4–20 mA Converter: Read a 4–20 mA Transmitter with Arduino & ESP32

Is this your problem?

If any of these match your 4–20 mA sensor setup, read on:

• The ADC reads 0 with the transmitter connected
• Readings stop rising near full scale
• The value jumps or drops when the pressure is high
• The transmitter’s display flickers or resets
• You can’t tell a broken wire from a real zero
• An ESP32 or Arduino pin died after a wiring mistake

How a 4–20 mA loop works, in one picture

A 2-wire transmitter doesn’t output a voltage. It sits in series with a power supply and sets the current through the whole loop: 4 mA at the bottom of its range, 20 mA at the top. The same current flows through every part of the loop, so cable resistance doesn’t change the signal. You read it by passing the current through a known resistor and measuring the voltage across it.

Calculate your burden resistor

Pick your board and your transmitter’s range. The calculator picks a standard burden value with room to detect faults, checks that the transmitter still gets enough voltage at 20 mA, and shows what happens if the loop shorts. Drag the current slider to see the reading at any loop current.

The 2-minute test: is it the transmitter or the ADC side?

Before you blame the sensor, test the burden and code with a fake loop. Disconnect the transmitter and its supply, then connect two AA cells in series with a 150 Ω resistor from the top of the burden to GND (cell + to the resistor, cell − to GND). With a 107 Ω burden that pushes about 11–12 mA, a known, quiet current. Check it with a multimeter in its mA range. If your code reads the same current, the ADC side is right and the problem is in the loop or the transmitter.

// Print loop current once a second (ESP32, ADC1 pin 34, 107 ohm burden)
void loop() {
  uint32_t mv = 0;
  for (int i = 0; i < 32; i++) mv += analogReadMilliVolts(34);
  float mA = (mv / 32.0) / 107.0;
  Serial.printf("%.2f mA\n", mA);
  delay(1000);
}

Wire it right in 3 steps

1. Put the burden on the low sideLoop supply + to the transmitter's + terminal. Transmitter − to the top of the burden. Bottom of the burden to GND, and the loop supply's − to the same GND. The ADC reads the top of the burden through a 1 kΩ resistor with 100 nF to GND at the pin.
2. Check the voltage budgetThe supply must cover the transmitter's minimum voltage plus the drop across the burden and cable at 20 mA: V_supply ≥ V_min + 0.02 A × (R_burden + R_cable). Most 2-wire transmitters need around 8 to 12 V; check yours. A 24 V supply usually leaves plenty of margin.
3. Protect the pin and detect faultsAdd a PTC fuse in the loop and a TVS across the burden, and keep the 1 kΩ in series with the pin. In code, flag anything below 3.6 mA (broken wire, no power, sensor fault) or above 21 mA (short or sensor fault) instead of showing it as a value.

2-wire, 3-wire or 4-wire?

Which board are you using?

The common 250 Ω value gives 1 to 5 V. That suits a 5 V Arduino only if you don't need to see over-range faults, and it will damage a 3.3 V board at 20 mA.

4–20 mA converter options

Still reading wrong? Other common causes

Frequently asked questions

© Ramanu Systems · Precision electronics modules, Made in India · Fault thresholds from NAMUR NE43. Board limits from the Microchip ATmega328P, Espressif ESP32 and ESP-IDF, Raspberry Pi RP2040 and Texas Instruments ADS1115 documentation. HART loop resistance from the HART physical layer specification. Results are estimates; verify with your own measurement. Found an error? Tell us and we will fix it and credit you.