Ecd Electronics Circuit Description - Scion Instruments 436-GC Service Manual

436-GC/456-GC

ECD Electronics Circuit Description

Power supplies and grounds.
There are four grounds serving the ECD electronics circuitry. Ground 1 is the return for the +5V digital
supply. It is connected directly to Ground 4, the unregulated +24V return, at the card edge connector on
the Mother board. The analog return is Ground 2, which provides a low-noise return for the ±15V, +5.25V,
and -5V supplies. Ground 3 is the reference for analog signal distribution, which carries almost no DC
current. All of the grounds must be tied together externally for the board to function properly.
Four RC filters remove noise from the analog power supplies. These consist of R43 with C32, R44 with
C29, R45 with C30, and R46 with C31.
Digital circuits.
One half of U5 decodes bus address information to access latch U7, buffer U6, and dual DAC U8. The
latch holds all of the digital control signals for the board, while the buffer transmits the board identification
number (02H) to the bus. R47 and R48 allow the inputs of U6 to be pulled high or low for automatic test.
The other half of U5 is used only as an inverter.
Power converter.
A switching regulator, connected in the inverting configuration, generates -50V from the unregulated +24V
input. The maximum current drain from the -50V supply is less than 10mA in normal operation. U2 (a
TL494) provides all of the functions needed to control the supply: an oscillator, pulse-width-modulation
comparator, voltage reference, dual error amplifiers, and output transistors. C4 and R12 set the oscillator
frequency to 50kHz, thus applying a 50kHz sawtooth to one input of the pwm comparator. The other input
to the comparator is driven by the outputs of both error amplifiers, which are connected in such a way that
the amplifier calling for the lower output from the supply dominates. The minimum dead time between
output pulses, regardless of the error amplifiers, is set to about 5% by grounding the DT input of U2 (pin 4).
Error amplifier #1, whose inputs are on pins 1 and 2, is used as the primary loop control amplifier. Its
inverting input is connected to the +5V±5% reference voltage (pin 14) through R1, and to the output of the
supply through R2. Since the noninverting input is connected to ground, the 1C will adjust its duty cycle to
drive the inverting input to zero volts also. This will occur only when the output voltage is -50V, since R2 is
ten times as large as Rl. Feedback from the error amplifier output (U2-3) is taken through C3 and R4,
which stabilizes the loop with a very high DC gain. CR8 prevents pin 2 from being driven to such a low
voltage that the 1C quits functioning properly.
Both output transistors are driven in phase, because the Output Control pin (U2-13) is tied to ground. The
emitters of the transistors (pins 9 and 10) are grounded, and the collectors (pins 8 and 11) drive Q1
through R10. R11 speeds the turn-off of Q1 in between pulses. When Q I is turned on, the +24V supply is
connected to Ll, causing the current through Ll to ramp upward. As soon as Ql is turned off, its collector
voltage falls rapidly until CR1 turns on, allowing the inductor current to continue flowing. After a number of
pulses, C2 charges to the desired output voltage, and the pulse width stabilizes at the value needed to just
maintain the output at -50V.
R13 and R14 divide the output voltage of the -50V supply by 51 for measurement by the system ADC
through P 1-3.
Current-to-frequency converter.
The circuitry across the top of the schematic page, including the sections labeled ECD ELECTROMETER,
V TO F CONVERTER, and ECD PULSER, forms a feedback loop with the external electron capture
detector cell. We will begin the description of this circuitry in the middle, where AR2 and AR3 comprise a
current-to-frequency converter.
SCION Instruments
436-GC/456-GC Service Manual Revision B February 2019
610 Hardware description
Page: 85