Black Horse F2101 Multiple Relay Channels Not Energizing – Common Coil Supply Failure in Planar F Systems - Excellent PLC

Fault Overview

In a Planar F control cabinet, three channels of the Black Horse F2101 4x Relay Amplifier suddenly stopped energizing.

Observed symptoms:

PLC outputs active
No LED illumination on affected channels
No relay click
Fourth channel still working
Field power stable

The simultaneous failure of multiple channels strongly suggested a shared circuit problem.

Understanding Internal Coil Supply Design

In the F2101 module:

Relay coils share a common supply rail
Each channel has individual driver transistor
Common return path connected internally
LED indicators powered from coil supply

If common coil voltage disappears, multiple channels stop working.

Step 1 – Verify Individual Driver Function

DRIVER_COMPARISON:

Compare control signal to working channel.

Check base/gate drive voltage.

Confirm PLC output stable.

Driver control signals were present on all failed channels.

Step 2 – Measure Coil Supply Voltage

COIL_SUPPLY_TEST:

Activate failed channel.

Measure voltage at coil supply rail.

Compare with working channel.

Measured:

0V at failed channels’ coil supply
24V at working channel coil rail

This indicated segmented supply failure.

Step 3 – Inspect Terminal Block and Internal Bus

BUS_INSPECTION:

Power down module.

Inspect terminal screws.

Measure continuity of coil supply path.

Found:

Loose terminal screw at coil supply input
Slight carbon discoloration
High resistance at input connector

This caused voltage drop across internal bus.

Root Cause – High-Resistance Connection at Coil Supply Input

Because three channels shared the same internal bus branch:

Poor contact increased resistance
Under load, voltage collapsed
Coils failed to energize

The fourth channel was connected to separate internal branch.

Why No Diagnostic Alarm Appeared

The module:

Does not monitor coil supply voltage directly
Only indicates channel activation
Lacks supply fault detection

Therefore, failure appeared as silent output loss.

Step 4 – Voltage Drop Verification

DROP_TEST:

Energize working channel.

Measure voltage before and after terminal.

Calculate voltage drop.

Measured 6V drop across loose connection under load.

Corrective Action Plan

CORRECTIVE_ACTION:

– Clean terminal contact surface.

– Tighten supply terminal to specified torque.

– Replace damaged connector if necessary.

– Perform full load voltage drop test.

After correcting terminal issue, all channels energized normally.

Engineering Best Practices

Periodically torque-check terminal screws.
Avoid using undersized conductors.
Inspect for heat discoloration.
Document coil supply wiring clearly.

Grouped failures often originate from shared supply connections.

Conclusion

When multiple channels on the Black Horse F2101 4x Relay Amplifier fail simultaneously, the most common cause is interruption of the common coil supply rail. Structured voltage tracing and terminal inspection quickly isolate the issue and restore stable operation in Planar F systems.