Model 12-4 Technical Manual Section 8
Ludlum Measurements, Inc. Page 8-3 May 2016
HV Supply Board (Drawing 464 × 243)
Switching Convertor
HV 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 V from the main board.
Feedback
The high-voltage output is reduced 99.9% by voltage dividers 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 V. If voltage at pin
2 is zero, voltage at pin 1 of U1 must be 2.541 V to maintain 1.24 V at pin 8
of U2. To obtain 2.541 V at pin 1 of U2 the output voltage must be 2.541 V.
In like manner, as the voltage at pin 2 of P1 increases to 2.181 V, the HV
output will be driven to zero.
Filtering
The HV 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%.
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