Sigma – Using MZ20A Actuators with the Surface Mount Version of the S-UNC-AO Card

[Product_Support]

Issue

When using MZ20A or MZ20A-R actuators with the surface-mounted S-UNC-AO card, the actuator may stop functioning, and the analogue output remains at 0 volts after a power cycle.

Product Line

Satchwell BAS & Sigma

Environment

• Satchwell Sigma S-UNC-AO Card
• MZ20A Actuators

Cause

If both the controller power and battery switches are turned off, upon restoring power, the analogue output may fail to resume normal operation, remaining at 0V. This is due to the absence of a discharge path for the analogue output channels.

The precise root cause of the controller lock-up is currently undetermined. However, it has been observed that certain conditions during editing can trigger this behavior.

Resolution

To prevent this issue:

1. Install a 2.2kΩ resistor (2K2 ohm) between each analogue output channel and the common 0V rail

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2. The resistor provides a discharge path, ensuring the output voltage returns to normal after power is restored.

Resistor Specifications:

• Type: Carbon Film
• Rating: ¼ watt, 5% tolerance
• Example: RS Stock No. 135-881 (or equivalent)

💡 Note: This modification is only required for MZ20A actuators. If the actuator is replaced with a different model, the resistor should be removed.