Model 15
Technical Manual
Section 8
Ludlum Measurements, Inc.
Page 8-3
June 2020
Low Voltage Supply
Battery voltage is coupled to U7 and associated components (a switching
regulator) to provide 5 V at pin 5 to power all logic circuits. Unregulated
battery voltage is used to power the meter drive (Q6) and the high voltage
blocking oscillator Q145.
Low Voltage Reference
U101 provides a 1.22 V precision reference for HV supply. This unit also
biases Q96.
High Voltage Supply
High voltage is developed by blocking oscillator Q145-T165 and rectified by
voltage multiplier CR166, 167, 169, and 175. Output voltage increases as
current through Q44 increases, with maximum output voltage with Q44
saturated.
High voltage is coupled back through R47, R90 to opamp pin 6 of U6.
R147 completes the high-voltage circuit-to-ground. High-voltage output is
set by front-panel control HV, which sets bias of pin 5 of U6. During stable
operation, the voltage at pin 6 of U6 will equal the voltage at pin 5 of U6.
Pin 7 of U6 will cause conduction of Q44 to increase or decrease until the
high voltage seeks a level for stability.
