1. What Are Analog Control Signals?
Analog control signals are used to represent a continuously variable measurement or command. Unlike a simple digital signal that is either on or off, an analog signal can represent values across a range.
Two of the most common analog signal types encountered in industrial controls, HVAC controls, building automation, and instrumentation are 4–20 mA and 0–10 V.
Both can represent the same engineering range. For example, a pressure transmitter could represent 0–100 PSI using either 4–20 mA or 0–10 V.
The important difference is how the information is electrically represented: 4–20 mA uses current, while 0–10 V uses voltage.
2. How 4–20 mA Works
A 4–20 mA signal represents the measured value using current flowing through the signal loop.
For a standard linear transmitter:
| Signal | Percent of Span |
|---|---|
| 4 mA | 0% |
| 8 mA | 25% |
| 12 mA | 50% |
| 16 mA | 75% |
| 20 mA | 100% |
The general scaling equation is:
For example, a 0–100 PSI transmitter producing 12 mA represents 50 PSI.
The 4 mA starting point is called a live zero. It allows the receiving system to distinguish a valid zero measurement from conditions such as an open circuit or loss of loop current.
3. How 0–10 V Works
A 0–10 V signal represents the measured value using voltage relative to the signal reference.
For a standard linear 0–10 V signal:
| Signal | Percent of Span |
|---|---|
| 0 V | 0% |
| 2.5 V | 25% |
| 5 V | 50% |
| 7.5 V | 75% |
| 10 V | 100% |
For a 0–100 PSI range, 5 V would therefore represent 50 PSI.
The basic scaling equation is:
0–10 V signals are particularly common in building automation and HVAC applications for commands such as valve position, damper position, actuator position, and speed references.
4. 4–20 mA vs 0–10 V
The two signal types can represent the same information, but they behave differently electrically.
| Characteristic | 4–20 mA | 0–10 V |
|---|---|---|
| Signal type | Current | Voltage |
| Typical range | 4–20 mA | 0–10 V |
| Zero signal | 4 mA | 0 V |
| Full-scale signal | 20 mA | 10 V |
| Loop wiring | Current loop | Voltage signal and reference |
| Voltage drop in wiring | Can affect loop voltage available to devices | Can directly affect the measured signal |
| Common applications | Instrumentation and industrial controls | HVAC, BAS/BMS, actuators and control outputs |
These are common applications rather than absolute rules. Either signal type can appear in a wide variety of control systems.
5. Wiring, Voltage Drop, and Long Cable Runs
One important practical difference is how cable resistance affects the two signal types.
4–20 mA
A current transmitter regulates the loop current. Cable resistance causes a voltage drop, but the current can remain at the intended value as long as the transmitter and receiving devices have enough voltage available to operate.
The loop therefore needs enough supply voltage to overcome the voltage requirements of the transmitter, input burden, wiring resistance, and other devices in the loop.
0–10 V
A voltage signal is measured directly as a voltage difference. Resistance in the wiring can become important when it causes a significant voltage drop or when the receiving input loads the source.
6. The 250 Ω Connection
A 250 Ω resistor provides a convenient way to convert a 4–20 mA signal into a 1–5 V signal.
This follows directly from Ohm's law:
With a 250 Ω resistor:
| Current | Voltage Across 250 Ω |
|---|---|
| 4 mA | 1 V |
| 8 mA | 2 V |
| 12 mA | 3 V |
| 16 mA | 4 V |
| 20 mA | 5 V |
This is why a 250 Ω resistor is commonly encountered in instrumentation systems where a 4–20 mA signal needs to be represented as a 1–5 V signal.
7. Sourcing and Sinking
Terms such as source and sink describe how current flows between control devices and are especially important when wiring analog I/O.
A sourcing output provides current to the connected circuit. A sinking input or output provides a path for current to return through the circuit.
The exact wiring arrangement depends on the particular transmitter, PLC, analog input module, controller, or actuator.
8. Measuring 4–20 mA and 0–10 V
Measuring 4–20 mA
Current must be measured in series with the circuit. The meter becomes part of the current path.
Before connecting a meter, verify the meter lead position, measurement function, expected current range, and the circuit arrangement.
Measuring 0–10 V
Voltage is measured across the signal and its reference point.
A typical measurement might therefore be made between the analog signal terminal and the appropriate signal common/reference terminal.
Always follow the equipment manufacturer's wiring and measurement instructions.
