Scion Instruments 436-GC Service Manual page 98

436-GC/456-GC
Igniter Supply
The igniter power supply provides a regulated current of 3.2ADC to a load of about 0.5 to 0.7 Ohms. The
+24VDC system supply voltage is stepped down through an LT 1074 switching regulator (U2), connected
in the buck topology. The regulator operates at 100kHz, connecting and disconnecting the +24V input with
its output (pin4). When pin 4 is switched to +24V, the current in L1 rises. When the switch is opened, the
voltage at pin 4 drops rapidly to -0.5V, and the inductor current, which had been flowing through VR4, flows
through CR9. C13 smooths the output voltage as the current in L1 rises and falls.
Load current flows through R14, creating a voltage drop of 0.32V at 3.2A. The difference between this
voltage and the voltage set by R17 and R18 is amplified in AR1 and applied to the PWM comparator input
of U2. The duty cycle of the regulator shifts in the appropriate direction to force the two voltages to be the
same. R16 and C18 set the bandwidth of the feedback loop to prevent oscillations. The error amplifier in
U2 is not used, since it requires an input voltage of +2.2V, which is much too large for this application. C17
keeps high-frequency noise out of the error amplifier input, so that it does not interfere with the external
amplifier control.
The igniter power supply is turned on and off with a signal from latch U12-19. This signal is inverted by U7
(pins 8-13), which is a flip-flop connected as an inverter. (The inversion is required to turn the supplies off
when latch U12 is reset by the SYSENAB signal.) When the control signal from U7-9 is high, diode CR10 is
turned on. Pin 6 of AR1 is pulled well above pin 5, forcing the output of AR1 to ground and lowering the
output current close to zero.
U11 (pins 9-15) decodes two signals from latch U12 (pins 15-16), to select which one of three diagnostic
voltages is applied to the Detector Miscellaneous input of the system ADC. Selecting the Y2\ output of U11
(pin 10 low) turns on analog switch U8 (pins 911). The voltage at the top of R14, which is equal to the
igniter current (in amps) divided by 10, is applied to the ADC multiplexer input. R39 limits current through
U8 if there is a short at the multiplexer input, preventing destruction of the switch and pcb traces. Selecting
the Y1 \ output of U11 (pin 11 low) applies the voltage on the high side of the igniter, divided by 11 through
R37 and R38, to the ADC input. The division is necessary, because the igniter voltage will rise to the full
+24VDC supply voltage when the igniter is disconnected.
High Voltage Supply (sheet 2)
The high voltage power supply is a balanced inverter which is adjustable from -300 to -900 Volts. A TL494
switching regulator controller (U1) furnishes the reference voltage, two error amplifiers, a pulse-width
modulator, and the power switching transistors on a single chip.
One error amplifier regulates the high voltage output, which is summed with the output of a DAC (U13,
section B) through R7 and R8, and applied to the inverting input of the amplifier (U1-2). Since the non-
inverting input (U1-1) is held at ground, the inverting input must also remain at zero volts, forcing the high
voltage output at J2 to vary from 0 to -900 VDC as the DAC output voltage varies from 0 to +10VDC.
Feedback from the amplifier output (U1-3) through R4 and C2 determines the regulator loop characteristics
and maintains stable operation. Capacitor C4 reduces coupling of output switching noise and nearby digital
signals into the amplifier input.
If the detector's photomultiplier tube is exposed to ambient light with high voltage applied, excessive anode
current will flow. Damage is prevented under such conditions by shutting down the high voltage supply
whenever the detector output signal exceeds ten to twelve volts. Under normal signal conditions, the non-
inverting input of the second error amplifier (U1-16) is held negative through R11. CR7 prevents the input
from dropping below -300mV, which would cause the device to malfunction. When the signal voltage rises
above +10V, current through R10 pulls U1-16 positive. The error amplifier output (U1-3) also rises, with a
gain of three (determined by R3 and R9), thus reducing the supply voltage. Note that the outputs of both
error amplifiers appear at U1-3. Since they share a common pull-down resistor, the amplifier whose output
is more positive controls the composite output terminal.
Pulse-width modulation at a 20kHz rate (set by R1 and C3) controls the supply output voltage. The ends of
the primary winding of T1 are alternately connected to ground through R2 and R6 by the switching
transistors in U1. The resistors limit the primary current, allowing pulse-width control of the output voltage
and protecting the switching transistors if the supply is shorted. Diodes CR1 and CR2 prevent the ends of
the primary from dropping below ground, which also clamps the positive excursions at +30V, due to the
SCION Instruments
436-GC/456-GC Service Manual Revision B February 2019
610 Hardware description
Page: 97