Temperature Transmitter Explained: How It Works and Wiring
A temperature transmitter explained — how it converts an RTD or thermocouple to 4-20mA, head vs rail mount, why use one over a direct sensor, and PLC wiring.
A temperature transmitter is a signal-conditioning device that accepts a raw sensor input — an RTD resistance or a thermocouple millivolt — and converts it to a standardized 4-20 mA current loop that a PLC analog input card can read. Instead of running a fragile, noise-susceptible sensor signal hundreds of metres to the control room, the transmitter does the heavy lifting at the measurement point and sends a robust industrial signal the rest of the way.
Temperature transmitters sit between the sensor and the PLC in virtually every serious process plant: chemical reactors, heat exchangers, pasteurizers, boilers, and HVAC air-handling units. Getting comfortable with how they work — and when to use one rather than a direct sensor card — separates competent automation engineers from exceptional ones.
What Is a Temperature Transmitter?
A temperature transmitter is a two-part device inside a single enclosure:
- Input stage — accepts the raw signal from an RTD (resistance) or thermocouple (millivolt EMF) and performs sensor-type-specific signal conditioning.
- Output stage — converts the conditioned measurement to a 4-20 mA DC current proportional to a user-configured span, with an optional HART digital signal superimposed on the same wires.
The transmitter is configured for a specific sensor type, range, and engineering units. A transmitter ranged 0–150 °C outputs 4 mA at 0 °C and 20 mA at 150 °C. Every degree of span maps linearly to 0.1067 mA (16 mA ÷ 150 °C). The PLC analog input card reads that current, and the PLC program scales it back to engineering units.
Quick definition: A temperature transmitter measures a sensor's raw electrical output, linearizes it, compensates for errors, and transmits a proportional 4-20 mA signal to a controller.
Why Use a Temperature Transmitter Instead of Running the Sensor Direct to the PLC?
This is the first question most engineers ask. The answer comes down to four practical realities of industrial installations.
1. Noise and Signal Integrity
RTDs produce a resistance change of roughly 0.385 Ω per °C (for a Pt100). A thermocouple Type K produces about 41 µV per °C. Both signals are tiny and vulnerable to electromagnetic interference from motor drives, contactors, and fluorescent lighting. Routing these signals over long cable runs without shielding and careful installation is asking for reading errors.
A temperature transmitter located at the sensor converts that fragile millivolt or milliohm change to a 4-20 mA current loop immediately at the measurement point. The current loop is highly immune to induced noise — for the same reasons explained in 4-20 mA current loop fundamentals. Only robust industrial current travels the long distance to the PLC.
2. Lead-Wire Resistance Error (RTDs)
A Pt100 RTD has a nominal resistance of 100 Ω at 0 °C. Cable resistance of even 1 Ω introduces a 2.6 °C error in a 2-wire RTD configuration. A 3-wire or 4-wire RTD connection at the transmitter eliminates this error in the electronics before the signal leaves the field — the long cable from transmitter to PLC carries only the current loop, which is immune to resistance effects.
3. Distance and Thermocouple Extension Wire
Thermocouples require extension grade or compensating cable matched to the thermocouple type all the way from junction to cold-junction compensation point. This special cable is more expensive than standard instrumentation cable, less mechanically robust, and must be routed to avoid thermal gradients. A head-mount transmitter places the cold-junction compensation at the sensor head, allowing ordinary shielded twisted-pair cable for the entire run from there to the PLC — a significant cost and installation advantage over long distances.
4. Linearization Done at the Source
Thermocouple output is non-linear. The relationship between millivolt output and temperature follows a polynomial curve that requires a lookup table or algorithm to invert. A dedicated RTD/TC card at the PLC handles this, but so does a temperature transmitter — and the transmitter does it right at the sensor, transmitting a linear 4-20 mA span to the PLC. This means the PLC only needs a standard analog input card, not a specialized (and more expensive) RTD or thermocouple card, to process the signal.
How a Temperature Transmitter Works
Step 1 — Sensor Input and Excitation
For an RTD input, the transmitter applies a small constant excitation current (typically 0.2–1 mA) through the RTD element and measures the resulting voltage drop. Using Ohm's Law (R = V ÷ I), it calculates resistance. The 3-wire or 4-wire configuration allows the transmitter to cancel lead-wire resistance by measuring it separately and subtracting it from the total.
For a thermocouple input, the transmitter presents a high-impedance millivolt input — typically > 1 MΩ — to avoid loading the thermocouple junction. It measures the small EMF produced by the thermocouple.
