Transcript Presentation Title Here

4-20mA Basics and
2-Wire vs. 3-Wire Transmitters
Material Created by Scott Hill
Presented by Ian Williams
May 23, 2011
Background
• Prior to the 1950s, control of industrial sites was
achieved with pneumatic systems
• 3 psi – 15 psi was used as the standard signal span
–
–
–
–
3 psi  0%
15 psi  100%
3 psi used as a “live zero”
< 3 psi considered a fault condition or “dead zero”
• The 4-20mA current loop was developed to emulate
the old 3-15 psi pneumatic system
Typical 4-20mA Applications
Report a process
variable from a
remote sensor to
a control station.
RL
3-Wire
XTR111
XTR
– Temperature,
pressure, flow
– 2-wire transmitter
XTR
Transmit control
signals from a
control station
out to a remote
device.
– Valve, actuator, heater
– 3-wire transmitter
2-Wire
XTR112
°T
4-20mA Overview
0 to 100%
Signal
Sensor
4 to 20mA
Signal
Transmitter
Receiver
250Ω
1V to 5V
• 4mA represents 0% input level
– Allows up to 4mA to power external input circuitry
– 4mA zero level allows under-scale settings and fault detection
• 20mA represents 100% input level
– Provides sufficient current to power electromechanical devices
– Over-scale can also be used to detect fault conditions
Why Use Current Transmitters?
• Immunity to noise
– Multiple unknown noise sources can exist between
transmitter and receiver
– Low impedance system prevents noise from impacting
the accurate regulation of loop current
• Long distance transmission
– Signals must often travel distances > 1 mile
– Impedance of long wires would severely attenuate a
voltage signal
– 4-20mA current loops are lossless, even over long
distances
• Kirchoff’s Current Law states that the current in a loop is equal
at any point within the loop
Current Transmitter Function
Signal Conversion/
Conditioner
Circuitry
T
Current
Output
Why Is Signal Conditioning Required?
Bridge
Output
• Linearity errors on most
sensors creates an
unacceptable error.
Ideal
Output
1.8
- Full
Scale
RTD
+ Full
Scale
Input
Thermistor
1.4
1.2
Linearity (%)
Linearity (%)
1.6
1
0.8
0.6
0.4
0.2
0
0
200
Temperature (°C)
400
-50
50
Temperature (°C)
100
Transmitter Type Diagrams
> 1000 feet
50-60 Hz
Noise
+8 V to +40V
+24 V typical
VLOOP
+VS
2-Wire Transmitter
-
XTR
I OUT
4-20mA
RL
VOUT
250typ
1V-5 V typ
+
>1000 feet
50 - 60 Hz
Noise
+7V to +44V
+24V Typical
3-Wire Transmitter
+VS
IOUT
XTR
+
4-20mA
RL
250typ
VOUT
1V-5V typ
-
2-Wire vs. 3-Wire Transmitter
• Application-specific parameters determine which
transmitter type should be used
– Accessibility and location
– System power requirements
– Sensor input or voltage/current input
2-Wire vs. 3-Wire Transmitter
• 2-Wire Transmitter (Loop powered)
– Transmitter and sensor remotely located
– Local power supply not practical
– Input circuitry floats with respect to loop supply ground
• 3-Wire Transmitter (Locally powered)
– Transmitter located close to power supply
– Input is referenced to power supply ground
– Also known as a voltage-to-current converter
Common 2-Wire Transmitter Applications
Implementation Of Current Transmitters
• 2-Wire Transmitter
RL
– Submersible temperature sensor
– Remote location prevents local
power supply
– Sends data back to control
station
XTR
3-Wire
XTR111
XTR
2-Wire
XTR112
°T
• 3-Wire Transmitter
– Sends control signal to element
at remote location
– Local power supply is available
2-Wire Transmitters
2-Wire Typical Input Scaling
XTR117
+5V
40uA
– Input span is 160µA
VREG
IIN
IIN
2
0 to 160μA
A1
-
