Voltage related information for BMXSDO0802

[YONGHO-KIM]

1. The configuration settings for BMXSDO802 are as follows.

2. The cable between the field device and the PLC panel is currently not connected.

3. Voltage is being measured at the terminal block and the output module in the PLC to check the health of each channel.

4. When each channel of the BMXSDO0802 is controlled and measured using a Multi-meter, the following voltages are observed: 

  - 0 (False): 18 - 19VDC

  - 1 (True): 23 - 24VDC

5. The question is: when the output is set to 0(False), the Multi-meter still reads approximately 18.5VDC. Why is voltage present even when the output is turned off?

[YONGHO-KIM · Accepted solution]

Importantly, this 19v reding does not indicate a fault of risk of unintended activation. The BMXSDO0802 module complies with IEC 61131-2 standards, which specify that a de-energized output must not supply more than 0.5mA of current. The module adheres to this requirement, ensuring that the leakage current remains below this threshold. As a result, the output remains in a true de-energized state and cannot activate any connected actuator.

1) Characteristics of Solid-State Output

• Solid-state outputs typically use transistors to control ON/OFF states.
• Even when the output is OFF, a small leakage current (up to 0.5mA) may flow due to the nature of transistors.
• As a result, the circuit may not appear to be completely open in a physical sense.

2) Residual Voltage When Load Is Not Connected

• If no load is connected to the output terminal, the terminal may become floating, and the potential difference in the internal circuit can be detected by a multimeter.
• This can result in a residual voltage being measured (e.g., approximately 18V).
• On the other hand, when a load is connected, the leakage current flows to ground through the load, and the output terminal voltage drops to nearly 0V.
• In other words, the output is not actually ON, and this is a normal behavior based on circuit characteristics.

3) High Impedance Characteristics of Digital Multimeters

• Digital multimeters generally have a high input impedance (typically over 10MΩ), allowing measurements with minimal impact on the circuit.
• Due to this high impedance, residual voltage can be sensitively detected.
• Therefore, measuring around 18V at the output terminal in the OFF state is a normal phenomenon caused by both the circuit and multimeter characteristics."