Ludlum Measurements 12, Technical Manual

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Model 12 Technical Manual
Section 8
Ludlum Measurements, Inc. Page 8-3 July 2017
HV Supply Board (Drawing 464 × 243 in back of
manual)
Switching Convertor
High voltage is developed by voltage multiplier CR1 through CR10 and
associated capacitors. This multiplier is driven by switching convertor U2
and T1. The convertor is powered by regulated 6 volts from the main board.
Feedback
The high-voltage output is reduced 99.9% by voltage divider R4, R5, and R6.
The remaining voltage is coupled to the HV meter circuit through R7 and
pin 1 of P1. Also, the voltage is coupled through R8 to the switcher
feedback input pin 8 of U2.
Voltage Control
The switching convertor U2 and T1 will develop an output voltage at C3
and R4, such that the voltage at pin 8 of U2 stays at 1.24 volts. If voltage at
pin 2 is zero, voltage at pin 1 of U1 must be 2.541 volts to maintain 1.24
volts at pin 8 of U2. To obtain 2.541 volts at pin 1 of U2, the output voltage
must be 2.541 volts. In like manner, as the voltage at pin 2 of P1 increases to
2.181 volts, the high-voltage output will be driven to zero.
Filtering
The high voltage is filtered by R2, R3, C1, and C2. R2 and R3 cause the
power supply to have an output resistance of 1.5 MEG. This will cause a
metering error for heavy loads. For the typical 100 MEG detector, this error
will only represent 1.5%.