learn · South Africa
Analog Signal Types: 4–20 mA, 0–10 V, RTD and Thermocouple
Compare analog signal types with current-loop, voltage and RTD examples, thermocouple principles and questions for South African instrumentation training.
Analog signal types describe how a measurement reaches a controller. A 4–20 mA interface carries information as current; a 0–10 V interface uses voltage. An RTD supplies a temperature-dependent resistance, while a thermocouple produces a small thermoelectric voltage that needs the appropriate reference compensation and conversion.
For a South African PLC or instrumentation learner, the important skill is identifying the complete measurement path. A temperature sensor can connect to a temperature input directly, or to a transmitter that sends a current signal onward. The sensor technology and the signal arriving at the PLC are therefore separate choices.
This guide compares those choices and develops fictional numerical exercises for current-loop headroom, voltage loading, lead resistance and configuration errors. It provides a study record for training and supervised practical assessment. The illustrations are conceptual and the calculations are not validated installation designs.
Start with the measurement path and its units
Write the process quantity, sensor, any transmitter, electrical interface, input channel configuration and software representation in order. Keep units at each stage. A number labelled Temperature is insufficient if nobody knows whether it contains resistance, tenths of a degree or an already converted engineering value.
| Interface or sensor | Quantity presented to suitable measurement hardware | Configuration question |
|---|---|---|
| 4–20 mA interface | Current | What process range and current range are configured? |
| 0–10 V interface | Voltage relative to the specified reference | What loading and reference conditions apply? |
| RTD | Temperature-dependent resistance | Which element curve and lead arrangement are supported? |
| Thermocouple | Thermoelectric voltage | Which thermocouple type and reference compensation apply? |
Consider two possible temperature paths. One uses an RTD connected to a compatible resistance-temperature input. The other uses an RTD connected to a transmitter, whose 4–20 mA output reaches a current input. Both begin with an RTD, but the PLC channels need different capabilities.
A signal inventory should distinguish these paths before a programmer chooses a scaling formula. The scaling and resolution tutorial explains why current, raw counts and engineering values cannot share an unexplained multiplier.
Understand the purpose of a 4–20 mA live zero
In a configured linear 4–20 mA measurement, four milliamps represents the lower process endpoint and twenty represents the upper endpoint. The non-zero lower signal is called a live zero. A value outside the nominal interval needs interpretation using the device and input-channel documentation.
The NI current-loop fundamentals guide describes a transducer regulating loop current and explains the relationship between the power source, signal and receiver. A loop-powered arrangement obtains its operating energy through the loop; other transmitter arrangements have separate power connections.
Do not label every low current “broken wire”. A failed acceptance check describes an observation, while a physical diagnosis requires supporting evidence. Record the measured or simulated current, supplied quality state, configured limits and relevant diagnostics separately.
For a fictional pressure range of zero to sixteen bar, the nominal linear relationship is particularly simple: pressure equals current in milliamps minus four. Thus 4, 8, 12, 16 and 20 mA correspond to 0, 4, 8, 12 and 16 bar. This convenience belongs to the chosen sixteen-bar span; it is not a general scaling shortcut.
Check current-loop voltage headroom with a stated model
A current loop still needs enough voltage to operate. Analog Devices' current-loop explanation describes the supply budget as the transmitter's operating requirement plus voltage drops in the receiver and wiring. Constant current does not mean unlimited resistance is acceptable.
Use a fictional steady-state series model with a 24 V supply, a transmitter needing at least 12 V, a 250-ohm receiver resistance and 50 ohms total cable resistance. Ignore other devices, tolerance, transient behaviour and environmental effects for this arithmetic exercise only.
At 20 mA, convert the current to 0.020 A before using Ohm's law. The receiver drop is 0.020 times 250, or 5 V. The cable drop is 0.020 times 50, or 1 V. The model leaves 24 minus 5 minus 1, or 18 V across the transmitter.
Relative to the assumed 12 V minimum, that is 6 V of headroom. At 4 mA the receiver and cable together drop 1.2 V, leaving 22.8 V. Under these assumptions, the largest nominal current is the tighter operating-voltage check.
| Model case | Supply V | Total series resistance ohms | Current mA | Transmitter voltage V |
|---|---|---|---|---|
| Base, low signal | 24 | 300 | 4 | 22.8 |
| Base, high signal | 24 | 300 | 20 | 18 |
| Lower supply | 18 | 300 | 20 | 12 |
| Additional resistance | 24 | 700 | 20 | 10 |
The lower-supply row reaches the assumed minimum with no margin. The last row fails the model's operating requirement. Neither row predicts the exact output of a real transmitter once its limits are exceeded.
A real review must include the actual equipment limits and all relevant drops and tolerances. The exercise demonstrates the budget calculation, not permission to select a supply or install an extra loop device from these invented values.
