In industrial automation and control, temperature transmitter serve as core devices connecting the physical world to control systems. Their essential function is to convert the physical quantity of temperature into standard 4–20 mA current signals or voltage signals for long-distance transmission to control rooms, based on the fundamental electrical relationships among voltage, current and resistance.

Whether you are an instrument engineer, automation technician, or electronics enthusiast, understanding the relationship between voltage, current and resistance in temperature sensor transmitters is a required course to master industrial temperature measurement technology. This article uses the most concise language to clarify this core principle for you.

I. Electrical Basics of Sensors

Different types of temperature sensors have different electrical output characteristics, but all ultimately follow Ohm’s Law.

RTDs (e.g., Pt100): Temperature change → Resistance change

Principle: Apply a constant excitation current to the sensor and measure the voltage across it.

Relationship: V=I×R. The measured voltage can be used to calculate resistance, and thus temperature.

Thermocouples: Temperature difference change → Thermoelectric potential (voltage) change

Principle: Directly measure the millivolt-level voltage signal across the thermocouple, convert it to temperature after cold-junction compensation.

Summary: Whether it is resistance change or voltage change, the relationship between voltage, current and resistance runs through the entire process.

II. Signal Conversion Chain of the Transmitter

With an understanding of sensor output characteristics, let us look at how a transmitter processes these signals step-by-step into standard industrial signals.

1. Excitation and Measurement (Taking RTD as an Example)

The transmitter provides a constant current source (usually < 1 mA to avoid sensor self-heating) that flows through the Pt100 RTD. By Ohm’s Law, a voltage develops across the sensor:

Vsense​=Iexc​×(Rwire​+Rt​+Rwire​)

This raises a critical issue: wire resistance affects measurement accuracy. The 3-wire connection commonly used in industry uses voltage–current relationships to cancel line resistance by adding an extra wire, ensuring the transmitter measures only the sensor voltage.

2. Signal Amplification and Conditioning

The voltage signal obtained from the sensor is very weak (millivolt level) and must be amplified by a high-precision operational amplifier. This process essentially amplifies small voltage changes into a manageable range (e.g., 0–5 V).

3. V/I Conversion: Voltage to Current

The amplified voltage signal is converted to the industrial standard 4–20 mA current signal. This stage best illustrates the voltage–current–resistance relationship.

The core of the V/I conversion circuit uses shunt current negative feedback. The input voltage Vi​ determines the output current Io​:

Io​=Rf​Vi​​

where Rf​ is the sampling feedback resistor. With sufficiently high precision Rf​, the output current has a strict linear relationship with the input voltage.

III. Why Convert to 4–20 mA?

Beyond the conversion principle, it is important to understand the reason. The advantages of the 4–20 mA current loop also rely on electrical properties:

Strong anti-interference: Current signals are unaffected by line resistance and voltage drop, suitable for long-distance transmission. 2-wire power supply: The transmitter’s own current consumption is below 4 mA; the remaining 4–20 mA carries the signal, unifying power and signal transmission. Fault self-diagnosis: An open circuit (infinite resistance) gives 0 mA; a short circuit (very low resistance) gives abnormal current. Monitoring whether the current stays within 4–20 mA allows quick fault detection.

IV. Core Relationships in One Table

Stage Input Quantity Output Quantity Core Electrical Relationship RTD Sensing Temperature T Resistance R R=f(T) Constant-Current Excitation Current I, Resistance R Voltage V V=I×R Amplifier Circuit Small Voltage Vi​ Large Voltage Vo​ Vo​=A×Vi​ (A: gain) V/I Conversion Voltage Vo​ Current Io​ Io​=Vo​/Rf​ 4–20 mA Transmission Current I Receiver Voltage Vrec​=I×Rload​

Conclusion

The relationship between voltage, current and resistance in temperature sensor transmitters is not a set of isolated electrical formulas, but a complete signal conversion chain. From the physical effect of the sensor, through excitation, measurement and amplification, to final V/I conversion, every step relies on Ohm’s Law and basic circuit analysis principles.

Mastering this relationship allows you not only to select and use temperature transmitters correctly, but also to quickly diagnose on-site faults (inaccurate readings, broken signals) and determine whether the problem lies in the sensor itself, the transmission line, or the transmitter circuit.