PLC Analog Signals and Scaling Explained: 0–10 V, 4–20 mA and Raw Values

Table of Contents

• Introduction
• What Is an Analog Signal?
• Digital vs Analog Signals
• 0–10 V Signal
• 4–20 mA Signal
• Why Is 4–20 mA Commonly Used?
• What Is a Raw Analog Value?
• What Is Analog Scaling?
• Analog Scaling Formula
• Practical Scaling Example
• Tank Level Example
• Temperature Example
• VFD Speed Reference Example
• Common Beginner Mistakes
• How to Troubleshoot Analog Signals
• How to Practice Analog Scaling
• Important Safety Considerations
• Conclusion

Introduction

Analog signals are an important part of industrial automation.

While digital signals normally represent simple ON and OFF conditions, analog signals represent continuously changing values such as temperature, pressure, flow, level, speed, and position.

PLCs receive analog signals from field devices such as transmitters and sensors. The PLC then converts the received signal into a numerical value that can be used by the control program.

One of the most important skills in PLC programming is understanding analog scaling.

In this article, we will learn about 0–10 V, 4–20 mA, raw analog values, scaling formulas, and practical industrial examples.

What Is an Analog Signal?

An analog signal is a continuously variable electrical signal used to represent a physical measurement.

For example, a temperature transmitter may produce a signal that changes according to the measured temperature.

Common industrial measurements include:

• Temperature
• Pressure
• Flow
• Tank level
• Speed
• Position
• Weight

Common analog signal standards include 0–10 V and 4–20 mA.

Digital vs Analog Signals

Digital signals generally have discrete states such as ON and OFF.

Analog signals can have many values within a defined range.

For example:

Digital Signal → ON / OFF

Analog Signal → 0–10 V or 4–20 mA

A proximity sensor used for object detection may provide a digital signal, while a pressure transmitter may provide an analog signal.

0–10 V Signal

0–10 V is a common analog voltage signal used in automation systems.

The voltage represents a measurement within a defined engineering range.

For example, a 0–10 V signal could represent a motor speed reference from 0% to 100%.

A simple relationship could be:

0 V → 0%

5 V → 50%

10 V → 100%

The exact relationship depends on the device configuration and application.

4–20 mA Signal

4–20 mA is widely used for industrial analog signals.

A transmitter can use the current range to represent a physical measurement.

For example, a pressure transmitter configured for 0–10 bar could use:

4 mA → 0 bar

12 mA → 5 bar

20 mA → 10 bar

The actual measurement range depends on the transmitter configuration.

Why Is 4–20 mA Commonly Used?

4–20 mA is popular in industrial environments because current signals can be transmitted over long cable runs with good resistance to voltage-drop effects.

The 4 mA starting point also provides a live-zero signal rather than using zero current for the lower measurement value.

This can help distinguish a normal low measurement from certain wiring or transmitter faults, although the actual fault-detection method depends on the instrument and PLC configuration.

What Is a Raw Analog Value?

A PLC normally does not directly use the physical voltage or current value as the final engineering value.

The analog input module converts the electrical signal into a digital numerical value.

This numerical value is often called the raw value.

For example:

Sensor Signal → Analog Input Module → Raw Value → Scaling → Engineering Value

The raw value range depends on the PLC, analog module, configuration, and signal type.

Always check the manufacturer's documentation for the actual raw-value range of the hardware you are using.

What Is Analog Scaling?

Analog scaling is the process of converting a raw PLC analog value into a useful engineering value.

For example, a PLC may receive a raw value from a level transmitter.

The program can convert that raw value into a tank level such as 0 to 10 meters.

The basic process is:

Raw Value → Scaling Calculation → Engineering Value

This allows the PLC program and HMI to display meaningful process information.

Analog Scaling Formula

A general linear scaling formula can be written as:

Engineering Value = ((Raw Value − Raw Minimum) × (Engineering Maximum − Engineering Minimum) ÷ (Raw Maximum − Raw Minimum)) + Engineering Minimum

For example, suppose:

Raw Minimum = 0

Raw Maximum = 27648

Engineering Minimum = 0

Engineering Maximum = 10

The PLC can use the formula to convert the raw input into a 0–10 engineering value.

