4200 Series Transmitters
Instruction Manual
Form 5596
November 2006
21
NOTES:
THIS EQUIVALENT CIRCUIT IS TYPICAL FOR ALL TYPES OF TRANSMITTERS.
GENERALLY, IF A PORTION OF THE CIRCUIT IS NOT USED, IT IS OMITTED.
RECEIVING DEVICE MAY BE ANALOG CURRENT INPUT TO DISTRIBUTED CONTROL SYSTEM,
PROGRAMMABLE LOGIC CONTROLLER OR INDICATING DEVICE. INDICATING DEVICE CAN BE A
VOLTMETER ACROSS 250 OHM RESISTOR OR A CURRENT METER.
FIELD WIRING FOR POSITION SWITCHING CIRCUITS CAN BE A TWO OR THREE WIRE SYSTEM
DEPENDING ON THE APPLICATION REQUIREMENTS.
SEPARATE RETURN REQUIRED FOR RELAYS. RETURN WIRE MUST BE CONNECTED EVEN IF
RELAY OUTPUTS ARE NOT USED.
2
4
1
2
3
4
+
+
−
−
24 VOLT
DC POWER
SUPPLY
DEVICE
RECEIVING
4 TO 20 MA
DC SIGNAL
DEVICE
RECEIVING
HI POSITION
SWITCH OUTPUT
(CONTACT
CLOSURE)
DEVICE
RECEIVING LO
POSITION
SWITCH OUTPUT
(CONTACT
CLOSURE)
B1946-4/IL
MECHANICAL
CONNECTION
TO DEVICE
TRANSMITTER
PRINTED WIRING
BOARD
FIELD CIRCUIT
PRINTED WIRING
BOARD
SWITCHING
CIRCUIT
TRANSMITTER
CIRCUIT
K1
RELAY
K2
RELAY
TRANSMITTER
Figure 9. Transmitter Operation
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.
Note
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
element for the position switch circuit input. On an
instrument with position switches only, the single
element potentiometer is used for the position switch
