CURRENT REGULATION

Before we go to the next section, there is one type of regulation that we should discuss-current regulation. In most power supplies, current is not regulated directly. Fuses and other circuit protection devices are used to set an upper limit on the amount of current that can flow in a power supply. Once this limit is exceeded, the fuse simply opens and the power supply is deenergized. Beyond this, current is usually left unregulated because the load will draw from the power supply only the amount of current that it needs. Decreases and increases in the power supply voltage caused by the variations in load current are usually controlled by the voltage regulator.

The Amperite Regulator

There are some cases in which current must be regulated or kept at a relatively constant value. The best example of this is the filament supply of a power transformer located in a power supply that is designed to supply filament power to many tubes. You can see this in view (A) of figure 3-49, which is a portion of a power supply designed to supply 50 vacuum tubes with both plate and filament voltages. Under normal conditions, circuit current will not exceed 2.5 amperes. For this reason, the power supply has been fused at 3 amperes. Because you are only interested in current regulation at this time, only the portion of the power supply that deals with current regulation is shown; namely, the power transformer and four of the 50 parallel connected vacuum-tube filaments. At operating temperatures, the resistance of each filament is 1 kilohm. Because the filaments are connected in parallel, the total filament resistance at operating temperature is 20 ohms. Ohm's law,

Figure 3-49. - Amperite regulator.

tells you that the filaments draw .315 amperes. You should know from your previous study that as conductors are heated, their resistance increases. Therefore, the cold resistance of the filaments is considerably lower than the hot resistance. In this case, assume 100 ohms per filament. The total resistance of the 50 parallel filaments is then 2 ohms when the power supply is first energized, and the filaments draw 3.15 amperes of current. If the current for the rest of the power supply is added to the filament current, the surge current will cause the power supply to draw 5 amperes when it is first energized. Unfortunately, the power supply is fused at 3 amperes. Under these conditions, it would be impossible to keep the power supply on the line long enough to get the filaments up to operating temperature.

There are three possible solutions to this problem. The first is simply to fuse the power supply at 5 amperes, but this could allow excessive current to flow in the power supply. Another solution is to use a slow-blow fuse. Unfortunately, the duration of the current surge may exceed the time limit that a slow-blow fuse can handle. Therefore, current regulation is the best solution to this problem.

Because of its quick-heating ability, the amperite tube is ideal as a current regulator. The amperite regulator is nothing more than an iron wire enclosed in a hydrogen-filled envelope. Because of its construction, the iron filament will heat quickly when current is applied to it.

View (B) of figure 3-49 shows the amperite regulator connected in series with the filaments of the load. When the power supply is first energized, the iron wire of the amperite gets hot quickly and presents a large resistance connected in series with the 2 ohms of filament resistance. As a result, most of the voltage is dropped across the amperite. Because of the large resistance of the amperite regulator, current in the circuit is held to an acceptable level in accordance with Ohm's law:

As the filaments warm up, their resistance increases, which causes circuit current to decrease. The decreasing circuit current allows the iron wire of the amperite to cool. As it cools, its resistance decreases until it reaches the approximate resistance of the circuit wiring. You might think that decreasing the resistance of the amperite would allow circuit current to increase again, but this does not happen. As the iron wire of the amperite cools and its resistance decreases, the resistance of the warming tube filaments increases. Throughout the entire heating cycle of the filaments, the total resistance of the series circuit, consisting of the amperite and tube filaments, remains fairly constant. When power is first applied, most of the resistance is in the amperite. Therefore, most of the voltage is dropped across the resistance of the amperite. Halfway through the cycle, the resistance of the amperite and the resistance of the filaments are approximately equal, and the voltage drops across the two series elements are equal. Finally, when the filaments have reached their operating temperature, most of the resistance is in the filaments of the tube. Therefore, most of the voltage is dropped across the tube filaments.

The important thing to note is that the total circuit resistance remains approximately the same throughout the heating cycle. As the cycle progresses, the resistance of the amperite decreases as the resistance of the tube filaments increases. Because resistance and voltage (6.3 volts) remain constant, current remains constant, except for the slight surge in the beginning of the heating cycle, which is necessary to heat up the iron wire of the amperite.

Now that we have discussed the different types of regulators, you should be able to see that there are many variables that affect good regulation.

Although you may not be required to design regulators, you will be required to maintain them because your electronic equipment depends upon good regulation to operate property.

Up to this point we have discussed only the individual sections of the electron tube power supply. In the next section, we will discuss the techniques of troubleshooting these individual sections and the total power supply.

Q.41 What is the purpose of the amperite regulator?
Q.42 As the tube filaments in the load heat up, will the circuit current increase or decrease?