Step 2 — Cold-Junction Compensation (Thermocouples)
A thermocouple measures the temperature difference between its hot junction (the sensing tip) and its cold junction (where the thermocouple wire terminates at the transmitter terminals). The transmitter contains an on-board reference temperature sensor — typically a small Pt100 or silicon bandgap sensor — that measures the terminal temperature. The firmware adds this reference temperature to the thermocouple's differential output to produce an absolute temperature reading. This process is called cold-junction compensation (CJC).
Placing the transmitter directly in the sensor head means CJC happens at a thermally stable, well-characterized point, which is why head-mount transmitters often achieve better thermocouple accuracy than long extension-wire runs to a PLC cold-junction card.
Step 3 — Linearization
The transmitter firmware applies the appropriate linearization algorithm:
- RTD (Pt100/Pt1000): Callendar-Van Dusen equation, which corrects for the slight non-linearity of platinum resistance over temperature.
- Thermocouple: ITS-90 polynomial coefficients for the specific thermocouple type (J, K, T, E, N, R, S, B).
The result is a true temperature value in the transmitter's internal representation.
Step 4 — Range and Output Conversion
The transmitter maps the linearized temperature onto the configured output span:
Output (mA) = 4 + [(T − T_low) ÷ (T_high − T_low)] × 16
Where T_low is the low-range temperature (→ 4 mA) and T_high is the high-range temperature (→ 20 mA). The electronics drive the 4-20 mA current loop output accordingly.
Head-Mount vs DIN-Rail vs Field Transmitters
Temperature transmitters come in three physical form factors, each suited to different installation scenarios.
| Form Factor | Mounting | Best For |
|---|---|---|
| Head-mount | Inside the sensor connection head (Form B / Form C housing) | Process plants; standard RTD/TC installations; minimizes extension cable |
| DIN-rail (rail-mount) | 35 mm DIN rail inside a panel or junction box | Multiple sensors in one enclosure; easier access for maintenance; cleaner panel |
| Field/wall-mount | Standalone field enclosure (IP65/IP67 rated) | Remote locations; high vibration; sensors far from any panel or junction box |
Head-Mount Transmitters
A head-mount transmitter is a small PCB module — typically 40–50 mm in diameter — that fits directly inside a standard DIN 43729 Form B or Form C sensor connection head (the round or square housing mounted at the top of a thermowell or surface-mount fitting). The sensor wires connect on one side; the 4-20 mA loop terminals are on the other.
This arrangement is the most common in process plants because:
- No extension cable is needed between sensor and transmitter.
- CJC is at the measurement point, improving thermocouple accuracy.
- The installed footprint is minimal — the transmitter occupies space that would otherwise be empty inside the head.
- Lead-resistance cancellation for RTDs is done immediately, before any cable run.
The tradeoff is access: adjusting configuration requires opening the sensor head, which may be in a hot or confined location. Most modern head-mount transmitters can be configured via HART communicator without opening the head.
DIN-Rail Transmitters
Rail-mount transmitters accept sensor wiring from the field and are housed inside a junction box or marshalling cabinet. They suit installations where:
- Multiple sensors terminate in a common cabinet near the process (e.g., a motor control center, a local panel).
- Easy access to transmitter configuration is important for commissioning.
- Sensor extension wire runs are short — within the same building or skid.
DIN-rail transmitters are physically larger, often include displays, and typically support a wider range of input types on a single device. They are also easier to replace during maintenance without disturbing the sensor installation.
Field/Wall-Mount Transmitters
Field-mount transmitters combine the transmitter electronics with a robust weatherproof housing (IP65 or IP67, often ATEX/IECEx rated for hazardous areas). They suit installations where sensors are far from any panel, where vibration or harsh environment rules out a head-mount form factor, or where the transmitter must survive direct weather exposure.
Configuration and Ranging
Before a temperature transmitter can be used, it must be configured for:
- Sensor type — RTD (Pt100, Pt1000, Ni100) or thermocouple type (J, K, T, E, N, R, S, B), and wiring configuration (2-wire, 3-wire, 4-wire for RTDs).
- Range (span) — the low-range value (4 mA point) and high-range value (20 mA point) in the chosen engineering units. Wider spans reduce resolution; narrower spans increase it.
- Damping — a digital low-pass filter time constant (0–32 seconds typical) that smooths out rapid fluctuations without affecting steady-state accuracy.
- Units — °C, °F, or K.