9. Worked Comparison: 0–100 PSI
Suppose the same 0–100 PSI measurement is represented by either a 4–20 mA transmitter or a 0–10 V signal.
| Pressure | 4–20 mA | 0–10 V |
|---|---|---|
| 0 PSI | 4 mA | 0 V |
| 25 PSI | 8 mA | 2.5 V |
| 50 PSI | 12 mA | 5 V |
| 75 PSI | 16 mA | 7.5 V |
| 100 PSI | 20 mA | 10 V |
The engineering information is identical. The electrical representation is different.
0–10 V at 5 V → 50 PSI
This is an important concept when troubleshooting: the controller cannot correctly interpret either signal unless its input configuration and scaling match the signal being supplied.
10. Common Field Problems
4–20 mA stuck near 4 mA
The transmitter may actually be reporting a value near the bottom of its configured range. Check the process condition and transmitter configuration before assuming the signal is faulty.
4–20 mA below 4 mA
Depending on the transmitter and configuration, a current below 4 mA can indicate an underrange condition or a diagnostic/fault state. Check the device documentation.
4–20 mA above 20 mA
Similarly, a signal above 20 mA may indicate overrange operation or a configured diagnostic condition.
0–10 V reads incorrectly
Check the signal reference, controller input configuration, wiring, source output, and scaling.
Signal changes but displayed value is wrong
This often points toward a scaling or configuration issue. Verify the transmitter range and compare it with the receiving input and PLC/HMI scaling.
Ground-reference problems
Voltage signals depend on the relationship between the signal and its reference. Differences in reference potential can therefore create measurement errors.
A current loop can also encounter grounding and wiring problems, so the complete signal circuit should always be considered rather than assuming one signal type is immune to field wiring problems.
11. When You'll Commonly Encounter Each Signal
Neither signal type is universally used for every application. The equipment and control architecture determine what you will encounter in the field.
4–20 mA
- Process instrumentation
- Pressure transmitters
- Flow transmitters
- Level transmitters
- Temperature transmitters
- Industrial analog inputs
- Instrumentation systems
0–10 V
- HVAC controls
- Building automation systems
- Damper actuators
- Valve actuators
- Fan speed commands
- Controller analog outputs
- Equipment control interfaces
These are common examples, not limitations. Always verify the actual signal type from the equipment documentation and wiring diagram.
12. Practical Field Workflow
When troubleshooting an analog signal, work through the signal path instead of immediately changing components.
- Identify the signal type. Determine whether the system is using 4–20 mA, 0–10 V, or another signal range.
- Determine the expected range. Find the transmitter or controller LRV and URV and identify what the signal should represent.
- Measure the actual signal. Measure current in series for a current loop or voltage across the appropriate points for a voltage signal.
- Convert the signal to engineering units. Determine what the measured signal should represent.
- Check the receiving input. Verify the analog input range, wiring, signal type, and configuration.
- Check PLC scaling. Make sure the controller is interpreting the input using the correct range and units.
- Check the HMI or supervisory system. If the PLC value is correct but the display is wrong, investigate the tag mapping, scaling, communications, and display configuration.
13. Common Mistakes
- Assuming every analog signal is 4–20 mA.
- Treating 0 V as automatically equivalent to a valid zero measurement without checking the signal type.
- Measuring a current loop incorrectly with a multimeter.
- Forgetting the signal reference when measuring voltage.
- Using the wrong input configuration on a PLC or controller.
- Scaling the PLC for a different range than the transmitter.
- Assuming cable resistance has the same effect on current and voltage signals.
- Adding a resistor to a current loop without checking the available loop voltage.
- Replacing a transmitter before verifying the actual signal and configuration.
14. Key Takeaways
4–20 mA and 0–10 V can both represent the same engineering information, but they do so using different electrical signals.
- 4–20 mA represents the signal using current and uses 4 mA as its standard zero point.
- 0–10 V represents the signal using voltage from 0 to 10 V.
- A 250 Ω resistor converts 4–20 mA into 1–5 V.
- Current is measured in series; voltage is measured across the signal and reference.
- Correct scaling requires the transmitter range and receiving system configuration to agree.
- The best troubleshooting approach is to follow the signal from its source through the receiving system and find the first point where the value becomes incorrect.
Related ControlsCalc Tools
Use these calculators and guides to work through analog signal calculations and troubleshooting.