Dynalyzer – High Voltage Unit
OPERATION MANUAL
23
Figure 12:
Schematic Diagram of a Typical X-ray Generator Circuit
Single-Phase Radiographic Current Measurements using Oscilloscope
When the current level is sufficiently high, the effect of cable capacitance may be neglected. If the
load is resistive, the current waveform would follow the voltage. For a full-wave rectified sine wave,
the average current should be:
l
o
= 2l
m
/ = 0.636 x l
m
However, close inspection will reveal that this is not the case. Because the current is emission
limited, the current will increase with the lower kV values, but at some point, will saturate (see
Figure 12). The formula above is thus no longer valid and graphical integration of similar areas will
yield a better value. Figure 13 illustrates the sectioning of the current waveform into triangular and
rectangular areas for integration. Division of the waveform into equal width segments is a second
approach to graphical integration as shown in Figure 14. The easiest approach would be to take 10
values of the current, equally spaced in time in the period of one cycle, and add them together and
divide by ten.
The peak current is different in each pulse, while the form factor or waveshape remains the same.
If graphical integrations are used at maximum accuracy, the ratio of peak to average value of one
pulse will be obtained, and this ratio used to determine the average value of each pulse. Then the
average value of the entire exposure can be approximated.
Under certain conditions it may be possible to make extended radiographic exposures without
exceeding the instantaneous loading and the target heat storage capacities. In general this
procedure would be limited to exposures under 200 mA, and under 6 seconds. The rating sheet of
the tube unit should be consulted.
A DVM may be used to measure the current. The user is cautioned that the current measured
during long exposures may not accurately equal the current measured in shorter exposures,
especially in falling load type generators, or in generators with mA stabilizers. Heating effects in
the target and cathode assembly may affect the exposure as well.
CABLE
CAP
CAP
CABLE
CATHODE
ANODE
2
1
