Black Horse F2 DO 16 02 Output Stuck ON – MOSFET Short Circuit Failure and Inductive Spike Analysis in HIMatrix Systems - Excellent PLC

Fault Overview

In a HIMatrix-controlled automation system, one channel on the Black Horse F2 DO 16 02 Remote I/O Module remained permanently energized.

Observed symptoms:

Output command from controller = OFF
Physical output voltage present (24V)
Load device continuously powered
No communication fault
Other channels normal

Unlike logic errors, this behavior indicated hardware-level conduction failure.

Output Hardware Architecture

The F2 DO 16 02 typically uses:

Low-side or high-side MOSFET switching
Integrated overcurrent monitoring
Thermal shutdown
Diagnostic feedback

When MOSFET fails shorted, it becomes permanently conductive.

Step 1 – Confirm Software State

LOGIC_VALIDATION:

Monitor controller output variable.

Disable forcing.

Download and verify program consistency.

Controller confirmed output OFF.

Step 2 – Voltage Verification

OUTPUT_TEST:

Measure voltage between output and return.

Disconnect field load.

Re-measure output terminal voltage.

Voltage remained at supply level even without load connected.

Step 3 – Resistance Comparison

HARDWARE_CHECK:

Power down module.

Measure resistance between output and supply.

Compare to healthy channel.

Faulty channel showed near-zero resistance.

Healthy channels showed high resistance.

Root Cause – Inductive Load Without Flyback Protection

The connected device was:

A DC solenoid valve
No flyback diode installed
Long cable routing
Frequent switching cycles

When de-energized, inductive collapse generated high-voltage spike.

Over time, repeated spikes exceeded MOSFET voltage rating, causing internal short circuit.

Why Protection Did Not Prevent Damage

Electronic overcurrent protection protects against excessive current, not voltage spikes.

Inductive overvoltage can:

Punch through MOSFET junction
Create permanent conductive path
Destroy internal gate insulation

This type of failure is cumulative.

Corrective Action Plan

CORRECTIVE_ACTION:

– Replace F2 DO 16 02 module.

– Install flyback diode across solenoid.

– Verify diode polarity.

– Consider RC snubber for long cable runs.

After adding proper suppression, replacement module operated normally.

Preventive Engineering Guidelines

Always protect inductive loads with suppression devices.
Avoid long unshielded output cables.
Review MOSFET voltage rating vs load characteristics.
Perform switching transient analysis during commissioning.

MOSFET-based outputs are efficient but sensitive to unmanaged inductive energy.

Conclusion

A permanently energized output on the Black Horse F2 DO 16 02 Remote I/O Module is most commonly caused by MOSFET short circuit failure due to repeated inductive voltage spikes. Proper suppression design and structured diagnostic testing ensure reliable operation in HIMatrix safety systems.