Principle Of Operation; Transmitter Circuit; Position Switch Circuit - Emerson Fisher 4200 Series Instruction Manual

Instruction Manual
Form 5596
July 2005
position switches, turn the high position switch
potentiometer (HIGH, R3) fully clockwise, and turn
the low position switch potentiometer (LOW, R6)
fully counterclockwise. To restore the position switch
circuits to operation, follow the high and low position
switch adjustment procedures described in this
section.
Both position switch relays are
energized in this condition. The trip
points cannot be adjusted to prevent
the relays from energizing.

Principle of Operation

The basic concept of the electronic position
transmitter can be understood by visualizing the
entire transmitter as a variable resistor in series with
a load and connected to a power supply as shown in
figure 9. A linkage or cable/pulley assembly
transmits linear motion of the device into rotary
motion of the potentiometer. The potentiometer is
connected to the input of the transmitter circuit. The
voltage present at the wiper of the potentiometer
supplies input voltage to the electronic circuitry in the
transmitter to produce a current output.
The position switch circuit is driven by the same
input system as the transmitter (except for the Type
4215 transmitter). The position switch circuit acts as
an electronic switch that controls two relays. The
voltage present at the wiper of the potentiometer is
compared to the settings of the high and low position
switch set points and provides an on or off condition
for the relays. Instruments that use a single element
potentiometer use the same element for both the
transmitter input and the input to the position switch
circuits. A Type 4215 transmitter with dual element
potentiometers uses one element of the
potentiometer for the transmitter input and the other
Note
4200 Series Transmitters
element for the position switch circuit input. On an
instrument with position switches only, the single
element potentiometer is used for the position switch
circuit input. Each of the position switch circuit
outputs is connected to a relay on the field circuit
printed wiring board, and depending on the
potentiometer position, the relays are energized or
de-energized.

Transmitter Circuit

At all times, the current flow into the transmitter
equals the current flow out of the transmitter. The
current flowing into the transmitter divides between a
regulator circuit and transistor Q1.The regulator
circuit establishes two voltages one of which is
(VREF). This voltage is applied across the input
potentiometer. The other voltage, (VREG) powers
the hybrid circuit.
On units with position switches, the transmitter
voltage regulator, 4 to 20 mA signal, and relay coil
currents flow in the wire between the power supply
(+) terminal and the (+) terminal on TB3. The
transmitter voltage regulator, switching, and relay
coil currents are returned to the power supply
common from the (R) terminal on TB3. The 4 to 20
mA output signal flows between the (−) terminal on
TB3 and the power supply common. This prevents
the relay coil switching currents from appearing on
the 4 to 20 mA output.

Position Switch Circuit

A regulated voltage is setup to power the position
switch circuit. The position switch circuit senses the
position of the potentiometer and triggers the
appropriate position switch relay (K1 or K2, figure
10) when travel reaches the trip points established
by the adjustments. The Type 4215 transmitter uses
dual potentiometers which allows individual
potentiometers for the transmitter circuit and the
position switch circuit.
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