21
Other Circuit Components 3250/4250 Circuit Description
Note This instrument requires the use of a null modem RS-232C cable. There are several
variations on null modem cables. Advanced Instruments recommends that you
purchase a RS-232C cable direct from our factory.
Supervisor/Operator Keyswitch Interface: A PLD is used to interface the supervisor/ operator
keyswitch to the microprocessor. In Operator position, the user cannot change setup or
calibration settings.
Other Circuit Components
Drive Circuitry, Serial Suffix A - C: Drive circuitry is provided to turn on and off the four high
current loads such as the stir wire, the head motor, the thermoelectric cooler, and the fan.
The stir wire coil is controlled by the rear panel-mounted darlington transistor (PCB522). The
drive circuit consists of a programmable timer, D/A converter, and interface op-amps. The
timer provides a square wave of approximately 71Hz, while the D/A converter controls the
output amplitude to the darlington transistor.
The head motor is controlled by two relays located at the rear of the chassis that interface
the 120 VAC motor to the DC logic. One relay raises the head, while the other relay lowers
the head. The relays get their commands from the application board via two FETs and
the application logic. LEDs are provided on the application board for monitoring the drive
signals. LEDs are also provided to monitor the signals from the head sensor board, described
below.
The thermoelectric cooler is controlled by the FET driver transistor mounted on the rear
panel (PCB520). The FET is, in turn, controlled by the microprocessor through the PLD
application logic. The processor varies the duty cycle square wave in response to software
commands and block probe resistance.
Drive Circuitry, Serial Suffix D and Higher: Drive circuitry is provided to turn on and off four
system loads such as the stir wire, the head motor, the thermoelectric cooler, and the fan.
The stir wire coil drive circuit consists of a programmable timer, D/A converter, and
interface op-amps on the application board (PCB605). The timer provides a square wave of
approximately 71 Hz, while the D/A converter controls the output amplitude to the darlington
transistor mounted on the driver board (PCB523).
The thermoelectric cooler is controlled by the microprocessor through the PLD application
logic. The processor varies the duty cycle square wave in response to software commands
and block probe resistance. These signals are provided to the FET driver transistor mounted
on the driver board (PCB523).
The head motor is controlled by the application board via two FETs and the PLD logic.
LEDs are provided on the application board for monitoring the drive signals. LEDs are also
provided to monitor the signals from the head sensor board, described below. Two active low
head up/head down signals are sent to the driver board (PCB523) where they are converted
into acceleration, direction, and step commands for the motor drive chip. The motor drive
chip translates these commands into current controlled outputs that move the DC stepper
motor and provide idle torque when no movement is required. The following adjustments
may be available for the motor circuit: R10 and TP16 are used to set the initial acceleration to
a rise time of 4uS. R21 and TP15 are used to set the run frequency to 8 Khz.
The fan is controlled by a FET transistor on the application board, that is interfaced to the
processor through the application logic.
Head Transition Board (M20450) Description: The stir/freeze coil and the sample probe are
connected to the head transition board which allows easy sample probe replacement.
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