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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 250typ 1V-5 V typ + >1000 feet 50 - 60 Hz Noise +7V to +44V +24V Typical 3-Wire Transmitter +VS IOUT XTR + 4-20mA RL 250typ 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!
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