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
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.