POWER SUPPLIES

LEARNING OBJECTIVES

Upon completion of this chapter you will be able to:

INTRODUCTION

In the early part of this century when electronics was first introduced, most electronic equipment was powered by batteries. While the use of batteries allowed the equipment to be portable (to some degree), it also placed several limitations on how the equipment could be used. Because of their general inefficiency, batteries had to be either replaced frequently or, if they were rechargeable, kept near a battery charger. Thus, the advantage of having portable equipment was more than offset by the need to replace or recharge the batteries frequently.

Users of electronic equipment needed a power supply that was reliable, convenient, and cost effective. Since batteries failed to satisfy these requirements, the "electronic power supply" was developed.

In today's Navy, all electronic equipment, both ashore and on board ship, require some type of power supply. Therefore, this chapter is of extreme importance to you. We will discuss the sections and individual components of the power supply and their purposes within the power supply. We will also discuss troubleshooting of each section and its components.

THE BASIC POWER SUPPLY

Figure 3-1 shows the block diagram of the basic power supply. Most power supplies are made up of four basic sections: a <emphasis type="b">TRANSFORMER</emphasis>, a RECTIFIER, a FILTER, and a REGULATOR.

Figure 3-1. - Block diagram of a basic power supply.

As you can see, the first section is the TRANSFORMER. The transformer serves two primary purposes: (1) to step up or step down the input line voltage to the desired level and (2) to couple this voltage to the rectifier section. The RECTIFIER section converts the ac signal to a pulsating dc voltage. However, you will see later in this chapter that the pulsating dc voltage is not desirable. For this reason, a FILTER section is used to convert the pulsating dc voltage to filtered dc voltage. The final section, the REGULATOR, does just what the name implies. It maintains the output of the power supply at a constant level in spite of large changes in load current or in input line voltage. Depending upon the design of the equipment, the output of the regulator will maintain a constant dc voltage within certain limits.

Now that you know what each section does, let's trace a signal through the power supply and see what changes are made to the input signal. In figure 3-2, the input signal of 120 volts ac is applied to the primary of the transformer, which has a turns ratio of 1:3. We can calculate the output by multiplying the input voltage by the ratio of turns in the secondary winding to turns in the primary winding. Therefore, the output voltage of our example is: 120 volts ac X 3, or 360 volts ac. Depending on the type of rectifier used (full-wave or half-wave), the output from the rectifier will be a portion of the input. Figure 3-2 shows the ripple waveform associated with a full-wave rectifier. The filter section contains a network of resistors, capacitors, or inductors that controls the rise and fall time of the varying signal so that the signal remains at a more constant dc level. You will see this more clearly in the discussion of the actual filter circuits. You can see that the output of the filter is at a 180-volt dc level with an ac RIPPLE voltage riding on it. (Ripple voltage is a small ac voltage riding at some dc voltage level. Normally, ripple voltage is an unwanted ac voltage created by the filter section of a power supply.) This signal now goes to the regulator where it will be maintained at approximately 180 volts dc to the load.

Figure 3-2. - Block diagram of a power supply.

Q.1 What are the four basic sections to a power supply?
Q.2 What is the purpose of the regulator?

THE TRANSFORMER

The transformer has several purposes: In addition to coupling the input ac signal to the power supply, it also isolates the electronic power supply from the external power source and either steps up or steps down the ac voltage to the desired level. Additionally, most input transformers have separate step-down windings to supply filament voltages to both power supply tubes and the tubes in the external equipment (load). Such a transformer is shown in figure 3-3. Because the input transformer is located in the power supply and is the ultimate source of power for both the load and the power supply, it is called the POWER TRANSFORMER. Notice that the transformer has the ability to deliver both 6.3 and 5 volts ac filament voltages to the electron tubes. The High-voltage winding is a 1:3 step-up winding and delivers 360 volts ac to the rectifier. This transformer also has what is called a center tap. This center tap provides the capability of developing two high-voltage outputs from one transformer.

Figure 3-3. - Typical power transformer.

Q.3 What are the purposes of the transformer in a power supply?
Q.4 For what are the low voltage windings in a transformer used?
Q.5 For what is the center tap on a transformer used?