Theory Of Operation; Function Description; Simplified Block Diagram - Emerson Rosemount 1181pH TRANSMITTER Instruction Manual

MODEL 1181pH/ORP
4.1 FUNCTION DESCRIPTION (see Figure 4-1). The
Model 1181 operates from the power it receives from
the loop current. At 4 milliamps, with 5 volt drop in the
Model 1181, available power to operate the Model
1181 is 20 milliwatts typical (40 milliwatts maximum).
The power supply (AR3) is a free CMOS multivibrator
which drives the transformer (T1) at a frequency of 20
kHz. A transformer (T1) generates the ±5 volts power
on the loop side (F) and on the sensor side (S). T1 also
provides proper polarity by phase modulating the mod-
ulator and demodulator.
4.1.1 An external preamplifier is used which receives
±5 volts power from P.S. (S). ARl receives the signal
SECTION 4.0

THEORY OF OPERATION

FIGURE 4-1. Simplified Block Diagram
input from the external preamplifier (+2.5 volts for pH;
±1.0 volt for ORP) and its gain is modified by the inter-
nal range select switches. The modulator is a current
device and is coupled to the demodulator by an isola-
tion transformer (T2). The output amplifier (AR5) drives
the current generator which returns a 4 to 20 milliamp
signal which is proportional to the demodulated signal.
The external zero and span potentiometers calibrate
the 4 to 20 milliamp output.
4.1.2 Diode (VR6) is a current diode which provides
the initial start-up current. Diodes (CR10 and CR11)
are meter protection diodes. The indicator is a 4 to 20
milliamp meter.
SECTION 4.0
THEORY OF OPERATION
31