Converting 4-20mA to Voltage: The 250 Ohm Resistor Method

A voltage-only input does not need a different transmitter. It needs a known place for loop current to develop a voltage. A 250 ohm precision resistor provides that place: the standard 4-20mA span becomes 1-5V, and the receiver can measure that voltage with its input circuitry.

The resistor is small, but it is part of the loop load. Its actual value, heating, placement, and voltage drop all affect what the receiver sees. This guide works through the conversion, then shows how to choose and wire the resistor without spending the loop's available voltage by accident.

TL;DR: Put a precision 250 ohm burden resistor in series at the receiver and use V = I x R: 4mA becomes 1V and 20mA becomes 5V. The resistor dissipates 0.1W at 20mA, but its resistance error transfers directly into the voltage reading, so verify tolerance, heat, wiring, and loop headroom (NotebookLM, Analog Signal Standards notebook, 2026).

This guide pairs with 4-20mA versus 0-10V analog signals for the signal-standard tradeoff before conversion. For an introduction to a related industrial protocol, see the HART protocol guide.

How Does a 250 Ohm Resistor Convert 4-20mA to 1-5V?

A 250 ohm burden resistor turns the 4-20mA loop into a 1-5V signal because voltage equals current times resistance. At the live zero of 4mA, the drop is 1.0V; at 20mA, it is 5.0V. A voltage input reads the drop across the resistor rather than trying to measure current directly (NotebookLM, Analog Signal Standards notebook, 2026).

The conversion is simply Ohm's law:

[ V = I \times R ]

For the lower endpoint, substitute 0.004A and 250 ohm:

[ 0.004\text{ A} \times 250\ \Omega = 1.0\text{ V} ]

For full scale, substitute 0.020A:

[ 0.020\text{ A} \times 250\ \Omega = 5.0\text{ V} ]

Citation capsule: A 250 ohm burden resistor implements V = I x R, converting 4mA to 1.0V and 20mA to 5.0V. The resulting 1-5V signal preserves the loop's live-zero offset while giving an ADC, PLC, or microcontroller a proportional voltage to measure (NotebookLM, Analog Signal Standards notebook, 2026).

Why Is 250 Ohm the Standard Value?

A 250 ohm resistor is the common choice because it maps the standard current endpoints to the standard 1-5V range with simple arithmetic and limits receiver burden to 5.0V at 20mA. A 500 ohm resistor reaches 10.0V instead, which uses twice the receiver voltage at the same current (NotebookLM, Analog Signal Standards notebook, 2026).

The value is a compromise, not magic. Every series resistance consumes voltage from the loop supply. The receiver's drop is 5.0V at 20mA with 250 ohm, compared with 10.0V with 500 ohm. That difference leaves more voltage available for the transmitter and the wiring. If the available voltage becomes too small at high process levels, the transmitter may not have enough terminal voltage to operate and the signal can roll off.

Citation capsule: The 250 ohm standard maps 4-20mA directly to 1-5V and consumes 5.0V at 20mA. At the same current, a 500 ohm resistor consumes 10.0V, so the 250 ohm choice preserves more voltage for the transmitter and cable run (NotebookLM, Analog Signal Standards notebook, 2026).

What Precision Does the Burden Resistor Need?

Burden-resistor tolerance transfers directly to voltage error, so self-heating and temperature drift belong in the measurement budget.

The resistor's percentage error becomes the converted voltage's percentage error at a one-to-one ratio. A 1% resistor therefore adds a ±1% initial calibration offset to the voltage measurement before transmitter drift, cable losses, or ADC noise are considered. Select the resistor against the whole measurement error budget, not as an isolated part choice (NotebookLM, Analog Signal Standards notebook, 2026).

The relationship is direct because resistance is one side of Ohm's law:

[ \frac{\Delta V}{V} = \frac{\Delta R}{R} ]

That is why a nominal 250 ohm part is not automatically a precision 250 ohm part. The packet uses a 0.1% overall measurement target to show the mismatch: a 1% resistor alone consumes ten times that total error allowance. The resistor must leave room for the other contributors to the measurement result.

Temperature adds a different kind of error. Ambient cabinet or junction-box temperature can change resistance. The resistor also warms itself while carrying loop current. This temperature-driven resistance change becomes a moving voltage error, which is harder to remove with a one-time calibration than a fixed initial offset. The research packet does not prescribe a universal tolerance or temperature-coefficient number; use the accuracy requirement and the component data to make that selection.

