Dynalyzer – Digital Display/Printer OPERATION MANUAL
14
Figure 9: Use of 75% Trigger Level for Three-Phase Exposure Time Measurement
The mA trigger capability is especially useful in grid pulsed systems, as pulsed-grid tubes such as
sine units are, in general, operated with a constant potential capacitor bank or a continuously
present three phase voltage. Triggering on voltage is therefore not possible. Because of the fast
current rise time, the trigger level is not critical, and may be set according to the manufacturer’s
instructions or at 75% to be consistent with the voltage trigger point. The exposure should consist
of only one pulse or the Dynalyzer will reset.
Capacitor discharge generators operate by turning off the negative bias present on the grid of the
x-ray tube. The exposure time is a function of this negative bias pulse width. The anode current
discharges the capacitors until the voltage drops below some predetermined threshold. A kV
trigger cannot be used in this case because the voltage is present before the exposure begins. mA
trigger will give the true exposure time if the trigger level is properly set. The mA waveform will
almost be a square wave with fast rise and fall times.
An external trigger may be used if an exposure signal is supplied by the generator.
mA trigger is also useful in systems where long cable lengths and moderate mA’s (25-100) cause
the cable to hold a charge at the end of the exposure. The kV waveform will appear to have a long
tail, and the measured exposure time will be unnecessarily long if kV triggering is employed. See
the following figure:
75% TRIGGER
LEVEL
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