Fluke Biomedical 942A-200L-M4 - Manual (Page 35)

Theory of Operation
High Voltage Supply (942-200-63, Appendix B)
2
2-21
2.3 High Voltage Supply
(Schematic 942-200-63, Appendix B)
The high voltage is utilized in a scintillation detector by the photomultiplier tube (typical range 750 volts to
1500 volts) or by a GM detector (typical range 500 volts to 650 volts). The adjustment range of the HV
supply is 300 VDC to 1800 VDC. The HV output is short circuit proof in that it will current limit the
oscillator section within ten seconds of the output being shorted. The board plugs into the main circuit
board at the J8 connector.
R5 and associated circuitry provide the DC voltage adjustment to U1 - C. U1 device 3-10, normally held
at ground by R17, allows an error input in applications using the optional americium regulator option
board. The output U1-8 will vary under control of either R5 or the error input voltage. Table 2-17 shows
the effect of the error input voltage on the high voltage.
Table 2-17. Error Input Voltage Range
+1 Volt (HV) +135 Volts
+2.5 Volts (HV) +335 Volts
-1 Volt (HV) –135 Volts
-2.5 Volts (HV) –335 Volts
+10 mV (HV) +1.35 Volts
Operation amplifier U1-A drives transistor Q1 which in turn drives the oscillator section transistor Q2, the
transformer primary and feedback windings, and associated circuitry. As R5 is adjusted to increase the
high voltage, U1-C voltage increases which causes U1-A to increase transistor Q1 base current. This
increases the emitter/collector current, raising the voltage on the emitter. As this control voltage
increases, the voltage developed across the transformer primary also increases. The transformer
secondary increases in voltage, which causes the high voltage output to increase. The voltage
quadrupler operation is illustrated in Figure 2-6.
R2 is a 1000:1 voltage divider which provides feedback to control the high voltage. U1-B is used as a
buffer between the 1000:1 divider and the 1000:1 output signal utilized by the V/F converter on the main
circuit board. This output signal is also utilized by U1-A to regulate the high voltage and is sent to the HV
test jack. The shutdown signal (provided by the controller) when low has no effect on the high voltage.
When the shutdown signal goes high, Q3 conducts causing Q1 to turn off. This action forces the
oscillator to turn off, effectively shutting down the high voltage.
Short circuit protection is provided by the positive temperature coefficient thermistor (PTC). The PTC
resistance in normal operation is nominally 5 ohms. When the high voltage output is shorted, the control
circuitry U1-A attempts to maintain regulation by increasing the base drive for transistor Q1. Excessive
current flows through the PTC, causing the internal temperature to increase. As the temperature
increases, the PTC resistance also increases dramatically. The effect is that the control voltage to the
oscillator is decreased to a minimum level.
The response of the PTC is approximately ten seconds. Removal of the short circuit condition results in
restoration of the high voltage to the preset level.