Examine voltage loading instead of using a distance cutoff
A voltage signal is interpreted between specified input terminals or references. Its suitability depends on the source, receiving input, wiring and environment. There is no cable length in this lesson below which every 0–10 V arrangement is automatically accurate.
The same NI guide discusses voltage-signal loss and high input impedance. To see the loading issue directly, use a fictional ideal 10 V source with 100 ohms of total series resistance feeding a purely resistive input. Treat the source resistance and wiring together; assume no noise or reference offset.
The received voltage is ten multiplied by the input resistance, divided by the sum of input and series resistance. With a 100000-ohm input, the result is approximately 9.990010 V. With a 1000-ohm input, it is approximately 9.090909 V.
| Input resistance ohms | Received voltage V | Loss relative to 10 V |
|---|---|---|
| 100000 | 9.990010 | About 0.099900% |
| 1000 | 9.090909 | About 9.090909% |
The cable model is unchanged, but the receiving load changes the result substantially. Conversely, changing a cable alone does not address a configuration mismatch or a reference problem. Name the suspected mechanism before proposing a remedy.
These are ideal DC divider calculations, not predictions of interference immunity or input protection. Input impedance, source capability, reference limits and permitted connections must come from the equipment documentation. A training answer should explain both the arithmetic and the omitted effects.
Test a 1–5 V signal against the wrong configuration
A 1–5 V interface and a 0–10 V interface do not have the same endpoints. Use a fictional zero-to-one-hundred-percent process range to expose the mismatch without involving a particular PLC's raw encoding.
The correct mapping for the supplied 1–5 V signal is (Voltage - 1) / 4 * 100. The incorrectly assumed 0–10 V mapping is Voltage / 10 * 100. Both formulas produce plausible numbers, making an endpoint test important.
| Supplied voltage V | Correct process percent | Wrong 0–10 V interpretation percent |
|---|---|---|
| 1 | 0 | 10 |
| 2 | 25 | 20 |
| 3 | 50 | 30 |
| 4 | 75 | 40 |
| 5 | 100 | 50 |
At the bottom the error is plus ten percentage points. At the top it is minus fifty percentage points. The error changes across the range; adjusting a single display offset will not repair the wrong span.
Retain both columns as expected results in a software exercise. Then correct the configuration and repeat all five points. A value becoming believable at one operating point is weaker evidence than a complete endpoint and midpoint record.
Distinguish RTD element identity from its lead arrangement
An RTD measures temperature through resistance. Pt100 and Pt1000 identify nominal platinum-element resistances of 100 and 1000 ohms at zero degrees Celsius respectively, but element identity alone is insufficient configuration information. The applicable resistance-temperature characteristic also matters.
The Fluke platinum-resistance calculator asks for the thermometer type and nominal resistance when converting between resistance and temperature. Record those selections in a learning exercise rather than assuming that every resistance sensor uses one straight-line conversion.
The NI temperature-sensor guide distinguishes two-, three- and four-wire RTD measurements. Two-wire measurements include lead-resistance effects. Three-wire correction depends on the measurement arrangement and can suffer from unequal lead resistances. Four-wire measurement separates excitation and voltage-sensing paths and requires compatible hardware.
Do not count terminal labels in a generic illustration and call that a module wiring specification. The exact sensor and input documentation must agree about lead arrangement. A module called “analogue input” or “universal” still needs an explicit list of supported measurement modes.
Calculate lead-resistance bias without inventing accuracy
Use a separate fictional resistance exercise. The element is exactly 100 ohms and each of two leads is exactly one ohm. Assume an ideal two-wire resistance measurement that includes both leads and no other error sources.
The measured resistance becomes 100 plus one plus one, or 102 ohms. If each lead is instead 0.2 ohms, the measured result is 100.4 ohms. These results expose the resistance bias directly before any temperature conversion is considered.
For an explicitly simplified classroom sensitivity of 0.4 ohms per degree Celsius, the two-ohm bias corresponds to five degrees and the 0.4-ohm bias to one degree. That sensitivity is an invented local linear model, not a Pt100 calibration equation or an accuracy specification.
Keeping the resistance and temperature steps separate prevents a common explanation error: dividing by a remembered coefficient and presenting the result as exact across the sensor's entire range. A real conversion needs the selected characteristic and its applicable range.
The instrument-calibration training guide develops the distinction between checking a numerical transformation and assessing a measuring instrument. Lead correction does not, by itself, prove that the complete temperature measurement meets a specified uncertainty.
Understand what thermocouple compensation contributes
A thermocouple's small voltage depends on its type and the measurement and reference junction temperatures. The relationship is not a universal constant voltage per degree. The Analog Devices cold-junction compensation guide explains why the reference temperature must be known and accounted for.
