Rosemount 1154
Reference Manual
OSCILLATOR
VOLTAGE REGULATOR
ZERO AND SPAN
ADJUSTMENTS
CURRENT CONTROL
CURRENT LIMIT
REVERSE POLARITY PROTECTION
The oscillator has a frequency determined by the capacitance of the sensing element and the inductance of the transformer windings.
The sensing element capacitance is variable. Therefore, the frequency is variable about a nominal value of 32 kHz.
An integrated circuit amplifier is used as a feedback control circuit and controls the oscillator drive voltage such that:
fV Iref
p – p = --------------------
C1 + C2
The transmitter uses a zener diode, transistor, and resistors to provide a constant voltage of 6.4 V dc for the reference and 7 V dc for the oscillator.
Zero adjustment components consist of a potentiometer and resistor that develop a separate adjustable current that sums with the sensor current. The coarse zero switch switches resistors into the circuit as needed.
Span adjustment is performed with a potentiometer which determines the amount of loop current which is sensed and fed back to the current control amplifier.
The current control amplifier consists of an integrated circuit, two transistors, and associated components. The IC reference voltage is established at the junction of a resistor network. The current control amplifier drives the current control to a level such that the current detector feeds back a signal equal to the sum of the zero current and the variable sensor current.
A current limiter prevents the output current from exceeding 30 mA in an overpressure condition.
A zener diode provides reverse polarity protection.
