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MODEL 2224-1 Technical Manual
Section 7
Ludlum Measurements, Inc. Page 7-3 January 2012
Detector Overload
A voltage drop is developed across R121 and sensed by comparator U012 as
detector current increases. When the voltage at pin 3 of U012 goes below
pin 2, pin 1 goes low, illuminating the
OL LED
and driving the meter to full
scale. R211, Overload, provides adjustment for the overload set point.
Meter Drive
Pulses are coupled from the μ P board (refer to μ P theory of operation) to
the gate of Q302. Q302 inverts the pulses, and R403 and C401 provides
integration. Integrated meter drive voltage is coupled from P1-13 via the
battery (
BAT
) and
HV
test switch to pin 5 of U311. The meter is driven by
the emitter of Q111, coupled as a voltage follower in conjunction with pin 6
and 7 of U311. R406, Meter Cal, is adjusted to calibrate the ratemeter
reading corresponding to the incoming count rate. R407 and R408 provide
temperature compensation for changes in the meter resistance due to
temperature variations.
Refer to Processor Board Schematic, Drawing 390 × 107 for the
following:
Power supply
Battery voltage is coupled to switching regulator U321 and associated
components to p5 V to power the μ P and display drivers U211,
U212. R101, C101, Q101, and Q201 form a delay switch, which allows
U321 to stabilize before the load current is connected to the +5 V supply.
Microproccessor (μP)
U311, Intel N87C51FA, controls all of the data, control inputs, and display
information. The clock frequency is crystal controlled by Y211 and related
components at 6.144 MHz. The μ P incorporates internal memory (ROM),
storing the program information. C311 resets the μ P at power-up to initiate
the start of the program routine. During the program loop, the
μ
P looks at
all of the input switches for initiation or status changes and responds
accordingly.
The
μ
P uses pulse-width modulation to control the analog ratemeter. The
analog output,
RATE
(P3-3), is divided into 255 increments in a 166 μ s