In the conceptual conversion, translate the reference-junction temperature into its equivalent thermoelectric voltage for the selected type, add it to the measured voltage, then convert the compensated voltage to temperature. Simply adding a reference temperature to an arbitrary linear voltage conversion is not a general solution.
The NIST ITS-90 thermocouple database supplies type-specific reference functions and tables. A table's temperature interval should not be confused with the service rating of a complete probe, sheath or cable assembly.
For a training record, identify the thermocouple type, conversion reference, reference-junction arrangement and location where the thermocouple materials transition to the measurement connection. Follow the actual terminal and cable instructions. Moving a transition while assuming compensation remains correct needs examination, not a generic copper-splice rule.
Ask the instructor to distinguish a wrong selected type, an incorrect reference-temperature assumption and a genuine process-temperature change. A shifting displayed value alone does not identify which of those occurred.
Select an interface from a written requirement
Begin a selection exercise with the measurement quantity and required range. Then specify the required uncertainty or permissible error, response expectations, environment, installation constraints and available input hardware. Keep missing information visible rather than selecting a sensor from one attractive headline specification.
For example, a fictional temperature study requires readings over twenty to two hundred degrees Celsius and individual comparison with a reference. That brief is incomplete until the permitted error, response needs and measurement arrangement are defined. “Use an RTD because it is accurate” is not yet an engineering justification.
A second fictional brief concerns a remote level transmitter and an existing current-input channel. That suggests investigating a compatible current interface, but does not establish transmitter suitability. The sensor's process compatibility and the electrical path still need their own evidence.
A third concerns a position signal inside a training rig. If the source provides 0–10 V, compare its documented output requirements with the receiving input. Do not replace that evidence with the assumption that anything inside one enclosure is immune to measurement error.
The instrumentation technician learning pathway helps organise these decisions into skills to practise. A useful assessment rewards identifying unknowns as well as obtaining the correct arithmetic answer.
Record faults as observations before naming causes
Create a worksheet with separate fields for the supplied signal, software value, quality information, configuration and suspected explanation. This prevents an expected test result from quietly becoming an unverified statement about the physical installation.
For a current input, a low supplied value might fail the exercise's nominal-range policy. Record that failed policy and the documented diagnostic information. For an RTD exercise, an added lead resistance changes the measured resistance under the specified model. For a voltage exercise, a wrong range changes the software interpretation even when the supplied voltage is correct.
Change one stated assumption at a time in the learning model, retaining the before and after records. If a test changes both the signal and the software range, its result cannot isolate which change caused the difference.
The PLC troubleshooting guide provides a broader investigation structure. Downstream alarms also require review when units or validity policies change; the alarm detection and priority lesson explains why a numeric threshold is only part of that behaviour.
Compare South African instrumentation courses by evidence
For training in Johannesburg, Pretoria, Durban, Cape Town or another South African location, request the actual syllabus and assessment format. Ask whether the course covers current and voltage interfaces, RTD lead arrangements, thermocouple compensation and configuration errors through individual exercises.
Confirm the venue or online delivery method, access duration, equipment available to each learner and instructor feedback. If physical measurement is important to your role, establish what supervised practical work is included. A browser calculation and a measured instrument result should have different evidence labels.
The South African instrumentation courses guide supports that comparison. These location references describe the learner's search context; they do not imply that this website operates a training branch in each city.
For related software learning, explore the instrumentation simulator overview. Confirm the supported model before assuming that a particular native RTD card, thermocouple configuration screen or electrical fault behaviour is reproduced.
The sensor learning material is another relevant starting point. Use it to develop signal-identification questions and retain observations from the actual exercise you complete.
Is 4–20 mA always better than 0–10 V?
Choose from the source and receiver specifications, required measurement performance and complete signal path. The current-budget and voltage-divider examples show different constraints. Neither supports a universal winner or a fixed cable-distance boundary.
Can an RTD connect to any analogue input?
Check the module's supported measurement modes, sensor characteristic and lead arrangement. A current or voltage input is not automatically an RTD input. A compatible transmitter can change the interface reaching the PLC, but introduces its own configuration and specifications.
Does a matching 4–20 mA range make transmitters interchangeable?
It establishes only part of compatibility. Review the process connection, materials, supply and load requirements, performance, diagnostics and applicable approvals. A matching output range alone is insufficient evidence for a replacement decision.
Why does my temperature change when the cabinet warms?
Record the observation and investigate the reference arrangement, connections, configuration and other possible influences. Do not diagnose a cold-junction problem solely from that symptom. Compare evidence with the documented measurement path.
What should my signal-types portfolio contain?
Keep a measurement-path inventory, the four-row current budget, two voltage-loading cases, five-point configuration comparison and lead-resistance calculations. Add the selected sensor references and unanswered hardware questions. These calculations were independently checked as mathematical models; retain separate evidence for any supervised practical measurements.