The actual raw limits must be taken from the PLC hardware documentation rather than assumed.

Practical Scaling Example

Suppose a PLC receives an analog signal representing tank level.

The transmitter range is:

4 mA → 0 meters

20 mA → 10 meters

The PLC analog module converts the signal into a raw value.

The program then scales the raw value to the engineering range of 0 to 10 meters.

The HMI can then display:

Tank Level = 6.5 meters

This makes the process easier for an operator to understand.

Tank Level Example

Tank-level measurement is a common analog application.

A level transmitter measures the liquid level inside a tank and sends an analog signal to the PLC.

The PLC receives the signal and performs scaling.

The HMI can then display the level as a percentage or engineering unit.

The sequence is:

Level Sensor → 4–20 mA → PLC Analog Input → Scaling → HMI Display

The PLC can also use the scaled value to control pumps, valves, alarms, or other equipment.

Temperature Example

Temperature measurement is another common analog application.

A temperature transmitter may be configured to represent a specific temperature range using 4–20 mA.

For example, a transmitter could be configured for:

4 mA → 0°C

20 mA → 100°C

The PLC converts the input signal into a raw value and then scales it to the temperature range.

The resulting temperature can be displayed on an HMI or used in control logic.

VFD Speed Reference Example

Analog outputs can also be used to control industrial equipment.

For example, a PLC can provide a 0–10 V reference to a VFD for motor-speed control.

A possible configuration could be:

0 V → 0% Speed Reference

5 V → 50% Speed Reference

10 V → 100% Speed Reference

The exact relationship depends on the VFD configuration and application.

The PLC can calculate the required reference and send the corresponding analog output.

Common Beginner Mistakes

Beginners often make several mistakes when working with analog signals.

One common mistake is assuming the raw-value range without checking the PLC hardware documentation.

Another mistake is using the wrong engineering minimum or maximum during scaling.

Incorrect wiring, incorrect signal type, and incorrect module configuration can also cause unexpected values.

Always verify the sensor range, PLC analog-module configuration, wiring, and scaling calculation.

How to Troubleshoot Analog Signals

If an analog signal is not working correctly, check the following:

• Check the sensor or transmitter power supply.
• Check the signal wiring.
• Check whether the transmitter is configured for 0–10 V or 4–20 mA.
• Check the PLC analog input configuration.
• Monitor the raw analog value.
• Check the scaling formula.
• Check the engineering minimum and maximum.
• Compare the PLC reading with a suitable reference measurement.
• Check for alarms or diagnostic information from the analog module.

Monitoring the raw value and scaled value at the same time can make troubleshooting easier.

How to Practice Analog Scaling

You can practice analog programming using simple applications.

• Scale a 0–10 V signal to 0–100%.
• Scale a 4–20 mA pressure transmitter to 0–10 bar.
• Scale a level transmitter to 0–10 meters.
• Scale a temperature transmitter to 0–100°C.
• Create a tank-level HMI display.
• Control a VFD using an analog speed reference.
• Create high and low analog alarms.
• Combine analog values with timers and PLC control logic.

These exercises will help you understand how real industrial analog signals are processed inside a PLC.

Important Safety Considerations

Analog circuits can be connected to industrial equipment operating at hazardous voltages or pressures.

Always follow the electrical drawings, manufacturer's documentation, isolation procedures, and applicable safety requirements when working with field wiring and control panels.

Do not make wiring changes to energized industrial equipment unless the applicable procedures specifically permit it and the work is performed by appropriately qualified personnel.

Conclusion

Analog signals are an important part of modern industrial automation.

Understanding 0–10 V, 4–20 mA, raw values, and scaling allows PLC programmers to work with real-world measurements such as temperature, pressure, flow, level, and speed.

Once you understand analog scaling, you can build more advanced PLC and HMI applications and display real process values correctly.

In the next article, we can learn about PLC HMI communication and understand how a PLC exchanges data with an HMI.

Meta Description

Learn PLC analog signals and scaling for beginners. Understand 0–10 V, 4–20 mA, raw values, scaling formulas, tank level, temperature, VFD speed control, and troubleshooting.