- Failure-mode output — what the transmitter outputs if it detects a sensor fault: drive high (> 21 mA), drive low (< 3.6 mA), or hold last value. Drive-low is the NAMUR NE43 convention for sensor failures and is preferred because it triggers the PLC under-range alarm.
Configuration is performed via:
- Local push-buttons and display (DIN-rail and field-mount units).
- HART communicator (handheld or connected via HART modem to a PC running configuration software).
- PC software (manufacturer-specific: Siemens SIPROM, Endress+Hauser FieldCare, Rosemount AMS Device Manager, etc.).
Smart Transmitters and HART
Most modern temperature transmitters are smart transmitters: they incorporate a microprocessor and support the HART (Highway Addressable Remote Transducer) protocol. HART superimposes a 1,200/2,200 Hz FSK digital signal on top of the 4-20 mA loop without disrupting the analog current. This allows:
- Remote configuration and ranging without opening the sensor head.
- Reading of secondary variables (terminal temperature, percent of range, loop current) alongside the primary 4-20 mA value.
- Continuous device diagnostics and predictive health monitoring.
- Multi-drop mode (multiple transmitters on one pair of wires, all digital, no analog component).
For deeper coverage of smart transmitter capabilities and HART integration, see what is a smart transmitter.
The Controls View: Transmitter into Standard Analog Input vs Dedicated RTD/TC Card
This is where the controls engineer earns their keep. You have two fundamentally different ways to get a temperature reading into a PLC:
Option A: Direct sensor wiring to a dedicated RTD or thermocouple input card
The sensor wires run directly to a specialized PLC card that handles excitation, lead-resistance cancellation, cold-junction compensation, and linearization in hardware and firmware.
Option B: Temperature transmitter converting to 4-20 mA, wired into a standard analog input card
The transmitter handles all sensor-specific processing in the field; the PLC sees a generic current loop signal.
| Consideration | Direct RTD/TC Card (Option A) | Transmitter + Analog Input (Option B) |
|---|---|---|
| PLC card cost | Higher (specialized card) | Lower (standard analog card) |
| Field wiring cost | Higher (extension cable, shielding over full run) | Lower (standard STP cable from transmitter to panel) |
| Sensor distance | Limited by noise and lead resistance | Effectively unlimited (current loop) |
| Wiring complexity | Higher (correct extension cable type, CJC at card) | Lower (any shielded pair) |
| Redundancy | Transmitter failure = card channel lost | Transmitter can be swapped without rewiring sensor |
| Diagnostics | Card-level diagnostics only | HART diagnostics at device level |
| Multi-sensor cabinets | Common with marshalling to centralized cards | Each sensor has its own transmitter |
| Best use case | Short runs (< 30 m), same panel, cost-sensitive count | Long runs, high noise, remote sensors, hazardous areas |
When to choose direct card wiring: The sensor is physically close to the PLC panel (same skid, same cabinet room), cable runs are short and well-shielded, and you have many temperature points that make per-channel transmitter cost significant. A dedicated 8-channel RTD card with short local wiring is clean, accurate, and low-cost per point.
When to choose a temperature transmitter: The sensor is remote — tens or hundreds of metres from the panel. The environment is electrically noisy (drives, contactors, welding equipment). The installation is in a hazardous area (ATEX/IECEx transmitters are standard). You want HART diagnostics and remote configuration. You are mixing temperature inputs with other 4-20 mA inputs on a standard analog card rather than purchasing a dedicated card type.
In practice, most large process plants use transmitters for field sensors and reserve direct RTD/TC cards for on-panel or on-skid sensors with short, controlled cable runs.
How to Wire a Temperature Transmitter to a PLC
The wiring procedure depends on the transmitter form factor and the PLC's analog input card type. The following covers the most common scenario: a 2-wire, loop-powered head-mount transmitter wired to a sourcing (current-sinking) analog input card.
Components Required
- Temperature transmitter (2-wire loop-powered)
- 24 VDC loop power supply (many PLC analog cards provide internal loop power)
- Shielded twisted-pair (STP) cable, minimum 0.5 mm²
- PLC analog input card with 4-20 mA input channels
Wiring Diagram (2-Wire Loop-Powered Transmitter)
24 VDC (+) ──────────── TX (+) terminal
[Transmitter]
TX (–) terminal ─────── AI card (+) input terminal
[PLC Analog Input Card]
AI card (–) / COM ────── 24 VDC (–) / 0 V
The loop current path is: supply positive → transmitter positive terminal → through transmitter electronics → transmitter negative terminal → AI card input positive → AI card input negative/common → supply negative.
