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4–20 mA Loop Resistor Calculator

Calculate the resistor required to convert a 4–20 mA current signal into a desired voltage.

This is commonly used when interfacing 4–20 mA instrumentation with voltage-based analog inputs, signal converters, data acquisition equipment, and test equipment.

Calculate Required Loop Resistor

The current at which you want to produce the target voltage.
The voltage you want across the resistor.

Result

Loop Resistor Formula

The required resistance can be calculated using Ohm's Law:

R = V ÷ I

Because the current is entered in milliamps, it must first be converted to amps:

Resistance = Voltage ÷ (mA ÷ 1000)

Where:

  • R = resistance in ohms
  • V = desired voltage
  • I = current in amps

Worked Example: 4–20 mA to 1–5 V

A common application is converting a 4–20 mA signal into a nominal 1–5 V signal.

To determine the required resistor, use the maximum signal: 20 mA and 5 V.

R = 5 V ÷ 0.020 A

Therefore:

R = 250 Ω

A 250 Ω resistor produces:

  • 4 mA → 1 V
  • 8 mA → 2 V
  • 12 mA → 3 V
  • 16 mA → 4 V
  • 20 mA → 5 V

Common 4–20 mA Resistor Values

Resistor 4 mA 12 mA 20 mA
100 Ω 0.4 V 1.2 V 2.0 V
150 Ω 0.6 V 1.8 V 3.0 V
250 Ω 1.0 V 3.0 V 5.0 V
500 Ω 2.0 V 6.0 V 10.0 V

Loop Voltage Burden

Adding a resistor to a 4–20 mA loop increases the voltage required from the loop power supply.

At the maximum loop current, the voltage dropped across the resistor is:

VR = I × R

For a 250 Ω resistor at 20 mA:

VR = 0.020 × 250 = 5 V

That means the resistor consumes 5 V of the available loop voltage at full scale. The transmitter and every other device in the loop must still have enough voltage available to operate.

Resistor Power Rating

A resistor also dissipates electrical power. Calculate it using:

P = I² × R

For a 250 Ω resistor at 20 mA:

P = (0.020)² × 250
P = 0.10 W

The resistor should have an appropriate power rating for the application. A resistor with a rating comfortably above the calculated dissipation is generally preferable.

Using a Loop Resistor for Troubleshooting

A known resistor can provide a convenient voltage measurement point when troubleshooting a current loop.

For example, with a 250 Ω resistor:

4 mA → 1 V
12 mA → 3 V
20 mA → 5 V

If the current is known but the measured voltage doesn't agree with Ohm's Law, verify the actual resistance and measurement points before assuming the transmitter is faulty.

Common Mistakes

  1. Forgetting to convert mA to amps. 20 mA equals 0.020 A.
  2. Calculating the resistor from the wrong current. For a 4–20 mA signal intended to produce a maximum voltage, use the 20 mA full-scale value.
  3. Ignoring loop voltage burden. A larger resistor requires more voltage from the power supply.
  4. Ignoring resistor power. Check the resistor's power dissipation at the maximum current.
  5. Assuming the resistor value is exact. Real resistors have tolerance. Consider the required accuracy of the application.

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Engineering Note

This calculator determines the ideal resistance using Ohm's Law. In an actual current loop, verify the power supply voltage, transmitter compliance voltage, input requirements, wiring resistance, and total loop burden before selecting or adding resistance.