3-41
According to Ohm's law, the value of the load resistance for this circuit figure will be 900 ohms if a
current of 100 milliamperes flows through RL. The internal resistance of the VR tube can be calculated in
a similar manner. With 22.5 milliamperes flowing and 90 volts dropped across the VR tube, its resistance
is 4 kilohms.
To determine the voltage regulation in the circuit for figure 3-44, assume a constant supply voltage
of 150 volts and a variable load resistance. If the value of RL were to decrease to 857 ohms, the load
current would increase to approximately 105 milliamperes to maintain 90 volts across the load resistance.
RS must drop 60 volts. To do so requires a current of 122.5 milliamperes flowing through the series
resistance. Since 105 milliamperes is now flowing through the load, the current through the VR tube must
decrease from 22.5 milliamperes to 17.5 milliamperes. We will discuss the sequence of events in more
detail to help you better understand how the tube current is made to vary.
The original load resistance was 900 ohms. Changes in this resistance will not occur instantaneously,
but will require some time to vary from 900 ohms to a new value. As resistance of the load begins to
decrease, load current begins to increase. The minute increase in load current will flow through the series
resistance RS causing a slight increase in ERS. This slight increase in voltage across RS will result in the
VR tube voltage dropping slightly. This slight drop in tube voltage will cause a decrease in the ionization
of the tube gas, which in turn increases the resistance of the tube. As a result, less current flows through
the tube.
Note that tube current can decrease only to a value of 5 milliamperes before deionization occurs.
Therefore, the load current cannot exceed 117.5 milliamperes, for beyond this value, tube current
becomes less than 5 milliamperes and regulation ceases.
If load resistance were to increase, load current would decrease. This would result in the VR tube
current increasing to maintain a current of 122.5 milliamperes. The VR tube current can only increase to
40 milliamperes. Beyond this value of current, the tube enters the abnormal glow region and tube voltage
increases.
The upper limit of the VR tube current will occur when load current decreases to a value of 82.5
milliamperes. When load current drops below this value, the VR tube ceases to regulate the load voltage.
Therefore, with a constant source voltage but variable load resistance, the limits of regulation will be
reached when current in the load exceeds 117.5 milliamperes or drops below 82.5 milliamperes.
The VR tube regulator can also compensate for changes in power supply voltage. Under these
conditions, the load resistance will remain constant while the power supply voltage will be variable. Refer
to figure 3-44 for the following discussion.
Assume that the source voltage begins to increase from an original value of 150 volts toward 155
volts. As this voltage increases, current through RS increases from its original value of 122.5
milliamperes. Initially, this additional current is drawn from the load, causing a slight increase in load
voltage. This increase in load voltage is felt across the VR tube and causes an increase in tube ionization.
This decreases the internal resistance of the VR tube with a resultant increase in tube current. When
source voltage reaches 155 volts, current through RS is approximately 133 milliamperes (RS = 490 ohms).
Most of the additional current through RS flows through the VR tube. As a result, approximately 33
milliamperes flows through the VR tube, maintaining the load voltage at 90 volts.
Since VR tube current decreases as source voltage decreases, tube current will drop below its lower
limit of 5 milliamperes at some point. When source voltage drops below 141.4 volts, tube current will be
less than 5 milliamperes and regulation will cease. The upper and lower limits of the supply voltage
variations that can be allowed and still provide regulation in the circuit are 158.6 volts and 141.4 volts,
