relation to the electrical potential felt at the emitter. A PNP transistor is just the opposite; it is forward biased when the potential at the base is LOW in relation to the emitter. To illustrate the principle of electrical potential, think of a garden hose with the water pressure turned on. With the nozzle turned off, no water flows through the hose, but water pressure is felt at the nozzle.

LOGIC CIRCUITS

Logic circuits function just like the transistorized circuits we just described; they just look different. The basic component of both systems is the same-the transistor. However, instead of individual transistors attached to a circuit board and wired together to form a circuit, logic circuits use silicon chips printed or etched with several transistors and a circuit. The etched circuits are designed to perform a standard summarizing function. They take several inputs in and provide one output. We will describe the operation of logic circuits as we examine the circuit used to close the breechblock of a Mk 45 Mod 1 gun mount.