Power Supplies

INTRODUCTION TO POWER SUPPLIES

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 supply that was reliable, convenient, and cost effective. Since batteries failed to satisfy these requirements, the "electronic supply" was developed.

In today's technically advanced world, all electronic equipment, require some type of 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. We will also discuss troubleshooting of each section and its components.

THE BASIC POWER SUPPLY

The illustration below shows the block diagram of the basic supply. Most supplies are made up of four basic sections: a TRANSFORMER, a RECTIFIER, a FILTER, and a REGULATOR.

Block diagram of a basic power supply.

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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 supply and see what changes are made to the input signal.

In the next illustration below, 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 × 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.

The illustration also 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 supply.) This signal now goes to the regulator where it will be maintained at approximately 180 volts dc to the load.

Block diagram of a power supply.

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THE TRANSFORMER

The transformer has several purposes: In addition to coupling the input ac signal to the supply, it also isolates the electronic 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 supply tubes and the tubes in the external equipment (load).

Such a transformer is shown in the illustration below. Because the input transformer is located in the power supply and is the ultimate source of power for both the load and the 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.

Typical power transformer.

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An Introduction to Rectifiers and the half-wave rectifier

Full-wave-rectifiers: the practical, conventional, and complete full wave rectifier.

The-bridge-rectifier and a comparison of the full wave and bridge rectifiers.

Filters and capacitor charge and discharge of a capacitor filter.

Inductive-filters: voltage drops in an inductive filter.

Capacitor-filters: half-wave and full-wave capacitor filters

The-LC-choke-input filter in a power supply: waveforms and filtering action.

LC-choke-input-filters-cont.-

Resistor-Capacitor (RC) Filters

LC Capacitor-Input Filter

Power supplies voltage and load regulation

Power-supply series and shunt voltage regulators

The basic VR tube regulator

VR-tubes connected in series and parallel

The electron tube voltage regulator

Current regulation and the amperite regulator

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