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Use a potentiometer-input 4–20 mA transmitter. A bare potentiometer connected to 24 VDC is only a voltage divider; it does not regulate the 4 mA-to-20 mA current required by a PLC or DCS. The transmitter measures the potentiometer wiper and controls loop current, normally using the same two-wire 24 V loop for power and signal.
+24 VDC → transmitter (+) → transmitter (−) → PLC current input (+) PLC current input (−) → 24 VDC 0 V
Connect the potentiometer to the transmitter’s dedicated pot high, wiper and pot low terminals—not directly across the 24 V supply.
What “4–20 mA potentiometer” means
The term is used for several different products. This article covers a physical potentiometer whose position is converted into a 4–20 mA output from a 24 VDC system. It is different from:
- a potentiometer wired to a PLC voltage input and scaled in software;
- a handheld 4–20 mA loop calibrator or manual current generator;
- a motorized potentiometer commanded by a current signal; or
- an integrated manual setpoint station that already contains current-regulating electronics.
Why a potentiometer alone cannot make 4–20 mA
A three-terminal potentiometer divides an excitation voltage. Its wiper voltage changes with shaft or slider position, but the device does not maintain 4 mA at one end, 12 mA at the midpoint and 20 mA at the other. A current loop needs an active sensing and regulating circuit.
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The signal chain is:
mechanical position → wiper voltage → transmitter electronics → regulated loop current
Loop-powered transmitter designs use an amplifier, reference and current-regulating output stage. The historical Analog Devices AD693 illustrates this type of loop-powered transmitter IC, but it is a design component rather than a complete field module.
The 4 mA lower limit is a live zero: a valid zero position is distinguishable from a broken wire or loss of loop power. A transmitter may deliberately drive a value below 4 mA or above 20 mA for a detected fault; the exact behavior is model-specific.
Required hardware
- Regulated 24 VDC supply rated for the total loop load.
- Linear potentiometer with resistance and mechanical ratings that match the transmitter.
- Potentiometer-input, 4–20 mA transmitter.
- PLC, DCS, indicator or recorder input configured for the correct current-loop type.
- Optional isolator, display or precision resistor, each of which adds voltage drop.
A practical example is the DIN-rail Datexel DAT2105 2W. Its datasheet specifies a loop-powered 4–20 mA output and potentiometer input up to 50 kΩ: DAT2105 2W datasheet. Verify the current specification, availability and load curve before ordering.
Standard scaling
For a correctly wired linear potentiometer, the nominal relationship is:
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- Accuracy to ±0.03%
Iout = 4 mA + 16 mA × position_fraction
| Potentiometer position | Nominal loop current |
|---|---|
| 0% | 4 mA |
| 25% | 8 mA |
| 50% | 12 mA |
| 75% | 16 mA |
| 100% | 20 mA |
Actual end points include the transmitter’s accuracy, calibration, resolution and mechanical end-stop tolerance.
Wiring the potentiometer and loop
Two-wire, loop-powered transmitter
24 VDC + ───────────────► Transmitter loop +
Transmitter loop − ───► PLC AI +
PLC AI − ───────────────► 24 VDC 0 V
The transmitter and PLC input are in series. Connect the potentiometer separately:
Transmitter pot high ─── potentiometer fixed high end Transmitter wiper ─── potentiometer wiper Transmitter pot low ─── potentiometer fixed low end
Use the transmitter’s excitation terminals. Do not apply 24 V directly to a pot input designed for a low internal reference. The API 4003GI documentation, for example, identifies separate high, wiper and low terminals and a low-voltage potentiometer excitation.
Separately powered transmitter
24 VDC → transmitter power terminals Transmitter active output + → PLC input + Transmitter output − → PLC input −
Some modules provide active (sourcing), passive (sinking), isolated or non-isolated outputs. There is no universal wiring diagram; match the transmitter output type to the PLC input manual. The API documentation shows examples of isolated and externally supplied configurations.
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Active and passive PLC inputs
A passive input normally needs an external supply; an active input may provide loop voltage itself. Identify who supplies the voltage and who regulates current before connecting anything. Never tie two active loop sources together, and do not connect a 24 V supply directly across an analog input unless the PLC wiring diagram explicitly requires it.
Selecting the potentiometer and transmitter
- Resistance: stay within the transmitter’s specified range. Products may be designed for 10 kΩ, 20 kΩ or 50 kΩ; a 10 kΩ pot is not universally interchangeable.
- Taper: choose linear unless the transmitter explicitly supports another characteristic.
- Construction: select suitable travel, shaft or slider style, mechanical life, enclosure and environmental protection.
- Electrical features: check 24 V operating range, two-wire or separate power, isolation, accuracy, response time, calibration and fault-current behavior.
- Installation: use shielded twisted pair where appropriate, route analog wiring away from motor and VFD cables, and terminate shields according to the site grounding standard.
For long runs or ground-potential problems, an isolated transmitter or a transmitter mounted near the potentiometer can be preferable. If high accuracy or harsh environmental protection is required, use an industrial position transmitter rather than a low-cost panel potentiometer.
Check the 24 V voltage budget
Calculate compliance at the maximum current, 20 mA:
Rload_max = (Vsupply − Vtransmitter_min − Vother_drops) / 0.020 A
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Include the PLC input, external resistor, isolator, display and wiring. With a 24 V supply and a 500 Ω input, the load consumes:
Vload = 0.020 A × 500 Ω = 10 V
Only 14 V remains for the transmitter and all other drops. Whether that is sufficient depends on the selected transmitter’s minimum operating voltage. Texas Instruments’ TIPD158 reference design illustrates a 10–36 V loop-supply range. Phoenix Contact publishes a device-specific example, Load ≤ (UB − 8 V) / 20 mA, and up to 800 Ω at 24 V for one isolator; neither figure is a universal rule.