Citation capsule: Because V = I x R, burden-resistor error propagates to converted-voltage error at a 1:1 percentage ratio. A 1% resistor creates a ±1% initial voltage offset, while a 0.1% overall measurement target leaves no room for that resistor error alone (NotebookLM, Analog Signal Standards notebook, 2026).

How Much Loop Voltage and Resistor Power Must You Allow?

At full-scale current, the 250 ohm resistor consumes 5V and dissipates 0.1W, so both load and heat require checking.

At 20mA, a 250 ohm burden resistor drops 5.0V and dissipates 0.1W under P = I² x R. That voltage is part of the loop's total load, while the heat can shift resistance and therefore voltage. Check both quantities at the highest current the loop must support (NotebookLM, Analog Signal Standards notebook, 2026).

The power calculation is:

[ P = I^2 \times R ]

At 0.020A and 250 ohm, the result is 0.1W. The packet identifies 1/4W and 1/2W metal-film precision resistors as suitable physical choices at that load. Choosing a resistor that can carry the wattage is necessary, but the reason is measurement quality as well as survival: self-heating can change resistance, and resistance change appears as a changed reading.

What should be checked first? Start from the maximum current, then list every series drop. At that current, a 250 ohm conversion resistor costs 5.0V. A 500 ohm resistor costs 10.0V. Long runs add their own loss. This approach finds an insufficient-voltage design before a transmitter flatlines at the top of its process range.

Citation capsule: A 250 ohm burden resistor dissipates 0.1W at 20mA and drops 5.0V from the loop supply. That drop must be included with transmitter and cable requirements, because a larger receiver burden can leave the transmitter without enough terminal voltage at high current (NotebookLM, Analog Signal Standards notebook, 2026).

Where Should You Wire the Conversion Resistor?

Put the burden resistor at the receiver so the voltage input measures directly across the converted 1–5V drop.

Wire the burden resistor at the receiving input. On a single-ended analog card, it sits between the signal channel and shared AI common; on a differential card, it sits directly across that channel's positive and negative terminals. The receiver measures the converted voltage at those terminals (NotebookLM, Analog Signal Standards notebook, 2026).

A differential card gives each channel separate positive and negative terminals. Place the resistor across those two terminals. The input measures the voltage across the resistor, rather than the voltage from one side to the shared PLC common. Common voltage present on both conductors is rejected by the differential measurement, subject to the card's common-mode limit.

Differential measurement rejects common voltage, but it has a limit: the packet gives a typical ±10V to ±15V common-mode range. Above it, the input can saturate. An isolator is required to break a ground path; a higher-impedance input does not remove it.

Citation capsule: At the receiver, place a burden resistor between signal and shared common for a single-ended input, or directly across positive and negative terminals for a differential input. Differential measurement rejects common voltage but typically has a ±10V to ±15V common-mode limit (NotebookLM, Analog Signal Standards notebook, 2026).

When Does a 500 Ohm Resistor Make Sense?

Use a 500 ohm burden resistor when the receiving voltage range calls for 2-10V and the loop has the extra voltage headroom. The same 4-20mA signal then produces 2.0V at live zero and 10.0V at full scale, but the resistor drops 10.0V at 20mA rather than 5.0V (NotebookLM, Analog Signal Standards notebook, 2026).

There is also a power consequence. Since P = I² x R, doubling the resistance doubles dissipation at the same current. A 500 ohm resistor at 20mA dissipates 0.2W by calculation from the packet's formula. Treat the different range as a loop-load decision, then check the receiver's voltage range and the full voltage budget together.

Citation capsule: A 500 ohm burden resistor converts 4-20mA to 2-10V: 4mA produces 2.0V and 20mA produces 10.0V. It also drops 10.0V at full scale and 11.0V at 22mA, doubling the voltage burden of a 250 ohm receiver (NotebookLM, Analog Signal Standards notebook, 2026).

What Measurement Errors Should You Check First?

Start with resistor value, voltage across the resistor, and the receiver reference. A 1% resistor causes a ±1% converted-voltage offset, a 250 ohm resistor at 20mA should show 5.0V, and ground differences can corrupt a single-ended measurement. These checks separate conversion error from scaling or process error quickly (NotebookLM, Analog Signal Standards notebook, 2026).

