Black Horse F2101 Relay Chattering Issue – Coil Hold Voltage Instability Analysis in Planar F Systems - Excellent PLC

Fault Overview

In a Planar F cabinet, one relay channel on the Black Horse F2101 4x Relay Amplifier began producing audible chattering.

Observed symptoms:

Relay repeatedly clicks ON/OFF rapidly
Output LED flickers
Load intermittently energizes
No PLC logic fluctuation
Other channels stable

This indicated unstable coil energization rather than contact welding.

Understanding Relay Coil Requirements

A mechanical relay requires:

Minimum pull-in voltage
Stable hold voltage
Adequate current supply
Clean return path

If coil voltage drops below hold threshold, relay releases momentarily, causing chatter.

Step 1 – Measure Coil Voltage Under Load

COIL_VOLTAGE_TEST:

Activate relay.

Measure voltage across coil.

Observe fluctuations over time.

Measured voltage fluctuated between 17V and 22V (nominal 24V system).

This fluctuation was enough to cross the relay hold threshold intermittently.

Step 2 – Check Power Supply Stability

POWER_SUPPLY_CHECK:

Measure 24V bus ripple.

Observe during heavy load switching.

Check supply headroom.

Supply ripple increased significantly when adjacent high-current load activated.

Voltage dipped briefly under load.

Step 3 – Ground Path Resistance Test

GROUND_PATH_TEST:

1. Measure voltage drop between module ground and supply ground.

2. Apply load and observe change.

Measured 1.8V drop across grounding terminal under load.

Loose terminal screw found on cabinet ground bar.

Root Cause – Combined Voltage Sag and Ground Resistance

Two contributing factors:

Undersized 24V power supply
Poor grounding connection increasing voltage drop

These conditions reduced effective coil voltage below hold threshold intermittently.

Why Only One Channel Affected

The affected relay controlled a heavier inductive load.

When energizing, it:

Drew higher initial current
Caused local voltage drop
Triggered unstable hold condition

Other channels carried lighter loads and remained stable.

Step 4 – Stability Verification

SYSTEM_VERIFICATION:

Tighten ground connections.

Temporarily upgrade supply.

Monitor coil voltage under full load.

After grounding correction and temporary higher-rated PSU, voltage stabilized at 24V ±0.5V.

Relay chattering stopped completely.

Corrective Action Plan

CORRECTIVE_ACTION:

– Replace undersized power supply.

– Tighten and clean grounding terminals.

– Separate high-current loads.

– Periodically check terminal torque.

Engineering Best Practices

Maintain minimum 30% PSU headroom.
Ensure low-resistance grounding path.
Avoid sharing control supply with heavy actuators.
Monitor voltage stability under dynamic load conditions.

Relay chatter is usually a supply integrity issue, not relay failure.

Conclusion

Relay chattering on the Black Horse F2101 4x Relay Amplifier is commonly caused by unstable coil hold voltage due to power supply sag and grounding resistance. Accurate voltage measurement under load conditions ensures reliable diagnosis and stable operation in Planar F systems.