4-20 mA pressure transmitter reads below 4 mA or zero on PLC, what to check?
I have a 4-20 mA pressure transmitter that is reading below 4 mA or even zero on my PLC analog input. The process pressure is definitely not zero. What should I check first to troubleshoot this?
When a 4-20 mA pressure transmitter reads below 4 mA or zero, it indicates a problem with the signal loop, power, or the transmitter itself. This usually means the PLC isn't receiving a valid signal. Start by verifying the loop power and checking the wiring connections.
Likely causes and checks
1Loss of Loop Power
- Points to it
- The transmitter requires external power to operate and generate the 4-20 mA signal. If the power supply is off, failed, or disconnected, the transmitter cannot send a signal, resulting in 0 mA.
- Check
- With the loop powered, using a meter rated for it, measure the DC voltage at the transmitter's power terminals. It should typically be 24 VDC. Also, check the fuse on the power supply or input card.
2Open Circuit or Wiring Fault
- Points to it
- A break in the wiring, a loose connection, or a damaged cable will prevent the current from flowing through the loop, causing the PLC to read 0 mA or a very low value.
- Check
- Isolate and lock out the circuit. Visually inspect all wiring from the power supply, through the transmitter, to the PLC input for damage or loose terminals. Use a multimeter to check for continuity across the loop, especially at junction boxes and terminal blocks.
3Failed Transmitter
- Points to it
- If the transmitter itself has failed internally, it may stop generating a current signal, even with proper power. This can happen due to internal component failure, damage, or age.
- Check
- Isolate and lock out the circuit. Disconnect the transmitter from the loop. Use a calibrated current source to inject a known 4-20 mA signal into the PLC input to verify the PLC and wiring are working. If they are, reconnect the transmitter and measure the current directly in series with the transmitter using a multimeter. If it reads 0 mA or an incorrect value despite proper power and pressure, the transmitter is likely faulty.
Next check: After checking power and wiring, if the problem persists, use a multimeter to measure the current directly in series with the transmitter to confirm if the transmitter is outputting a signal. If it's not, consider replacing the transmitter.
What it typically looks like in the data
Related questions
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