Excellent PLC
Incident Overview
In a conveyor automation system, a motor starter controlled by one channel of the Black Horse F1201 4x Relay Amplifier Module began rapidly switching ON and OFF.
Operators reported:
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Audible relay clicking at irregular intervals
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Motor contactor repeatedly engaging and dropping
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No logic command fluctuation in the controller
The issue was isolated to a single relay channel, while others operated normally.
What Is Relay Chatter?
Relay chatter refers to rapid, unintended switching caused by unstable coil energization. It is typically triggered by:
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Fluctuating input signal
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Power supply instability
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Excessive voltage drop
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Ground reference instability
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External electromagnetic interference
Unlike a stuck relay, chatter indicates unstable control conditions.
Structured Diagnostic Flow
To avoid replacing a healthy module prematurely, the following method was applied:
1. Monitor control logic signal stability.
2. Measure supply voltage during switching.
3. Inspect input signal integrity.
4. Check grounding continuity.
5. Perform load isolation test.
Step 1 – Confirm Stable Control Logic
Trend monitoring confirmed the controller output remained continuously ON.
This eliminated upstream logic instability.
Step 2 – Power Rail Stability Test
Using a high-resolution meter:
Measure DC supply at module.
Observe voltage during chatter event.
Check for dips below coil hold-in threshold.
Voltage briefly dipped during motor start events, approaching the relay drop-out threshold.
Step 3 – Load-Induced Voltage Drop Analysis
Further inspection revealed:
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Undersized DC power supply
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Shared supply feeding multiple inductive loads
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No dedicated suppression on motor contactor coil
When large loads activated, supply voltage sagged, causing relay coil to repeatedly lose holding current.
Controlled Isolation Test
Disconnect high-load devices temporarily.
Activate relay output.
Observe stability.
After isolating high-current loads, chatter stopped completely.
Root Cause
The primary cause was inadequate DC supply capacity combined with voltage drop across long wiring runs.
The relay coil requires stable voltage above its holding threshold. When voltage drops below this level—even briefly—the relay releases, then re-engages when voltage recovers, producing chatter.
Corrective Action Plan
– Upgrade DC power supply capacity.
– Shorten or increase gauge of supply wiring.
– Install suppression devices on inductive loads.
– Verify stable ground reference.
After implementing a higher-capacity regulated supply and installing flyback diodes, chatter was eliminated.
Preventive Engineering Recommendations
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Calculate total coil load before selecting DC supply.
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Separate control power from heavy inductive loads.
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Perform voltage drop analysis during commissioning.
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Periodically test supply stability under full operational load.
Power integrity is critical for relay stability in multi-channel modules.
Conclusion
Intermittent output chatter on the Black Horse F1201 4x Relay Amplifier Module is typically caused by unstable coil voltage due to supply sag, load-induced voltage drops, or grounding instability. Structured voltage monitoring under dynamic load conditions allows precise fault isolation and prevents unnecessary module replacement in Planar F control systems.