Sensor questions: PLC wiring, 4-20mA and recording to Excel
Short answers to the questions engineers ask us most, from connecting a thermistor, thermocouple, RTD or potentiometer to a PLC to recording sensor data on a PC. Each answer links to the full guide.
Connecting sensors to a PLC
How do I wire a thermistor to a PLC?
Through a thermistor transmitter. Most PLC analog inputs can’t read a thermistor’s resistance directly, so the transmitter converts it to a 4-20mA signal. Wire the thermistor to the transmitter input, the 4-20mA loop in series with the supply and a spare PLC analog input, then scale 4–20 mA to temperature in the PLC.
Thermistor wiring guide →How do I connect a thermocouple to a PLC or DAQ analog input?
Most standard PLC analog inputs can’t read a thermocouple directly. Put a signal conditioner or temperature transmitter between the thermocouple and the PLC, then wire the sensor, the power and the measurement channel. J, K, T and E are the common types; T is often used for environmental monitoring.
Thermocouple to PLC guide →How do I connect a Pt100 RTD to a PLC?
Use a loop-powered RTD transmitter that converts the RTD’s resistance to a 4-20mA signal, and wire it to a PLC analog input. Pt100 and Pt1000 both work, and with a 24 V DC supply and 22 AWG cable the transmitter can sit more than 1,000 feet from the PLC.
RTD to PLC guide →How do I connect a potentiometer to a PLC?
A 2-wire, loop-powered potentiometer transmitter turns position into a 4-20mA signal for a spare PLC input. Versions are available for 1K, 2K, 5K and 10K ohm potentiometers, for valve position, level floats and linear or rotary travel.
Potentiometer transmitters →4-20mA signals
What is live zero in 4-20mA?
Live zero means the lowest reading of a 4-20mA signal is 4 mA, not 0 mA. A reading below 4 mA therefore points to an electrical problem, such as a failed supply fuse or a tripped breaker, which can raise an alarm or be logged.
Live zero explained →How do I read 4-20mA on a voltage input?
Add a 250 ohm resistor across the input. By Ohm’s law, 4 mA then reads as 1 V and 20 mA as 5 V, so on a 0-5 V recorder any value below 1 V shows an electrical problem.
Shunt resistor details →How do I read a 4-20mA signal on a PC?
Plug a USB converter such as the ROI-USB20 into a Windows PC. It is powered from the USB port, reads 4-20mA sensors in real time, and works with the included software or with Excel.
4-20mA on a PC →Recording sensor data on a PC and in Excel
What is a simple way to record temperature with a PC?
Connect a USB temperature module, such as the 4-input ROI-USB, and link its readings into Excel with no programming. A keyboard wedge is the simplest method; DDE is more robust because it doesn’t depend on which window is active.
Record temperature in Excel →Can temperature monitoring data be exported to Excel or LIMS?
Yes. With DDE, live readings stream straight into Excel cells, where you can chart them, set alarms with formulas and produce scheduled reports automatically. Data saved from Excel as a CSV file can be imported by most other systems, including LIMS.
Live data into Excel →What is DDE?
DDE (Dynamic Data Exchange) is a Windows method that lets one application send data to others, once or continuously. A DDE server app reads the sensors, and Excel or another DDE client receives the live values. One server can feed several clients.
DDE connection guide →What is a data acquisition system?
A data acquisition system measures physical quantities such as temperature, pressure or position and records them on a computer. It combines sensors, signal conditioning that makes their signals readable, and hardware and software that convert, display and store the readings.
Data acquisition explained →Not seeing an answer to your question?
Send us a note and we’ll be happy to answer it, and perhaps add it to this page. For a system, tell us the sensors, the ranges and what they connect to, from one sensor or parameter to hundreds.