3-42
respectively. Remember that tube voltage varies slightly through its operating range, but this voltage
change is less than that which would exist without the use of a VR tube.
As the source voltage increases, the current through the VR tube increases. Since the upper limit of
tube current is 40 milliamperes, there is a limit in the ability of the tube to regulate increasing voltage.
When the supply voltage exceeds 158.6 volts, tube current will be greater than 40 milliamperes and
regulation will cease.
If the source voltage decreases from 150 volts to 145 volts, only 55 volts must be dropped across the
490-ohm series resistance (RS) to maintain the load voltage at 90 volts. Current through R
S
for a 55 volt
drop is 112 milliamperes. Since load current is 100 milliamperes, the remaining 12 milliamperes must
flow through the VR tube. This represents a decrease in the ionization level of the VR tube, with a
resultant increase in tube resistance. Under these conditions, 90 volts will be maintained across the load
resistance.
VR Tubes Connected in Series
In applications where a regulated voltage in excess of the maximum rating of one tube is required,
two or more tubes may be placed in series as shown in figure 3-45.
Figure 3-45.—VR tubes as voltage dividers.
In the figure, a VR75-30 and a VR105-40 are shown connected in series. The source voltage is 250
volts, and 82.5 milliamperes flows through the load resistance. Since current through the two VR tubes is
common, the limits of regulation are determined by the tube having the smaller current limitations. (In
this case, the VR 75-30). In computing Imean for this circuit, Imax and Imin will be 30 milliamperes and 5
milliamperes, respectively. Therefore, the mean current will be 17.5 milliamperes.
The value of RS in the figure can be computed using the source voltage of 250 volts and the total
current through R
S
(load current + Imean). Using these values, RS = 700 ohms. Note that the regulated
voltage to the load is 180 volts. This provides a regulated voltage greater than would be possible using
either VR tube by itself.
Another advantage of using VR tubes in series is illustrated in figure 3-46. In this circuit, several
values of regulated voltages are obtained from a single power supply.
