Siemens MOBILETT Plus Manual page 44

15 - 2
At the start of an exposure, the high-voltage inverter on board D13 is controlled by a pulse
frequency of 50 kHz.
This frequency is continuously decreased during 0.7 ms to 20 kHz, or 25 kHz (<60 kV).
This frequency is held during the rest of the exposure.
The frequency scan during the exposure start is intended to limit the inverter current.
By means of measuring resistors in the single tank, the actual high voltage values at the
anode and cathode "HT+" and " HT-" are fed to operational amplifiers on the CPU board
D1.
The outputs of these signals can be measured at test points " HT POS" and "HT NEG".
These two signals are converted by means of a differential amplifier into the signal "HT"
and by means of a summation amplifier into the signal "HTD".
The signal "HT" represents the voltage drop across the X-ray tube in the single tank.
It is transformed via another amplification circuit into the peak value of the tube high volt-
age "HT PEAK".
The signal "HTD" represents the difference between the two high voltage signals "HT
POS" and "HT NEG".
The high voltage signal levels are as follows:
The microprocessor 80C166 on the CPU board D1 measures the signal "HT PEAK" every
150 µs. The measured values are used as input to the feedback-control of the tube volt-
age.
The tube voltage is controlled by pulse-width modulation of the high-voltage inverter con-
trol signals, and of the filament-control signal.
The values of "HT PEAK" are checked to be within its tolerance range of ±5 kV with refer-
ence to the selected kV value.
The signal "HTD" is also checked against its limits of ±20 kV.
The maximum values of the signals "HTD" and "HT" are defined as follows:
MOBILETT Plus/ Plus E
"HT POS" +1 V / 10 kV
"HT NEG" –1 V / 10 kV
"HT" +1 V / 40 kV
"HTD" ±1 V / 40 kV
"HT PEAK" +1 V / 40 kV
"HTD" ±20 kV
"HT" 140 kV
Register 6 SPR8-215.850.02
Rev. 01 01.02
High-voltage circuit
Page 2 of 4 Siemens-Elema AB
SPS-UD Solna, Sweden