Step-by-Step Procedure
-
Confirm power: Verify the loop supply voltage falls within the transmitter's specified range (commonly 10–36 VDC for 2-wire devices). Most modern PLC analog input cards provide 24 VDC loop power on each channel — check the card manual.
-
Wire the transmitter: Connect the two loop wires (+ and –) to the transmitter's clearly marked terminals inside the sensor head. Observe polarity — reversing polarity will not damage most modern transmitters but will produce no output.
-
Wire to the PLC card: Connect the same loop wires to the PLC analog input channel. Connect the shield drain wire to the panel earth at the PLC end only — not at the transmitter end — to avoid creating a ground loop.
-
Configure the PLC card: Set the input channel to 4-20 mA mode. On some cards this is a hardware jumper; on others it is a configuration parameter in the PLC software.
-
Configure the transmitter: Set sensor type, range (low value = 4 mA, high value = 20 mA), damping, and failure mode using a HART communicator or local configuration interface.
-
Scale the PLC analog input: In the PLC program, scale the raw analog count to engineering units. For a 0–200 °C span on a 12-bit card (0–4095 counts):
Temperature (°C) = (Raw_Count ÷ 4095) × 200
For cards that resolve the 4-20 mA range to 3,277–16,383 counts (16-bit, 1–5 V burden):
Temperature (°C) = [(Raw_Count − 3277) ÷ (16383 − 3277)] × 200
Most PLC platforms provide a built-in scaling function block (SCL in Siemens, Scale in Allen-Bradley) that accepts the raw count and high/low engineering values and outputs the scaled result directly.
- Verify with a loop calibrator: Apply a known 4 mA, 12 mA, and 20 mA signal using a loop calibrator and confirm the PLC reads 0%, 50%, and 100% of span respectively before connecting the live sensor.
FAQ
What is a temperature transmitter?
A temperature transmitter is a field instrument that accepts a raw RTD resistance or thermocouple millivolt signal, linearizes and temperature-compensates it, and outputs a proportional 4-20 mA current loop signal to a PLC or DCS. It places sensor-specific signal conditioning at the measurement point rather than at the control system.
Why use a temperature transmitter instead of a direct RTD?
Running an RTD directly to a PLC over long distances introduces lead-resistance errors and makes the fragile milliohm signal vulnerable to electromagnetic noise. A temperature transmitter eliminates both problems: it cancels lead resistance at the sensor head using a 3-wire or 4-wire measurement, converts the result to a noise-immune 4-20 mA current loop, and allows the use of ordinary shielded cable for the remaining run to the PLC. It also lets you use a standard (lower-cost) analog input card rather than a specialized RTD card.
What is a head-mount transmitter?
A head-mount transmitter is a compact PCB module — typically 40–50 mm in diameter — designed to fit inside a standard DIN 43729 sensor connection head. The sensor terminals are on one side and the 4-20 mA loop terminals on the other. Mounting the transmitter inside the sensor head keeps the cold-junction compensation point at the measurement location, eliminates the need for thermocouple extension cable, and minimizes the cable run between sensor and transmitter to virtually zero.
How do you wire a temperature transmitter to a PLC?
For a 2-wire loop-powered transmitter, connect the 24 VDC supply positive to the transmitter positive terminal, run the transmitter negative terminal to the PLC analog input positive terminal, and connect the PLC analog input common/negative back to the supply negative. Connect the cable shield to earth at the PLC panel end only. Set the PLC analog input channel to 4-20 mA mode, configure the transmitter range, and scale the raw analog count to engineering units in the PLC program.
Summary
A temperature transmitter solves a fundamental instrumentation problem: RTDs and thermocouples produce small, sensor-specific signals that degrade over distance and in noisy environments. By placing the signal conditioning at the measurement point — in the sensor head, on a DIN rail near the process, or in a field enclosure — the transmitter converts that fragile signal to a robust 4-20 mA loop that travels reliably to the PLC over ordinary instrumentation cable.
For the controls engineer, the key decision is transmitter-plus-standard-analog-card versus direct-sensor-to-dedicated-card. Short, clean runs favor dedicated cards; long, noisy, or remote installations favor transmitters. When in doubt, a HART-enabled transmitter adds diagnostic depth at minimal extra cost and pays dividends at the first unexpected sensor fault.
For related reading, see thermocouple types explained, the full comparison of RTD vs thermocouple, and types of industrial sensors for context on where temperature measurement fits in a broader instrumentation strategy.
