Metal Samples MS2500L, Руководство оператора

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Current Loop Control
The circuitry for the current loop control is contained on the A2 Board and consists of:
• The current loop sense and balancing amplifier; AR5-B.
• The current loop control transistor; Q1.
• The current loop sense network; R23, R24, R25, and R41.
The output of the current loop must be maintained at the desired signal level regardless of the amount of
current drawn in order to power the circuitry. This is accomplished by using the R23 resistor placed in
the loop to develop a voltage drop from -1.0 to -5.0 volts for a current span of 4-20 mi11iamps. At an
output of zero, the sample and hold circuitry is at 1.0 volt (with reference to ground), this voltage is
subtracted from the negative voltage developed by R23 using the R24, R41, and R25 network. AR5-B
senses this difference and biases Q1 until the current loop develops a -1.0 volt drop across R23,
(4mA). At full scale output, the sample and hold develops a 5.0 volt signal thus requiring a 20mA loop
current to balance the circuit.
E. Operation Modes
Offset Mode
A mentioned earlier, the polarization current and the potential null circuitry are disabled when operating
in the and OFFSET- modes . The reference to the instrumentation amplifier, SW1-C, is set
to the common reference and the output of the instrumentation reflects the cell potential difference. This
value is sent directly to the output sample and hold circuitry in the mode. If OFFSET- mode
is selected, AR1-C inverts the polarity and amplifies the signal before it reaches the output sample and
hold circuit.
Track Mode
When the TRACK mode is selected, the output sample and hold circuitry is set to run repetitive conver-
sion cycles constantly updating its output on 2.5 second intervals.