Date post: | 24-Dec-2015 |
Category: |
Documents |
Upload: | george-hicks |
View: | 228 times |
Download: | 2 times |
4-20mA Basics and2-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
°T
XTRX
TR
3-WireXTR111
2-WireXTR112
RL
– Temperature, pressure, flow
– 2-wire transmitter
– Valve, actuator, heater– 3-wire transmitter
Transmit control signals from a control station out to a remote device.
Report a process variable from a remote sensor to a control station.
• 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
SensorReceiver
250Ω
0 to 100% Signal
4 to 20mA Signal
Transmitter 1V to 5V
4-20mA Overview
• 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
Why Use Current Transmitters?
T
Signal Conversion/Conditioner
Circuitry
Current Output
Current Transmitter Function
• Linearity errors on most sensors creates an unacceptable error.
Line
arity
(%
)
Temperature (°C)
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0
0.2
0 200 400
RTD Input
Ou
tpu
t
+ Full Scale
- Full Scale
Ideal Output
Bridge
Line
arity
(%)
Temperature (°C)-50 50 100
Thermistor
Why Is Signal Conditioning Required?
RL VOUTXTR+VS
IOUT4-20mA
>1000 feet
-
+
250typ 1V-5V typ
+24V Typical+7V to +44V 50 - 60 Hz
Noise
2-Wire Transmitter
3-Wire Transmitter
VLOOP
RL
+8 V to +40V+24 V typical
VOUT
XTR
+VS
IOUT4-20mA
> 1000 feet
-
+250typ 1V-5V typ
50-60 Hz Noise
Transmitter Type Diagrams
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
• 2-Wire Transmitter– Submersible temperature sensor
– Remote location prevents local power supply
– Sends data back to control station
• 3-Wire Transmitter– Sends control signal to element
at remote location
– Local power supply is available
°T
XTR
XT
R
3-WireXTR111
2-WireXTR112
RL
Implementation Of Current Transmitters
2-Wire Transmitters
• 4-20mA span is 16mA• XTR117 has current gain of
100– Input span is 160µA
• Offset current needed if input signal reaches zero
+
-
XTR117
R1
IIN
IRET
A1
2
3
8 VREG
IO = 100 * IIN
2.475 kΩ
+5V
40uA
0 to 160μA
125k
Ω
IIN
Offset Current
2-Wire Typical Input Scaling
XTR117
IIN
IRET
2
3
8
VREG
IOUT DAC
XTR117
IIN
IRET
2
3
8
VREG
VOUT DAC
XTR117
IIN
IRET
2
3
8
VREGREF3140
Input Circuitry
2-Wire Typical Input Interface
+
-
+5V Regulator
XTR117
RLIM
R1 R2
IIN
IRET
A1
E
B
VREG
2.475 kΩ 25Ω
100Ω
1kΩ
8kΩ
V+
IO
• IRET is not GND
• Current Input• Current Output
• Current gain set by R1 and R2
• Regulator and input circuitry floats at IRET potential
Typical 2-Wire Transmitter
XTR117
RIn
VIn
RL
VLoop
IRET
IIN
IOUT
V+
2-Wire Transmitter Common Abuse
R1
R2
25 R4 100
V1 25
R7
250
A+
I LOOP
-
++
4
3
5
1
2
R3
125k
IIN 160u
XTR117
2.47
5k
40uA
20mA
5V Regulator
Grounded Loop Supply with Floating Input Source
R1
R2
25 R4 100
V1 25
R7
250
I LOOP
-
++
4
3
5
1
2
R3
125k
IIN 160u
IR1 IR2
XTR117
2.475k
40uA
19.8mA200uA20mA
5V RegulatorIOFFSET
ILOOP = IR1 + IR2
IR2 = IR1 * 99
IR1 = IIN Total
IIN Total = IIN + IOffset
Internal Current Paths
VR1 = VR2
Equal Path Voltages Created
R1
R2
25 R4 100
V1 25
RL
250
-
++
4
3
5
1
2
R3
125k
I IN 160u
IR1 IR2 V Float
ILOOP
V IRET
XTR117
2.475k
40uA
5.495V
20mA5V19.8mA200uA
5V Regulator
IRET
VFLOAT = ILOOP * RL
VR2 = R2 * IR2
V IRET = VFLOAT + VR2
V IRET = 5.495V
Calculating IRET Level
R1
R2
25 R4 100
V1 25
RL
250
-
++
4
3
5
1
2
R3
125k
I IN 0
IR1 IR2 V FloatILOOP
V IRET
XTR117
2.475k
40uA
1.099V
4mA1V3.96mA40uA
5V Regulator
IRET
VFLOAT = ILOOP * RL
VR2 = R2 * IR2
V IRET = VFLOAT + VR2
V IRET = 1.099V
V IRET
V FloatIR2IR1
I IN160u
R3
12
5k
-
++
4
3
5
1
2
R7
25
0
VLoop 25V
R4 100
R2
25
R1
AM2 AM3
AM5
40uA
2.475k
XTR117
40uA
19.96mA160uA
200uA 19.8mA 5V
20mA
5.495V
5V Regulator
ILOOP
V IRET
ILOOP
V FloatIR2IR1
I IN160u
R3
12
5k
-
++
4
3
5
1
2
R7
25
0
VLOOP 25V
R4 100
R2
25
R1
AM3
AM5
40uA
2.475k
XTR117
36.082uA
40.64mA40.82mA
196.08uA-178.25uA
4.45mV
17.82uA
0V
5V Regulator
AM2
Effects of Grounding IRET
IRET Floating IRET Grounded
3-Wire Transmitters
+
-
XTR111 OD
A1
GND
VIN
VSP
3V
+
-
Current Mirror
+
-
EF
ISET
REGF
REGS
IS
VG
RSET
• Input and Output are referenced to GND
• Voltage Input• Current Output
• Current gain set by VIN and RSET
• Regulator referenced to GND
Typical 3-Wire Transmitter
+
-
XTR111 OD
A1
GND
VSP
+
-
Current Mirror
+
-
EF
ISET
+24V
REG F
SREG
3V
REF3140
VIN
VOUT DACμC
+5V
2kΩ
3kΩ
47
0nF
2kΩRSET
15Ω
LOAD
15Ω
Q1
Q2
VG
IS
10
nF
Short-CircuitProtection
XTR111 Typical Input Interface
2-Wire General Purpose
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
2-Wire RTD Conditioner
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
2-Wire Bridge Conditioner XTR106 Bridge Conditioner, Linearization Correction
3-Wire General Purpose
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
4-20mA Receiver RCV420 4-20mA input, 0-5V Output
4-20mA Product Family
Thank You!