• Offset current needed if
input signal reaches zero
8
125kΩ
Offset
Current
+
• 4-20mA span is 16mA
• XTR117 has current gain of
100
IRET
3
R1
2.475 kΩ
IO = 100 * IIN
2-Wire Typical Input Interface
VREG
8
IOUT DAC
VREG
8
REF3140
XTR117
IIN
2
XTR117
IRET
3
Input
Circuitry
IIN
2
VREG
8
IRET
3
VOUT DAC
IIN
2
IRET
3
XTR117
Typical 2-Wire Transmitter
VREG
XTR117
+5V
Regulator
V+
B
1kΩ
IIN
8kΩ
A1
-
IRET is not GND
Current Input
Current Output
Current gain set by R1
and R2
• Regulator and input
circuitry floats at IRET
potential
+
•
•
•
•
E
RLIM
100Ω
IRET
R1
2.475 kΩ
R2
25Ω
IO
2-Wire Transmitter Common Abuse
RIn
VIn
IIN
V+
VLoop
XTR117
IRET
IOUT
RL
Grounded Loop Supply with
Floating Input Source
XTR117
V1 25
3
IIN 160u
R2 25
R1 2.475k
4
+
-
5
1
+
R7 250
40uA
R3 125k
5V Regulator
2
R4 100
I LOOP 20mA
A
+
Internal Current Paths
XTR117
V1 25
3
IIN 160u
2.475k
R1
+
-
4
5
+
1
R7 250
40uA
R3 125k
5V Regulator
IOFFSET
2
R2 25
IIN Total = IIN + IOffset
IR1 = IIN Total
IR2 = IR1 * 99
ILOOP = IR1 + IR2
IR1 200uA
IR2 19.8mA
R4 100
I LOOP
20mA
XTR117 Created
Equal Path Voltages
40uA
R3 125k
5V Regulator
Virtual Short
+
3
R1
I IN 160u
-
4
+
-
5
1
+
2
Equal
+
R 4 100
R2
IR1
200uA
IR2
VR1 = VR2
19.8mA
I LOOP 20m
Calculating IRET Level
XTR117
XTR117
5V Regulator
4
5
+
1
2
R2 25
IRET
IR1 200uA
IR2
19.8mA
2.475k
R1
I IN 0
IRET
R4 100
V IRET
3
5.495V
V Float 5V
ILOOP
20mA
IR1
40uA
+
-
4
IR2
5
+
RL 250
I IN 160u
2.475k
R1
+
V1 25
R2 25
3
40uA
R3 125k
V1 25
RL 250
40uA
R3 125k
5V Regulator
1
2
R4 100
V IRET
3.96mA
1.099V
V Float 1V
ILOOP
4mA
VFLOAT = ILOOP * RL
VFLOAT = ILOOP * RL
VR2 = R2 * IR2
VR2 = R2 * IR2
V IRET = VFLOAT + VR2
V IRET = VFLOAT + VR2
V IRET = 5.495V
V IRET = 1.099V
Effects of Grounding IRET
R3 125k
VLoop
25V
160uA
4
AM3
1
2
19.96mA
3
2.475k
R1
I IN
160u
AM2
+
-
4
40.82mA
R4 100
R2 25
AM2
-
+
VLOOP
25V
AM3
5
2
40.64mA
5.495V
IR1
200uA
IR2
V IRET
V Float 5V
19.8mA
IRET Floating
ILOOP
20mA
IR1
196.08uA
1
+
R2 25
2.475k
R1
5
R7 250
3
+
5V Regulator
AM5 36.082uA
R3 125k
5V Regulator
AM5 40uA
I IN
160u
XTR117
40uA
R7 250
XTR117
40uA
R4 100
V IRET
V Float
0V
4.45mV
IR2
-178.25uA
ILOOP
IRET Grounded
17.82uA
3-Wire Transmitters
Typical 3-Wire Transmitter
• Input and Output are
REG
referenced to GND
REG
• Voltage Input
• Current Output
• Current gain set by VIN V
and RSET
• Regulator referenced
to GND
XTR111
VSP
OD
F
EF
+
S
-
IS
+
ISET
3V
+
VG
A1
-
IN
Current
Mirror
GND
RSET
XTR111 Typical Input Interface
+24V
XTR111
2kΩ
470nF
3kΩ
IS
15Ω
ISET
Q1
VG
Q2
A1
GND
RSET
2kΩ
15Ω
LOAD
10nF
VIN
Short-Circuit
Protection
-
VOUT DAC
Current
Mirror
+
μC
REG S
+
3V
-
EF
-
REF3140
OD
REG F
+
+5V
VSP
4-20mA Product Family
2-Wire General Purpose
2-Wire RTD Conditioner
2-Wire Bridge Conditioner
3-Wire General Purpose
4-20mA Receiver
XTR101
I_IN to I_OUT Converter, Current Excitation
XTR115
I_IN to I_OUT Converter, 5V Regulator, 2.5V Reference
XTR116
I_IN to I_OUT Converter, 5V Regulator, 4.096V Reference
XTR117
I_IN to I_OUT Converter, 5V Regulator
XTR105
RTD Conditioner, 800uA Current Sources
XTR108
RTD Conditioner, Digital Calibration, Input Mux
XTR112
RTD Conditioner, 250uA Current Sources
XTR114
RTD Conditioner, 100uA Current Sources
XTR106
Bridge Conditioner, Linearization Correction
XTR110
Selectable Input/Output Ranges, Voltage to Current Converter
XTR111
Configurable Input/Output Ranges, Voltage to Current Converter
XTR300
Configurable Input/Output Ranges, Current/Voltage Output Mode
RCV420
4-20mA input, 0-5V Output
Thank You!