Relay Output Stuck Closed on Black Horse F1201 4x Relay Amplifier Module – Failure Analysis and Repair Strategy - Excellent PLC

Incident Description

In a Planar F control cabinet, one output channel of the Black Horse F1201 4x Relay Amplifier Module remained energized even after the control signal was removed.

Field symptoms included:

Actuator remained active
Output status indicator mismatched logic command
Forcing output OFF in controller had no effect

Only one relay channel was affected; the remaining three operated normally.

Understanding the F1201 Relay Architecture

The F1201 module amplifies low-level control signals into isolated relay outputs. Each channel typically consists of:

Input driver stage
Coil energizing circuit
Mechanical relay contacts (NO/NC)
Arc suppression network

Because the output is mechanical, failures can be electrical or physical in nature.

Failure Classification

When a relay remains stuck closed, the root cause generally falls into one of three categories:

RELAY_STUCK_DIAGNOSIS:

Control signal continuously active (logic issue)

Coil driver transistor shorted

Mechanical contact welding due to overload

Step 1 – Eliminate Logic or Control Layer Issue

Before suspecting hardware:

LOGIC_VERIFICATION:

– Confirm output command state in controller.

– Check for forced outputs.

– Monitor real-time logic transitions.

If the control signal properly drops to OFF but the field output remains energized, proceed to hardware checks.

Step 2 – Coil Voltage Measurement

Using a multimeter:

COIL_TEST:

Measure voltage across relay coil.

IF voltage present → driver stage shorted.

IF no voltage present → mechanical contact welded.

In this documented case, no coil voltage was detected, yet the output remained active.

Root Cause: Contact Welding

High inrush current from an inductive load (solenoid valve without proper suppression) caused arc formation inside the relay. Over time, repeated switching welded the normally open contact closed.

Typical indicators:

Burn marks near output terminal
Slight overheating odor
Permanent continuity across contacts

Corrective Action

CORRECTIVE_ACTION_PLAN:

– De-energize cabinet.

– Remove F1201 module.

– Replace module (relay not field-repairable).

– Install proper flyback diode or RC snubber on inductive load.

After replacement and load suppression installation, output behavior returned to normal.

Preventive Engineering Recommendations

Always use suppression devices on inductive loads.
Avoid exceeding rated switching current.
Periodically inspect high-duty-cycle relay outputs.
Consider solid-state alternatives for frequent switching applications.

Relay modules are durable but sensitive to improper load protection.

Conclusion

A relay output stuck closed on the Black Horse F1201 4x Relay Amplifier Module is most commonly caused by contact welding due to excessive or unsuppressed inductive load switching. Structured voltage testing quickly distinguishes between logic faults, driver short circuits, and mechanical contact failure. Proper load suppression significantly extends relay life within Planar F systems.