How to Replace a 3-Wire Sensor?

Replacing a 3-wire sensor requires a step-by-step approach to ensure power-off safety, correct wiring, and signal compatibility. The following is a general replacement procedure applicable to sensors commonly found in industrial environments, such as resistance thermometers (e.g., PT100) and discrete sensors (e.g., proximity switches):

I. Preparation Before Replacement

Verify that the new sensor's model and parameters match the original:

Type: PT100/PT1000? PNP/NPN? 4-20mA? Ensure it matches the original model exactly;

The measurement range, accuracy, thread size, and protection rating (e.g., IP65) must be suitable for the specific operating conditions.

Prepare the necessary tools:

Multimeter (for testing voltage and resistance);

Screwdrivers and wrenches (for removing and installing terminal connections);

Insulation tape or terminal caps (to prevent short circuits);

Shielded cable (if an extension cable is required, use a 3-core shielded twisted-pair cable of the same specifications).

Safety Measures:

Strictly ensure that the power supply is disconnected to prevent short circuits or damage to the PLC input module caused by working on live circuits;

Mark the relevant points within the control system to prevent accidental operation.

II. Removing the Old Sensor

Disconnect the wiring:

Take a photograph to record the original wiring configuration (specifically noting the positions of the brown, blue, and black wires);

Loosen the terminal screws and remove the three wires one by one;

If replacing a resistance thermometer, pay close attention to the terminal markings (e.g., M+, I+, I-).

Remove the sensor body:

Use a wrench to loosen and remove the retaining nut;

Slowly withdraw the sensor and inspect it for any signs of corrosion, scale buildup, or mechanical damage.

III. Installing the New Sensor

Secure the sensor:

Insert the new sensor into the thermowell or mounting port;

Use a wrench to tighten it to the specified torque value, taking care not to overtighten, which could cause breakage. Wiring (Critical Steps)

Resistance Thermometers (e.g., 3-wire PT100)

Identify Wire Colors: Typically consists of two red wires and one blue wire, or two white wires and one black wire.

Wiring Rules:

Connect the two wires of the same color to the instrument's M+ and I+ terminals, respectively;

Connect the wire of the different color to the I- terminal;

Short-circuit (jumper) the M- and I- terminals (Note: Some modules have this connection pre-wired internally; consult the manual to verify).

Key Point: The three wires must be of the same material, same length, and same wire gauge; otherwise, the temperature compensation will fail.

Discrete Sensors (PNP/NPN Types)

PNP Type: Outputs a high-level signal (≈24V); matches a sinking-input PLC (where the COM terminal is connected to 0V);

NPN Type: Outputs a low-level signal (≈0V); matches a sourcing-input PLC (where the COM terminal is connected to +24V).

Error Example: Connecting an NPN sensor to a sourcing-input PLC will result in the signal failing to conduct (no signal detection).

IV. Post-Replacement Testing

Pre-Power-On Inspection

Use a multimeter to measure the voltage across the power terminals (Brown-Blue) to ensure it is within 24V ±10%;

Check that all wiring connections are secure and verify there are no short circuits or reverse connections.

Post-Power-On Verification

Resistance Thermometers: Observe the instrument display to ensure the temperature reading is plausible and stable;

Discrete Sensors: Manually trigger the sensor and observe whether the corresponding PLC input point responds;

Analog Sensors: Measure the output current (4-20mA) or voltage (0-10V) to verify that it varies in response to changes in the input parameter.

Recommended Procedure: Power off to check resistance → Power on to measure voltage → Trigger sensor to test signal → Check system feedback.