First, measure the loop current and the voltage directly across the burden resistor. Apply V = I x R with the actual installed resistor value. A 12mA current through 250 ohm should produce 3.0V. If the current and voltage disagree with that relationship, look at the resistor, its connections, or where the voltage probes are placed before changing software scaling.

Finally, distinguish an electrical conversion fault from a valid process signal or a loop-fault condition. The 4mA live zero represents the bottom of the calibrated span; zero current does not represent the same thing. Do not mask that distinction with a low-end clamp.

Citation capsule: A practical conversion check compares loop current with the voltage across the burden resistor: 12mA through 250 ohm yields 3.0V, while 20mA yields 5.0V. A mismatch can come from resistance error, wiring or probe placement, heating, or a ground-referenced measurement problem (NotebookLM, Analog Signal Standards notebook, 2026).

How Do You Scale the Converted Voltage Back to Engineering Units?

A 250 ohm conversion leaves a linear 1-5V signal in which 1.0V represents 0% span and 5.0V represents 100% span. Scale the measured voltage by its 1V-to-5V endpoints, then map the normalized result to the transmitter's engineering range. The voltage conversion changes the electrical form, not the process relationship (NotebookLM, Analog Signal Standards notebook, 2026).

Use the interpolation formula:

[ \text{Process Value} = \left(\frac{V_{\text{measured}} - V_{\text{min}}}{V_{\text{max}} - V_{\text{min}}}\right) \times (EU_{\text{max}} - EU_{\text{min}}) + EU_{\text{min}} ]

For the 1-5V conversion, set the voltage endpoints to 1V and 5V. A 3.0V reading is halfway through that four-volt span. If the transmitter covers 0-1000 psi, the formula produces 500 psi. The packet's example makes the relationship visible: 3.0V is 50% of the converted voltage span and therefore 50% of the engineering span.

Use the PLC raw-count scaling guide for the controller-side calculation and its checks. Keep the two questions separate during troubleshooting: first establish that current became the right voltage, then establish that the voltage became the right process value.

Citation capsule: With a 250 ohm conversion, 1.0V is 0% span and 5.0V is 100% span. A 3.0V measurement is halfway through the 1-5V range, so a 0-1000 psi transmitter scales to 500 psi using linear interpolation (NotebookLM, Analog Signal Standards notebook, 2026).

Frequently Asked Questions

Can I use a 250 ohm resistor with a voltage-only PLC input?

Yes. A 250 ohm precision burden resistor converts the 4-20mA loop to 1-5V, so a voltage input can measure the drop across it. The input must be connected across the resistor, and the loop must retain enough voltage for the transmitter and wiring (NotebookLM, Analog Signal Standards notebook, 2026).

Why does 4mA become 1V instead of 0V?

The 4mA live zero is the lower calibrated endpoint of the current loop. Across a 250 ohm resistor, 0.004A times 250 ohm is 1.0V, so the conversion retains the live-zero distinction rather than treating the lower process limit as zero volts (NotebookLM, Analog Signal Standards notebook, 2026).

Can I use a 1% resistor for a 4-20mA conversion?

A 1% resistor will produce a working conversion, but its resistance error becomes a ±1% voltage error at a one-to-one ratio. For a loop targeting 0.1% overall measurement precision, that resistor alone consumes ten times the total error allowance (NotebookLM, Analog Signal Standards notebook, 2026).

Does a 500 ohm resistor always produce a better voltage signal?

No. A 500 ohm resistor produces a 2-10V range, but it drops 10.0V at 20mA, compared with 5.0V for 250 ohm. Choose it only when the receiver needs that range and the supply can still support the transmitter and cable (NotebookLM, Analog Signal Standards notebook, 2026).

Should the resistor go at the transmitter or receiver?

Put the conversion resistor at the receiver where the voltage is read. A single-ended card uses the resistor between signal and shared common; a differential card uses it across independent positive and negative terminals (NotebookLM, Analog Signal Standards notebook, 2026).

Conclusion

A 250 ohm precision resistor converts a 4-20mA loop into 1-5V: 1V at 4mA, 3V at 12mA, and 5V at 20mA. Include its voltage drop in the loop budget and choose its accuracy against the total measurement allowance.

Install it at the receiver and measure across it before troubleshooting PLC scaling.