Siemens MOBILETT Plus Manual page 38

13 - 2
This pulse-width modulated signal can be measured at test point "FIL" on board D4 and is
used to control a MOS-FET on board D4.
The MOS-FET is connected in series with the filament transformer in the single tank, H1.
This MOS-FET supplies the filament transformer with regulated current pulses having a
frequency of 20 kHz.
The voltage of the current source can be measured between the test points "60 VDC" and
"60 V DC RET" on board D4. The actual value of this voltage can be measured at test
point "FILVOL" (1 V/14 V) on the CPU board D1.
A measuring resistor on board D4 measures the primary filament current.
The voltage drop, occurring across the resistors, is fed after a filtering to the CPU board
D1.
The filtered measured value can be measured at test point "FIL VAL" on the CPU board
D1 as a representative IH
In "stand by mode" the X-ray tube filament is heated with the so-called "preheating". In
this mode, it is possible to measure a pulse time of 6 µs at test point "FIL" on board D4.
The actual filament current value signal "FIL VAL" is measured and monitored every
52.5 ms by the microprocessor on board D1.
In "stand by mode" the signal "FIL VAL" must be between the limits 0.16 – 2.5 V.
If this measured value is outside of the predefined monitor limits,
•
the filament circuit will be immediately disabled,
•
the green "ready" LED on the Control and display panel will be turned off, and
•
an error message will be shown in the Control and display panel.
In "preparation mode" (with the primary contact of the exposure release switch closed) the
pulse width modulation is changed.
The pulse width for the trigger signal "FIL" on board D4 is continuously increased to a
value of about 20 - 30 µs, and then held constant.
The pulse width value used in the preparation depends on the selected kV value. In the
CMOS memory there are two tables with values, one contains fixed start values and the
other one contains the self adapted values of the generator.
MOBILETT Plus/ Plus E
in the filament of the X-ray tube.
actual
Register 6 SPR8-215.850.02
Rev. 01
Page 2 of 4
01.02 SPS-UD
Filament circuit
Siemens-Elema AB
Solna, Sweden