Issue
An Analogue Input (AI) point configured for a 0–10V signal in Sigma or BAS may display a default or erroneous value when the input voltage exceeds 10V DC.
Product Line
Satchwell BAS & Sigma
Environment
Satchwell BAS & Sigma
Cause
Sigma AI points are designed to operate within a 0–10V DC range. If the connected sensor or device outputs a voltage greater than 10V, the system interprets this as an out-of-range condition, causing the AI point to revert to its default value.
This behavior is commonly observed with certain sensors (e.g., SHD100 / SHD101 Humidity Sensors) that can output voltages above 10V, especially under specific environmental conditions.
Resolution
To prevent out-of-range values and ensure accurate readings, consider the following mitigation strategies:
1. Voltage Clamping Using a Zener Diode
- Component: Zener Diode (e.g., 9.1V rating)
- Installation:
- Can be installed at the sensor end (preferred for ease) or at the controller input.
- Operation:
- Limits the input voltage to the Zener’s breakdown voltage (e.g., 9.1V).
- Any voltage above this threshold is clamped, preventing overvoltage conditions.
- Note:
- Typically, a series resistor is used with Zener diodes. However, if the sensor has high impedance, the current is minimal, and a resistor may not be necessary.
Diagram:
2. Series Diode Voltage Drop
- Component: Standard Diode (e.g., 1N4001)
- Installation:
- Connect in series with the sensor’s output signal to the controller.
- Operation:
- Introduces a voltage drop (~0.6V), effectively reducing the input voltage.
- Requires a revised lookup table to compensate for the voltage drop.
- Example: 10V input becomes 9.4V at the controller.
3. Limiting Actuator Feedback Voltage
- Scenario: When feedback is sourced from an actuator.
- Solution:
- Restrict actuator output to a maximum of 9.5V.
- Ensures feedback signal remains within acceptable range.
- Caution:
- May slightly reduce valve/system performance if the actuator controls a mechanical device like a valve.
Additional Notes
- Always verify the sensor’s output characteristics before integration.
- Consider using lookup tables to calibrate and correct voltage offsets introduced by diodes.
- For humidity sensors like SHD100/101, refer to their datasheets for voltage output behavior under varying humidity levels.