Using resistors to obtain a voltage signal
| Resistor | Voltage at 4 mA | Voltage at 20 mA |
|---|---|---|
| 100 Ω | 0.4 V | 2 V |
| 250 Ω | 1 V | 5 V |
| 500 Ω | 2 V | 10 V |
These values follow V = I × R. A 500 Ω resistor dissipates 0.020² × 500 = 0.2 W at 20 mA, so a precision resistor rated at 0.5 W or more is commonly chosen for thermal margin. Its tolerance and temperature coefficient affect measurement accuracy, and its voltage drop must be included in the loop calculation.
Installation and commissioning procedure
- Identify the receiving input. Confirm whether it is active or passive, isolated or non-isolated, configured for 4–20 mA, and rated for the expected input resistance.
- Choose the transmitter and pot together. Confirm resistance range, supply range, output type, isolation and fault behavior in the same manufacturer documentation.
- Prove the voltage budget. Add every series voltage drop at 20 mA and leave design margin.
- Wire with power removed. Connect pot high, wiper and pot low to the corresponding functional terminals. Terminal numbers vary by model.
- Complete the loop. Follow the transmitter and PLC diagrams for the correct polarity and for any internal or external input resistor.
- Test the endpoints. Measure loop current in series with a calibrated meter or loop calibrator: minimum should be approximately 4 mA, midpoint approximately 12 mA and maximum approximately 20 mA.
Never place an ammeter directly across the 24 V supply; that shorts the supply through the meter. Some transmitters, including the DAT2105 2W, document response time and configurable burnout values outside the normal range; allow for those specifications during testing.
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- 1000V overload protection on V, Ohms, frequency, and 150V overload protection on mA, backed up by a 440 mA 1000V fuse
- Runs about 50 hours typical (measurement), 12 hours typical (sourcing 12 mA) on one 9-volt alkaline battery
- Clear LCD display with Backlight
- Current Measurement Range: 0 to 30 mA DC
Troubleshooting by symptom
0 mA
- Check supply polarity, fuse and voltage at the transmitter.
- Verify loop continuity, PLC wiring and transmitter power type.
- Determine whether the PLC is already supplying loop power; remove an incorrectly connected second active source.
- Check that the loop load does not exceed the transmitter’s compliance limit.
Approximately 4 mA at every position
- Inspect the wiper connection and pot terminal identification.
- Measure wiper voltage at the transmitter while moving the control. If it does not change, the fault is in the potentiometer or its wiring.
- Check input configuration, supported resistance and any transmitter low-fault state.
Approximately 20 mA continuously
- Look for a wiper short to the high end, an open low-end connection or reversed terminal identification.
- Check for a transmitter high-overrange fault and for a resistance mismatch.
4 mA works but current collapses near 20 mA
This usually indicates inadequate voltage compliance or excessive load. Measure voltage at the transmitter while current is 20 mA and check the PLC resistance, conversion resistor, isolator, display, cable drop and transmitter minimum voltage.
Span runs backwards
If the desired minimum produces 20 mA and the maximum produces 4 mA, swap the two fixed-end pot wires. Do not swap the wiper with an end terminal.
Current is correct but the PLC value is wrong
- Check for a channel configured as 0–20 mA, voltage mode or the wrong polarity.
- Verify raw-count range, 4 mA/20 mA engineering scaling and any external resistor setting.
Noisy or unstable signal
- Check supply ripple, loose terminals, worn pot track and excessive source impedance.
- Separate analog wiring from VFD and motor cables; use appropriate shielding and grounding.
- Confirm the transmitter’s filtering and cable limits.
When another architecture is better
| Requirement | Suitable approach |
|---|---|
| PLC or DCS requires a field 4–20 mA signal | Potentiometer-input 4–20 mA transmitter |
| Short local wiring and an available voltage channel | 5 V or 10 V potentiometer into a PLC voltage input, scaled in software |
| Commissioning or simulation | Dedicated 4–20 mA signal generator |
| PLC does not provide loop power | Separately powered transmitter with a correctly matched active output |
| Ground-loop risk or demanding installation | Galvanically isolated transmitter |
| Remote, accurate position measurement | Industrial position transmitter or encoder |
A voltage-input arrangement can be economical for a local panel, and ratiometric measurement can reduce sensitivity to excitation variation; Laurel explains this approach at Laurel Electronics. It does not, however, provide a 4–20 mA signal to another device.
Examples of suitable commercial hardware
Dedicated potentiometer transmitters
- Datexel DAT2105 2W: DIN-rail, loop-powered potentiometer-to-4–20 mA module; verify the 50 kΩ limit and complete datasheet before purchase.
- API 4003GI: potentiometer input, low-voltage excitation, current-output configurations and isolation options; generally more configurable than a basic two-wire module.
- Robert Owen ROI-XMA/ROI-RPT families: product information describes loop-powered potentiometer transmitters with stated 7.5–36 VDC supply ranges; match the exact model’s resistance and mounting requirements.
General-purpose conditioners
Phoenix Contact loop isolators and the MINI Analog Pro range are useful for isolation and standard signal conversion, but they are not automatically three-terminal potentiometer transmitters. The AD693 is an IC for custom electronics, not a ready-to-wire industrial module.
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